SQUARATE DYE CONJUGATE, PROCESS FOR MAKING IT AND ITS USE IN ASSAY METHODS AND KITS

Abstract: Novel assays for ligands and receptors employing novel compounds that are conjugates of squarates dyes and mem­ bers of a specific binding pair (sbp) are disclosed. The sbp members are selected from the group consisting of ligand and its complementary receptor. The sbp member is covalently or non-covalently bound to the squarate dye, which usually has an absorption maximum greater than 600 nanometers. The novel conjugates are employed in assays for determining the presence or amount of an sbp member analyte in a sample suspected of containing such analyte. Kits comprising such novel conjugates are also disclosed.

Description: Fluorescent compounds find wide application because of their ability to emit light upon excitation with energy within certain energy ranges. By virtue of this ability, fluorescers have found employment as labels in chemical or biological processes, e.g. assays. That is, various compounds can be conjugated to a fluorescent compound, the conjugate subjected to some type of partitioning, and the fate of the conjugate determined by irradiating the sample with light and detecting the zone in which the conjugate exists. This technique can be employed in immunoassays, involving specific binding pairs, such as ligands and receptors, e.g., antigens and antibodies. By conjugating a fluorescer to one of the members of the specific binding pair and employing various protocols, one can provide for partitioning of the fluorescer conjugate between a solid phase and a liquid phase in relation to the amount of antigen in an unknown sample. By measuring the fluorescence of either of the phases, one can then relate the level of fluorescence observed to a concentration of the antigen in the sample. Alternatively, one can avoid partitioning of the fluorescent label by providing for a mechanism which varies the fluorescence of the label, depending upon the label environment in a liquid medium. For example, in addition to labeling one of the members of the specific binding pair with the fluorescer, one may label the other member with a quencher, that is, a molecule which is able to absorb the excitation energy of the fluorescer molecule, preventing the emission of a photon. The quenching then will occur only when the two members of the specific binding pair are associated and the fluorescer and quencher thereby achieve the required spatial proximity for quenching. U.S. Pat. No. 3,998,943 discloses an immunoassay involving a ligand-fluorescer conjugate employing steric inhibition of simultaneous binding of antibody for ligand and antibody for fluorescer, where the antibody for fluorescer substantially quenches the fluorescence. U.S. Pat. No. 3,996,345 describes an immunoassay involving fluorescer-quencher pairs, which employs a conjugate of a fluorescer bonded to one member of a specific binding pair and a conjugate of a quencher bonded to the same or different member of a specific binding pair. The assay is dependent upon the degree to which the quencher and fluorescer are brought within quenching proximity based on the amount of analyte in the medium. Novel conjugates of fluorescers and quenchers with poly(amino) acids are disclosed in U.S. Patent Nos. 4,351,760 and 4,318,846. A dye tagged reagent is described in U.S. Patent No. 4,166,105. Digoxigenin immunogens, antibodies, labeled conjugates and related derivates are discussed in U.S. Patent No. 4,469,797. Various squarate dyes are discussed by Sprenger, et al. , Angew. Chem. , 80 , 541 (1968); Sprenger, et al., Angew. Chem. , 79 ; 581, 1967; Sprenger, et al., Angew. Chem. internat. Edit. , 5 :894, 1966; and Maaks, et al., ibid ., 5 :888, 1966. The use of laser beams and slits to differentiate particles based on their relative size by the correlation of fluorescence fluctuations in a relatively large sample volume is described by Briggs et al, Science , 212 :1266-1267, 1981, and by Nicoli et al., Proc. Natl. Acad. Sci., USA, 77 :4904-4908, 1980. The use of squarate dyes incorporated into cells as fluorescent agents in immunoassays is described in European Patent Application No. 85306054.9, published under EP 176252. The present invention concerns novel compounds that are conjugates of squarate dyes, usually having an absorption maximum greater than 600 nanometers, and members of a specific binding pair. The specific binding pair members are selected from the group consisting of ligand and its complementary receptor. The novel compounds are employed in assays for determining the presence or amount of an analyte in a sample containing the analyte. Novel assay methods employing the above conjugates or water compatible squarate dyes are also included in the invention. Kits comprising the novel compounds are also provided. The present invention provides an improvement in compounds used in an assay for determining an analyte in a sample where the compounds are conjugates of a dye and a specific binding pair member. The improvement comprises employing a squarate dye as the dye in the conjugate. The present invention also includes novel assay methods employing the squarate dye conjugates or water compatible squarate dyes in conjunction with a helium/neon laser. Before proceeding further with the description of the specific embodiments of the present invention, a number of terms will be defined. Analyte--the compound or composition to be measured, the material of interest, which is usually a member of a specific binding pair and may be a ligand, which is mono-or polyvalent, usually antigenic or haptenic, and is a single compound or plurality of compounds which share at least one common epitopic or determinant site. The polyvalent ligand analytes will normally be poly(amino acids), i.e., polypeptides and proteins, polysaccharides, nucleic acids, and combinations thereof. Such combinations include components of bacteria, viruses, chromosomes, genes, mitochondria, nuclei, and cell membranes. The precise nature of the analytes together with numerous examples thereof are disclosed in U.S. Patent No. 4,299,916 to Litman, et al., particularly at columns 16 to 23, the disclosure of which is incorporated herein by reference. For the most part, the polyepitopic ligand analytes employed in the subject invention will have a molecular weight of at least about 5,000, more usually at least about 10,000. In the poly(amino acid) category, the poly(amino acids) of interest will generally be from about 5,000 to 5,000,000 molecular weight, more usually from about 20,000 to 1,000,000 molecular weight; among the hormones of interest, the molecular weights will usually range from about 5,000 to 60,000 molecular weight. A wide variety of proteins may be considered such as the family of proteins having similar structural features, proteins having particular biological functions,or proteins related to specific microorganisms, particularly disease causing microorganisms. The monoepitopic ligand analytes will generally be from about 100 to 2,000 molecular weight, more usually from 125 to 1,000 molecular weight. The analytes of interest include drugs, metabolites, pesticides,. pollutants, and the like. Included among drugs of interest are the alkaloids. Among the alkaloids are morphine alkaloids which includes morphine, codeine, heroin, dextromethorphan, their derivatives and metabolites; cocaine alkaloids, which include cocaine and benzoyl ecgonine, their derivatives and metabolites, ergot alkaloids, which include the diethylamide of lysergic acid; steroid alkaloids; iminazoyl alkaloids; quinazoline alkaloids, isoquinoline alkaloids; quinoline alkaloids, which include quinine and quinidine; diterpene alkaloids, their derivatives and metabolites. The next group of drugs includes steroids, which includes the estrogens, gestogens, androgens, andreocortical steroids, bile acids, cardiotonic glycosides and aglycones, which includes digoxin and digoxigenin, saponins and sapogenins, their derivatives and metabolites. Also included are the steroid mimetic substances, such as diethylstilbestrol. The next group of drugs is lactams having from 5 to 6 annular members, which include the barbituates, e.g. phenobarbital and secobarbital, diphenylhydantonin, primidone, ethosuximide, and their metabolites. The next group of drugs is aminoalkylbenzenes, with alkyl of from 2 to 3 carbon atoms, which includes the amphetamines, catecholamines, which includes ephedrine, L-dopa, epinephrine, narceine, papaverine, and their metabolites. The next group of drugs is benzheterocyclics which include oxazepam, chlorpromazine, tegretol, imipramine, their derivatives and metabolites, the heterocyclic rings being azepines, diazepines and phenothiazines. The next group of drugs is purines, which includes theophylline, caffeine, their metabolites and derivatives. The next group of drugs includes those derived from marijuana, which includes cannabinol and tetrahydrocannabinol. The next group of drugs includes the vitamins such as A, B, e.g. B₁₂, C, D, E and K, folic acid, thiamine. The next group of drugs is prostaglandins, which differ by the degree and sites of hydroxylation and unsaturation. The next group of drugs is antibiotics, which include penicillin, chloromycetin, actinomycetin, tetracycline, terramycin, the metabolites and derivatives. The next group of drugs is the nucleosides and nucleotides, which include ATP, NAD, FMN, adenosine, guanosine, thymidine, and cytidine with their appropriate sugar and phosphate substituents. The next group of drugs is miscellaneous individual drugs which include methadone, meprobamate, serotonin, meperidine, amitriptyline, nortriptyline, lidocaine, procaineamide, acetylprocaineamide, propranolol, griseofulvin, valproic acid, butyrophenones, antihistamines, anticholinergic drugs, such as atropine, their metabolites and derivatives. Metabolites related to diseased states include spermine, galactose, phenylpyruvic acid, and porphyrin Type 1. The next group of drugs is aminoglycosides, such as gentamicin, kanamicin, tobramycin, and amikacin. Among pesticides of interest are polyhalogenated biphenyls, phosphate esters, thiophosphates, carbamates, polyhalogenated sulfenamides, their metabolites and derivatives. For receptor analytes, the molecular weights will generally range from 10,000 to 2X10⁶, more usually from 10,000 to 10⁶ For immunoglobulins, IgA, IgG, IgE and IgM, the molecular weights will generally vary from about 160,000 to about 106. Enzymes will normally range from about 10,000 to 6,000,000 in molecular weight. Natural receptors vary widely, generally being at least about 25,000 molecular weight and may be 10⁶ or higher molecular weight, including such materials as avidin, thyroxine binding globulin, thyroxine binding prealbumin, and transcortin. Ligand analog or analyte analog--a modified ligand or ligand surrogate or modified analyte or analyte surrogate which can compete with the analogous ligand or analyte for a receptor, the modification providing means to join a ligand analog or analyte analog to another molecule. The ligand analog or analyte analog will usually differ from the ligand or analyte by more than replacement of a hydrogen with a bond which links the ligand analog or analyte analog to a hub or label, but need not. The term ligand surrogate or analyte surrogate refers to a compound having the capability of specifically binding a receptor complementary to the ligand or analyte. Thus, the ligand surrogate or analyte surrogate can bind to the receptor in a manner similar to the ligand or analyte. The surrogate could be, for example, an antibody directed against the idiotype of an antibody to the ligand or analyte. Poly(ligand analog)--a plurality of ligand analogs joined together covalently, normally to a hub nucleus. The hub nucleus is a polyfunctional material, normally polymeric, usually having a plurality of functional groups, e.g., hydroxyl, amino, mercapto, ethylenic, etc. as sites for linking. The hub nucleus may be water soluble or insoluble, preferably water soluble, and will normally be at least about 30,000 molecular weight and may be 10 million or more molecular weight. Illustrative hub nuclei include polysaccharides, polypeptides (including proteins), nucleic acids, anion exchange resins, and the like. Water insoluble hub nuclei can also include walls of containers, e.g. glass or plastic, glass beads, addition and condensation polymers, Sephadex (™) and Agarose beads and the like. Member of a specific binding pair ("sbp member")--one of two different molecules, having an area on the surface or in a cavity which specifically binds to and is thereby defined as complementary with a particular spatial and polar organization of the other molecule. The members of the specific binding pair are referred to as ligand and receptor (antiligand). These will usually be members of an immunological pair such as antigen-antibody, although other specific binding pairs such as biotin-avidin, hormones-hormone receptors, nucleic acid duplexes, IgG-protein A, DNA-DNA, and DNA-RNA, are not immunological pairs but are included in the invention. Ligand-any organic compound for which a receptor naturally exists or can be prepared. Receptor ("antiligand")-any compound or composition capable of recognizing a particular spatial and polar organization of a molecule, e.g., epitopic or determinant site. Illustrative receptors include naturally occurring receptors, e.g., thyroxine binding globulin, antibodies, enzymes, Fab fragments, lectins, nucleic acids, protein A, and complement component Clq. Support or surface --a porous or non-porous water insoluble material. The surface can be hydrophilic or capable of being rendered hydrophilic and can be formed from inorganic powders such as silica, magnesium sulfate, and alumina; natural polymeric materials, particularly cellulosic materials and materials derived from cellulose, such as fiber containing papers, e.g., filter paper, and chromatographic paper.; synthetic or modified naturally occurring polymers, such as nitrocellulose, cellulose acetate, poly(vinyl chloride), polyacrylamide, cross linked dextran, agarose, polyacrylate, polyethylene, polypropylene, poly(4-methylbutene), polystyrene, polymethacrylate, poly(ethylene terephthalate), nylon, and poly(vinyl) butyrate).; either used by themselves or in conjunction with other materials; such as glass, ceramics, and metals. Binding of sbp members to the surface may be accomplished by well-known techniques, commonly available in the literature. See, for example, "Immobilized Enzymes," Ichiro Chibata, Halsted Press, New York (1978) and Cuatrecasas, J. Biol. Chem. , 245:3059 (1970) or Cautrecases, J. Biol. Chem. , 245,3059 (1970). Particles--particles of at least about 50nm and not more than about 50 microns, usually at least about 100 nm and less than about 25 microns, preferably from about 0.2 to 5 microns diameter. The particle may be organic or inorganic, swellable or non-swellable, porous or non-porous, preferably of a density approximating water, generally from about 0.7 to about 1.5g/ml, and composed of material that can be transparent, partially transparent, or opaque. The organic particles will normally be polymers, either addition or condensation polymers, which are readily disperisble in the assay medium. The organic polymers will also be adsorptive or functionalizable so as to bind, either directly or indirectly, an sbp member. The particles can be derived from naturally occuring materials, naturally occurring materials which are synthetically modified and synthetic materials. Among organic polymers of particular interest are polysaccharides, particularly cross-linked polysaccharides, such as agarose, which is available as Sepharose (™) , dextran, available as Sephaex (™) and Sephacryl (™) , cellulose, and starch; addition polymers, such as polystyrene, polyvinyl alcohol, homopolymers and copolymers of derivatives of acrylate and methacrylate, particularly esters and amides having free hydroxyl functionalities. Inorganic polymers include silicones, glasses, available as Bioglas. Natural or synthetic assemblies such as liposomes, phospholipid vesicles, and cells can also be employed. Where the particles are commercially available, the particle size may be varied by breaking larger particles into smaller particles by mechanical means, such as grinding, sonication, and agitation. The particles will usually be polyfunctional or be capable of being polyfunctionalized or be capable of being bound to a support or a compound of the invention through specific or non-specific covalent or non-covalent interactions. A wide variety of functional groups are available or can be incorporated. Functional groups include carboxylic acids, aldehydes, amino groups, cyano groups, ethylene groups, hydroxyl groups, and mercapto groups. The manner of linking a wide variety of compounds to particles is well known and is amply illustrated in the literature. See for example Cautrecasas, J. Biol. Chem. , 245 3059 (1970). The length of a linking group to a compound of the invention may vary widely, depending upon the nature of the compound being linked, the effect of the distance between the compound being linked and the particle on the binding of sbp members and the analyte. Squarate dye--a dye having the structural element of cyclobutenonolate, generally being a condensation product of squaric acid (dihydroxycyclobutenedione) and an active compound such as a pyrrole or a aniline. The squarate dye generally has an absorption maximum greater than 600 nanometers, and preferably is 620 to 650 nm. The molar extinction coefficient for the squarate dye at the wavelength of the exciting light should be as high as practical and should be greater than 10,000, preferably greater than 100,000 square centimeters per mole. The squarate dyes should have a high quantum yield, normally greater than 0.05, preferably greater than 0.3. Label--A member of the signal producing system that is conjugated to an sbp member. The label can be any squarate dye as defined herein. Signal Producing System--The signal producing system may have one or more components, at least one component being a squarate dye or precursor of a squarate dye. The signal producing system includes all of the reagents required to produce a measurable signal including means for causing electronic excitation of the squarate dye. A preferred means can be, for example, a He/Ne laser with an emission wavelength at 633 nm. However, other light sources having an excitation wavelength greater than 600 namometers can be employed. Other components of the signal producing system can include enzymes, chemiluminescent compounds, quenchers, and substrates. Quencher--those compounds having absorption maxima at or greater than 600 nanometers. Such compounds preferably have little or no observable fluorescence and efficiently quench the fluorescence of a squarate dye-sbp member conjugate in accordance with the present invention. Examplary of such compounds are Gallocyanine, Celestine Blue, Delphine Blue, and Methylene Green, Group or functionality imparting water solubility--a functionality incorporated into a compound of the invention which imparts water solubility to the compound, that is, renders the compound soluble in water to an extent of at least one nanomolar. Such functional group or functionality can include a sulfonate, phosphate, phosphonate, carboxylate, hydroxyl, amine, ether, or amide. The group imparting water solubility generally comprises from 1 to 30 atoms, preferably 1 to 12 atoms, other than hydrogen, which atoms are selected from the group consisting of carbon, oxygen, nitrogen, sulfur, phosphorus, and halogen of atomic number 9 to 53. Such group can be part of the squarate dye prior to the formation of the conjugate of the squarate dye and the sbp member. Consequently, the squarate dyes can be conjugated to a wide variety of sbp members including poly(amino)acids without significantly altering the water solubility of the sbp member or without having the spectroscopic properties of the squarate dye adversely affected. Ancillary Materials-- Various ancillary materials will frequently be employed in an assay in accordance with the present invention. For example, buffers will normally be present in the assay medium, as well as stabilizers for the assay medium and the assay components. Frequently, in addition to these additives, additional proteins may be included, such as albumins, or surfactants, particularly non-ionic surfactants, binding enhancers, e.g., polyalkylene glycols. The compounds of the invention are novel fluorescent compounds comprising a squarate dye conjugated to an sbp member. The squarate dye has an absorption maximum usually greater than 600 nanometers. For the most part the compounds of the invention will have the following formula - wherein: D and Y are the same or different and each comprises a chain of from 1 to 6 moieties selected from the group consisting of -CH= and -N=, said chain having alternating single and double bonds and terminating in a functionality selected from the group consisting of O, S, and disubstituted nitrogen, wherein the sum of the number of said moieties and said terminal functionality in both D and Y is an even number, preferably within the range of from 6 to 10, said chain, or part thereof including the terminal functionality, being aliphatic or part of one or more alicyclic or aromatic rings, or a combination thereof, said chain or rings, or functionalities having zero, one, or more substituents comprising from 1 to 30 atoms, usually 1 to 12 atoms, more usually 1 to 4 atoms, other than hydrogen, which atoms are selected from the group consisting of carbon, oxygen, nitrogen, sulfur, halogen of atomic numbers 9 to 53, arsenic, silicon, selenium and phosphorus, wherein said substituents may be taken together to form one or more rings which may be alicylic or aromatic, wherein at least one of D or Y is bonded to A; A is a group comprising at least one member of a specific binding pair (sbp) consisting of ligand and its complementary receptor; the sbp member can be bound to a polymeric backbone; preferred sbp members are antigens, antibodies and haptens; When the sbp member is not bound to a polymer, n is a number on the average of from about 1 up to the molecular weight of A divided by 100 and, when the sbp member is bound to a polymer, n is a number on the average from about 2 to 10⁴, usually 10 to 10³, preferably 20 to 100. Reference to "on the average" means that some sbp member molecules may have more or less than n number of dye molecules, but the mixture of conjugate molecules will have on the average, n number of dye molecules per conjugate; and q is 1 or 2. The squarate dye and the sbp member can be bound together either covalently or non-covalently. Covalent binding can result from a bond or a linking group. A wide variety of linking groups may be employed to bond the squarate dye and the sbp member. The choice of linking group will vary widely, depending upon the available functionalities or functionalities which may be present or readily introduced into the dye or sbp member, the desired length of the linking arm, the desirability of having the linking arm provide for a particular environment, chemical property or physical property, e.g. positively or negatively charged, solubility enhancement, dipole effects, or the like. The linking group preferably includes a non-oxo-carbonyl, carbamoyl, thiocarbamoyl, sulfonyl, amino, thio, particularly a functionality having a non-oxo-carbonyl, and sulfur analogs thereof. For the purposes of this disclosure non-oxo-carbonyl shall include the carbonyl group of carboxylic acids the nitrogen containing iminocarbonyl group of amidic acids and the sulfur containing thionocarbonyl group of thio acids The term noncovalent means that the bond is formed by other than the sharing of electrons. Such bonds are formed primarily by electrostatic interactions, for example, hydrogen bonding, dipole dipole interactions, or van der Waals interactions. Compounds included within the scope of the invention have the formula - wherein: m is 1 to 4; preferably 3 to 4; k is 1 to 4; preferably 3 to 4; m + k is an even number; X is O, S, or N(R₁)₂; p is (-) when X is O or S and (+) when X is N(R₁)₂; R₁ is hydrogen and when not hydrogen is independently selected from a group of substituents having from 1 to 30 atoms other than hydrogen, usually 1 to 12 atoms, more usually 1 to 4 atoms, selected from the group consisting of carbon, oxygen, nitrogen, sulfur, halogen of atomic number 9 to 53, arsenic, silicon, selenium, and phosphorus, wherein R₁ may be taken together with one or more other R₁s to form one or more rings, usually five or six membered rings, and wherein a hydrogen atom on at least one R₁ is replaced by a bond or a linking group to A₁; for example, when m and k are each 4, R₁ on carbon 1 and R₁ on carbon 4 can be taken together to form an ethylene group as part of a benzene ring, or, when m and k are each 2, X is N(R₁)₂ and one R₁ of N(R₁)₂ can be taken together with R₁ on the carbon bearing N(R₁)₂ to form a five member ring; additionally, a benzene ring can be fused to the five membered ring; n₁ is n; A₁ is A; and q₁ is q. Compounds of the following formula are within the scope of the invention: wherein: A₂ is A; n₂ is n; q₂ is q₁; D₁ is independently selected from the group consisting of Y₁ is independently selected from the group consisting of Z in D₁ and Z in Y₁ are independently selected from the group consisting of carbon, nitrogen, oxygen, sulfur, and selenium; R₂ is R₁; and s is 2 when Z is carbon, 1 when Z is nitrogen, and 0 when Z is oxygen, sulfur, or selenium. For example, D₁ can be- wherein R₂ bound to nitrogen is independently selected from the group consisting of methyl and a group imparting water solubility and R₂ bound to carbon is hydrogen, wherein a hydrogen atom or at least one R₂ is replaced by a bond or a linking group to A₂. In another example, D₁ can be wherein Z(R₂) s is isopropylidene. Preferred compounds are selected from the group consisting of and wherein: A₃ is a member of a specific binding pair, preferably antigen, hapten, or antibody; n₃ a number on the average from about 1 up to the molecular weight of A₃ divided by 5000, usually 2 to 15, preferably 4 to 10; t is 1 or 2; q₃ is q; R₃ is independently selected from the group of substituents having 1-10, preferably 1 to 6, carbon atoms, 0-6, preferably 1 to 2, oxygen atoms, and 0-3 preferably 0 to 1, nitrogen atoms, and may be taken together to form an alicylic or aromatic ring; exemplary of such substituents are carboxymethyl, aminoethyl, dihydroxy, propyl, ethyl, methyl. R₄ is independently selected from the group consisting of hydroxyl, hydrogen, phosphate, phosphonate, sulfonate, nitro, halogen, amino, and R₃; and L replaces any hydrogen of R₃ and is a bond or a linking group to A₃. Preferred compounds are those compounds wherein at least one R₃ on nitrogen has a hydrogen replaced by L, particularly where every remaining R₃ on nitrogen is lower alkyl of from 1 to 5 carbons, or wherein t is 1 and R₄ is hydroxyl, or wherein L is selected from the group consisting of non-oxo-carbonyl, carbamoyl, thiocarbamoyl, thio and amino. Generally, the squarate dye should contain a group or functionality imparting water solubility. However, water insoluble squarate dyes can be employed provided that a medium for conducting the conjugation of the squarate dye to the sbp member contains a substance that will render the squarate dye soluble. Such substance can be, for example, a detergent or a complexing agent such as cyclodextrin. Water insoluble squarate dyes can also be used when the sbp member does not require an aqueous medium for conjugation to the dye. Furthermore, when the squarate dye and the sbp member are non-covalently bound together, water solubility is not necessary and it may additionally be desirable to incorporate into the squarate dye substituents that impart hydrophobicity, such as, e.g., a hydrocarbon group having from 4 to 30 carbon atoms, preferably 6 to 20 carbon atoms, or substituents that provide for enhanced affinity of the dye for a receptor. For example, biotin can be attached to a squarate dye to provide for coupling to a ligand attached to avidin. A particular example of a compound of the invention has the formula - wherein: W is selected from the group consisting of antibody for digoxin and a digoxin analog capable of binding to such antibody. For example, the digoxin analog can be 3-deoxy-3-digoxigenin. The compounds of the present invention can be prepared by a reaction sequence, the individual steps of which are separately known in the art. Squarate dyes, and their preparation, are known in the art. Some of the squarate dyes of the present invention can be made according to the procedures described by Sprenger et al., Angew. Chem., 80 , 541 (1968); Springer et al., Angew. Chem., 79 , 581 (1967); Sprenger et al., Angew. Chem., internat. Edit., 5 , 894 (1966); and Maaks et al., Angew. Chem. internat. Edit., 5 , 888 (1966). In general, squaric acid (dihydroxycyclobutenedione) is condensed with an active compound such as a pyrrole or an aniline. The condensation is conducted under conditions for removing water from the reaction mixture. For example the condensation can be carried out under reflux in an alkanol/benzene solvent mixture. The resulting product can be collected and purified by, for example, recrystallization, distillation, or chromatography. The group or functionality imparting water solubility to the compound of the invention can be part of an initial reactant for the condensation or it can be introduced after the condensation by conventional techniques. The squarate dyes can be conjugated to sbp members by techniques that are known in the art. Such conjugation can be the result of direct bond formation between the squarate dye and the sbp member. On the other hand, a linking group as described above can be introduced into the squarate dye or the sbp member for attachment to the other component. A functionality for attachment such as carboxylic acid, hydroxyl, thio, amino, aldehydic, amido, activated ethylenes such as maleimide, and sulfonic acids, can be introduced into the squarate dye or the sbp member if such functionality is not originally present in the dye or the sbp member. Methods of conjugation involving sbp members are described in, e.g., U.S. Patent No. 3,817,837, the disclosure of which is incorporated herein by reference. The compounds of the invention have properties that are very desirable for their use in assays. The compounds have a high extinction coefficient, a high quantum efficiency, approaching one, chemical stability, and satisfactory Stokes shift. Furthermore, where the compounds are to be used in the presence of serum or other composition, which is in itself fluorescent, the compounds absorb energy in a substantially different range from that absorbed by the other compounds in the medium. As mentioned above, the present compounds have an absorption maximum usually greater than 600 nanometers. A further advantage is that the compounds can be used with He/Ne lasers, which allow use of small volumes and fibre optics. It is particularly surprising that squarate dyes can be used in the manner of the invention because they are as a class (relatively) hydrophobic and difficult to work with in aqueous environments. One aspect of the present invention involves an assay for a material of interest in a sample suspected of containing the material of interest. In the assay a fluorescent compound is employed to generate a signal in relation to the presence or amount of the material of interest in said sample and an energy source for excitation of said fluorescent compound is also employed. The improvement in such assay comprises employing a squarate dye, particularly a water compatible squarate dye having an absorption maximum usually greater than 600 nanometers, as the fluorescent compound and a helium/neon laser as the energy source. The squarate dyes can be rendered water compatible by incorporating a group or functionality imparting water solubility into the squarate dye. Alternatively, the squarate dye can be rendered water compatible by employing about 0.01 to 10%, of a detergent such as Triton) (™) X-100 or sodium dodecyl sulfate, or 1 x 10⁻⁴ to 1 x 10⁻²M cyclodextrin, in the assay medium. In another alternative, 0.02 - 20 µm latex particles or particles such as liposomes, cells and the like can be stained with the squarate dye to provide water compatibility. European Patent Application No. 85306054.9, published as EP 176 252 (equivalent to Canadian Patent Application No. 489455, Japanese Patent Application No. 189350/85, and Australian Application No. 46820/85) discloses the use of squarate dyes incorporated into cells, specifically blood cells. This aspect of the present invention extends to the use of squarate dyes other than by incorporation into cells. Another aspect of the present invention involves an improvement in an assay for an analyte in a sample suspected of containing said analyte where the analyte is an sbp member. The method involves an sbp member conjugated to a dye and the improvement of the present invention comprises employing as the dye a squarate dye. For example a fluorescent assay can employ as a reagent a fluorescent compound conjugated to a member of a specific binding pair. Such assay is for the determination of an analyte which is also a member of a specific binding pair. The binding of the conjugate to the analyte or a specific binding pair member complex of the analyte is indicative of the presence of the analyte. The present improvement comprises employing in the fluorescent assay a reagent that is a squarate dye conjugated to a member of a specific binding pair. Another example is a method for detecting the presence of a determinant site or a receptor by employing a fluorescent reagent having a fluorescent compound bound to a member of a specific binding pair. The binding of the fluorescent reagent to the determinant site or the receptor, or a specific binding pair member complex of the determinant site or the receptor, is determined as indicative of the presence of the determinant site or the receptor. The improvement of this invention comprises employing a fluorescent reagent that is a conjugate of a squarate dye and a member of a specific binding pair. The method has particular application where the determinant site or receptor is associated with a cell such as, e.g., being present on the cell surface. The present conjugates can be used for determining qualitatively, semiquantitively or quantitatively an analyte in a sample. Where compounds are to be detected in physiological fluids, the sample may include serum, urine, saliva, lymph or the like. Where the compound of interest is involved in chemical processing or ecological concerns, the sample may be an aqueous medium, or may be obtained by extraction from an organic medium, soil, inorganic mixtures. Another reagent in the assay can be a compound of the invention wherein the sbp member is a receptor for the analyte. As indicated previously, the compounds of this invention can include squarate dyes conjugated to compounds which may be measured by known immunoassay techniques. The conjugates are reagents which compete in an assay medium for the analyte in a sample. Therefore, the conjugate retains a sufficient proportion of the structure of the analyte to be able to compete with the analyte for a receptor for analyte. The assays may involve a change of spectroscopic properties due to a change in environment about the spectroscopically active compound or the bringing together of a fluorescer-quencher pair within sufficient proximity for the quencher to interact with the fluorescer. Alternatively, methods can be employed which involve the separation of associated and unassociated fluorescer and the detection of the fluorescer in one or both of the fractions. In carrying out the method an aqueous medium will normally be employed. Other polar solvents may also be employed, usually oxygenated organic solvents of from 1-6, more usually from 1-4 carbon atoms, including alcohols, ethers and the like. Usually these cosolvents will be present in less than about 40 weight percent, more usually in less than about 20 weight percent. The pH for the medium will usually be in the range of about 4-11, more usually in the range of about 5-10, and preferably in the range of about 5.4-9.5. The pH is chosen so as to maintain a significant level of binding between sbp members while optimizing signal producing proficiency. In some instances, a compromise will be made between these two considerations. Various buffers may be used to achieve the desired pH and maintain the pH during the determination. Illustrative buffers include borate, phosphate, carbonate, Tris, barbital and the like. The particular buffer employed is not critical to this invention but in individual assays, one buffer may be preferred over another. Moderate temperatures are normally employed for carrying out the method and usually constant temperatures during the period of the method. The temperatures for the determination will generally range from about 10°-50° C., more usually from about 15°-40° C. The concentration of analyte which may be assayed will generally vary from about 10⁻⁴ to 10⁻¹⁵M, more usually from about 10⁻⁶ to 10⁻¹³M. Considerations such as whether the assay is qualitative, semi-quantitative or quantitative, the particular detection technique, and the concentration of the analyte will normally determine the concentration of the other reagents. While the concentrations of the various reagents will generally be determined by the concentration range of interest of the analyte, the final concentration of each of the reagents will normally be determined empirically to optimize the sensitivity of the assay over the range of interest. The total binding sites of the members of the specific binding pair which are complementary to the analyte will be not less than about 0.1 times the minimum concentration of interest based on binding sites of the analyte and not more than about 10,000 times the maximum concentration of interest based on analyte binding sites, usually about 0.1-1000 times, more usually about 0.3-10 times, the maximum concentration of interest. For ligand analyte, where labelled ligand is employed, the concentration range of the labelled ligand based on equivalents will generally be not less than about 10⁻⁶, more usually not less than 10⁻², times the minimum concentration of interest and not greater than 100, usually not greater than 10, times the maximum concentration of interest. The concentration of the compound of the invention in the assay medium is dependent on the type of assay, e.g heterogeneous or homogeneous, competitive or direct, Normally, the compound of the invention will be present in the assay medium in a concentration about 10⁻⁶ to 10⁻¹⁵, usually about 10⁸ to 10¹³M. The order of addition of the various reagents may also vary and is dependent on much of the same considerations mentioned above. The present assay method has application both to hetereogeneous and homogeneous assays. Exemplary heterogeneous assays are found in U.S. Patent Nos. 4,256,834 and 4,261,968. Homogeneous immunoassays are exemplified by immunofluorescence methods such as those disclosed in U.S. Patent No, 3,993,345, enzyme channeling techniques such as those disclosed in U.S. Patent No. 4,233,402, and other enzyme immunoassays as discussed in Maggio, supra, and in U.S. Patent No. 3,817,837. The assay can be competitive or direct and can involve compounds of the invention that are either labeled ligand or labeled receptor. In one approach in accordance with the invention for detecting the presence or amount of analyte in a sample suspected of containing said analyte wherein said analyte is an sbp member consisting of ligand and its complementary receptor. The method comprises (1) combining in an assay medium the sample, as described above, a conjugate of a squarate dye and an sbp member, and a second sbp member, wherein the two sbp members are complementary to the analyte and (2) determining the effect of said sample on the fluorescence of compound I as related to the presence or amount of analyte in said sample. The second sbp member can be conjugated to a compound capable of quenching the fluorescence of the conjugate when both spb members are bound to the analyte. Alternatively, the second sbp member can be bound to a particle or to a surface or support to permit separation of conjugate that binds to the support from the conjugate remaining in solution. For example, in one technique, a quencher for the squarate dye is employed. One reagent is a compound of the invention comprising a conjugate of a squarate dye and an analog of a ligand analyte. Another reagent is a conjugate of quencher and an sbp member that is a receptor for analyte. The ligand analyte in the sample and the ligand analyte analog in the reagent compete with receptor for analyte. When the receptor for analyte binds to the labeled ligand analyte analog, the fluorescer and quencher are brought within quenching distance. A similar assay employing fluorescent compounds not within the scope of this invention is extensively described in U.S. Patent No. 3,996,345. The assay technique is described beginning with Column 17 and ending at Column 23, which description is incorporated herein by reference. The ratios of fluorescent compound to ligand and receptor is described in the above-cited patent at Columns 4-6, which description is incorporated herein by reference. In a related but different approach one reagent can be a compound of the invention that is a conjugate of a squarate dye and a receptor for the ligand analyte. Another reagent is a conjugate of a quencher for the squarate dye and a receptor for the ligand analyte. When the two reagents above are combined with the sample and brought together by the prescence of ligand analyte, the squarate dye and the quencher are brought within quenching distance. A typical quencher can be, e.g., gallocyanine, which can be conjugated through an amide bond to an sbp member. The assay is carried out by combining the squarate dye conjugate and the quencher conjugate in conjunction with the sample. The fluorescence is determined in comparison to an assay medium having a known amount of analyte. In another example the compound of the invention is a conjugate of a squarate dye and a receptor for the ligand analyte. Ligand or ligand analog is bound to a support or to a particle. Similar assays are described in U.S. Patent application Serial No. 964,099, now Patent No. 4,275,149, filed November 24, 1978. These assays are predicated upon having the fluorescer molecule available in bulk solution for interaction with a signal modulator or bound to a particle, where the particle environment prevents the interaction. Alternatively, the particle can provide an environment which modulates the fluorescent signals when the fluorescer conjugate is bound to the particle. Another approach involves steric exclusion in that receptors for the ligand and for the squarate dye are employed, where simultaneous binding of the receptor for the ligand and receptor for the dye is inhibited. Furthermore, when the receptor for the dye is bound to the dye, the fluorescence of the dye is substantially diminished. Further reduction, if not complete inhibition of fluorescence, can be achieved by conjugation of quencher to receptor for the dye. A similar assay is extensively described in U.S. Patent No. 3,998,943, issued December 21, 1976. The assay is described in columns 3-5 of the subject patent, which description is incorporated herein by reference. Generally, the method involves combining in an assay medium the sample suspected of containing the analyte, the conjugate of the sbp member and the dye, and other reagents in accordance with a particular assay protocol chosen. The sample is then exposed to a source of excitation. The fluorescence is determined either as a rate or equilibrium mode, readings being taken within about 1 second to 1 hour after all materials have been combined for a rate mode, while for an equilibrium mode, readings may be taken for as long as up to about 24 hours or longer. As a matter of convenience, the reagents employed in the present invention can be provided in a kit in packaged combination with predetermined amounts of reagents for use in assaying for an analyte in a sample. The reagents will include a compound of the invention as disclosed above, and, where appropriate, conjugates of quenchers and sbp members or other reaction partners for the compound of the invention required to provide the detectable signal. In addition, other additives such as ancillary reagents may be included. The relative amounts of the various reagents may be varied widely, to provide for concentractions in solution of the reagents which substantially optimize the sensitivity of the assay. The reagents can be provided as dry powders, usually lyophilized, including excipients, which on dissolution will provide for a reagent solution having the appropriate concentrations for performing the assay. EXAMPLES The invention is demonstrated further by the following illustrative examples. All temperatures are in degrees Centigrade (°C). Parts and percentages herein are by weight unless otherwise specified. The following abbreviations were used: DMF - dimethylformamide BGG - bovine gamma globulin PBS - phosphate buffered saline (.01M sodium phosphate, 0.15M NaCl, 0.005M NaN₃, pH7.0) tlc - thin layer chromatography NHS - N-hydroxy succinimide EDAC- ethyl-3-(dimethylaminopropyl) carbodimide hydrochloride PBS/BCD - PBS containing 0.01M β-cyclodextrin. PBS/10%BGG - PBS containing 100mgBGG/ml BGG - bovine gamma globulin PBS/BGG - PBS containing 10mg/ml of BGG PBS/BGG/Triton - PBS/BGG containing 10mg/ml of Triton X-100 PBS/SDS - PBS containing 50 mg/ml of SDS SDS - Sodium dodecyl sulfate EXAMPLE 1 Preparation of Aminodextran Into a 50-ml round bottomed flask fitted with a thermometer insert was placed 5.0g of Dextran T500 (Pharmacia), 15 ml of deionized H₂O, and a magnetic stir bar. The mixture was stirred and heated to 80° (oil bath). To this was added 1.875g of solid Zn(BF₄)₂°xH₂O (Alfa). When solution was complete and the temperature had stabilized at 80°, 10.03g of epichlorhydrin was added dropwise over 5-15 min. The mixture was stirred and heated at 80° for 3 hr. Heating was then stopped, and the reaction mixture was allowed to stir and cool to room temperature overnight. The clear solution was added dropwise with vigorous stirring to 250 ml of methanol. The white precipitate that formed was washed with methanol and dried under N₂. Further drying overnight in a vacuum dessicator gave 4.71g of chlorodextran. Elemental analysis of lyophilized material purified on a Sephadex® G-25 column packed and eluted with H₂O showed 3.385 mmol Cl/mg chlorodextran. A solution of 1g of chlorodextran in 1ml of deionized water was added dropwise with stirring to 40ml of concentrated aqueous NH₃. The reaction mixture was stirred 3 days. After concentration (rotovap) to 5 ml the solution was added dropwise to 500 ml of vigorously stirred methanol. The resulting white precipitate was collected by filtration, washed under N₂ with methanol, and dried under N₂. Additional drying in a vacuum dessicator overnight gave 1.13g of aminodextran as a white powder. Elemental analysis (C,H,N,Cl) corresponded to 2.30x10⁻⁴ mmol amine/mg aminodextran as the hydrochloride salt. Elemental analysis of a lyophilized sample that had been purified on Sephadex® G-25 eluted with aqueous 1M NH₃ showed 2.98x10⁻⁴ mmol amine/mg aminodextran. The crude material was used without further purification. EXAMPLE 2 Preparation of Digoxigenin-Linked Aminodextran Digoxin containing a general tritium label (500 µl of a 1 mCi/ml solution in 1:9 toluene:ethanol, 20 Ci/mmol) and unlabeled digoxin (2.0 g, 2.56 mmol) were dissolved in 50 ml of ethanol. To this was added 75 ml of ethanol, 100 ml of distilled H₂O, and 1 ml of concentrated aqueous HCl. The solution was refluxed for 45 minutes. The solution was then cooled, neutralized with aqueous 5 N NaOH, and evaporated to 100 ml. The resulting white precipitate was collected by filtration, recrystallized from 80% aqueous ethanol, and dried under vacuum to give 557 mg of tritium labeled digoxigenin: mp 205-207°; 0.169 mCi/mmol, 3.7x10⁸ cpm/mmol. The tritium-labeled digoxigenin was oxidized to the 3-ketodigoxigenin with Pt and O₂ by the procedure of Tamm and Gubler ( Helv . Chim . deta, 42, 239, (1959)). The tritium labeled 3-ketodigoxigenin was converted into the tritium-labelled carboxymethoxime by the following method: A clear solution of 3-ketodigoxigenin (228 mg, 0.59 mmoles), carboxymethoxylamine hemihydrochloride (140 mg, 0.64 mmole, Aldrich Chemical Company), and sodium acetate (294 mg, 3.6 mmole) in methanol (18 ml, dried over molecular sieves 3A) was allowed to stir at room temperature under nitrogen for 3 hours. The tlc of an aliquot of sample showed the complete formation of oxime derivative (Rf 0.33; 0.5:1:10/HOAc-MeOH-CHCl₃, Silica gel plate). The resulting reaction product was stripped to dryness, the residue dissolved in 32 ml 5% NaHCO₃ at 5-10°C, and was extracted with 3x20 ml chloroform. The chloroform extracts were discarded. The bicarbonate layer was acidified at 5-10°C with 28 ml of 1N hydrochloric acid to pH 2-3 and was extracted with 10x25 ml ethyl acetate. The ethyl acetate extracts were washed with saturated sodium chloride and dried over anhydrous sodium sulfate. Evaporation of solvent gave the tritium labeled 3-ketodigoxigenin carboxymethoxime as a solid which recrystallized from a mixture of methanol-ethyl acetate-hexane: mp. 202-220°(dec); IR (KBr): 3600 cm⁻¹-3200 cm ⁻¹ (acid). The NHS ester of ³H-digoxigenin carboxymethyloxime was prepared in situ by addition of 1.18 mg of EDAC to a stirred solution at 0° of 2.76 mg of ³H-digoxigenin carboxymethyloxime prepared as above with and 0.849 mg of NHS in 30µl of freshly distilled (CaH₂) DMF, followed by a second addition of 0.2 mg of EDAC after 1 hr. To 10 mg of aminodextran prepared in Example 1 in 1.0ml of 0.1M sodium pyrophosphate (pH8.5) at 0° was added over 2-3 min by microliter syringe 9.5 µl of the above solution of ³H-digoxigenin NHS ester in DMF. After two hr the reaction mixture was washed free of small molecules on an Amicon (™) filtration device by dilution of the reaction mixture to 15 ml with 14 ml of 0.1M sodium pyrophosphate containing 10 mg/ml sodium cholate (pH 8.5) and concentrating to 1.5 ml followed by 10 mM sodium phosphate buffer (pH7.0) as the dilution/concentration buffer. The final solution was diluted with the sodium phosphate buffer to 7.0 ml. Analysis for aminodextran by optical rotation gave 1.377 mg aminodextran/ml solution. Scintillation counting as analysis for digoxigenin gave 4.702x10-5 mmol digoxigenin/ml solution. Aminodextran and 3H-digoxigenin carboxymethyloxime were used as standards. A value of 3.415x10⁻⁵ mmol digoxigenin/mg aminodextran was thus determined. EXAMPLE 3 Preparation of Digoxigenin-Linked Magnetic Particles Magnetic particles prepared by polymerization of acrylic acid and acrolein derivatives in the presence of Rembaum magnetic iron oxide as described by Rembaum et al. in J. Immunol. Methods (1982) 52 : 341-351 were used as the magnetic particle. To 5.0mg of this material, reported to contain surface aldehyde groups, in a 1.5-ml polypropylene centrifuge tube was added 182µl of the solution of digoxigenin-linked aminodextran (dig-dextran) prepared as described in Example 2. The tube was capped and vertically rotated overnight. After a 2-min centrifugation to collect the particles, scintillation counting of the supernatant showed that 36.6 µmol of digoxigenin per mg of magnetic particles had adhered to the particles. The particles were washed with brief sonication with five 200µl portions of PBS followed by centrifugation and decantation of the washes. Scintillation counting of the washes showed about 45% of the counts to remain with the particles. This gave a final analysis of 6.90x10⁻⁷ mmol digoxigenin/mg particles and 1.73x10⁻⁶ mmol digoxigenin/ml suspension. EXAMPLE 4 Preparation of Squarate Dye-Linked Antidigoxin 1-[4-(diethylamino)-2-hydroxyphenyl]-3-[4-(N-ethyl-carboxymethylamino)-2-hydroxyphenyl]-2,4-dihydroxycyclobutenediylium dihydroxide, bis (inner salt) (DECAS) was prepared by refluxing with azeotropic water removal equimolar amounts of 3-diethylamino phenol, squaric acid, and N-ethyl-N-3-hydroxyphenyl glycine methyl ester prepared as in Example 6 in 2:1 (v/v) n-butanol: benzene followed by isolation of the monomethylester-dye by silical gel chromatography. The deep blue methyl ester was hydrolyzed with NaOH to give DECAS after acidification. The NHS ester of DECAS was generated by addition of 1.3 mg of EDAC to a solution of 3.0 mg of DECAS and 0.99 mg of NHS in 120 µl of freshly distilled (CaH₂) DMF at 0°. Tlc (silica, 90:10 v/v chloroform: methanol) indicated the presence of the NHS ester of DECAS and disappearance of starting material after 1 hr. To 8.7 mg of polyclonal sheep antidigoxin in 0.5 ml of PBS was added 0.5 ml of pH8.5 sodium pyrophosphate (0.1M). To the vigorously stirred 0° antibody solution was added slowly two 25-µl portions of the NHS ester solution of DECAS prepared above. Each addition took 2-3 minutes and was followed by a 20-30 pulse sonication at 0°. After 2 hr. the reaction was centrifuged and the supernatant (1ml) was chromatographed on an 18-ml bed of Sephadex®G-25 prepared in a pH7 buffer consisting of 50mM sodium phosphate, 0.15M NaCl, and 10 mg/ml of sodium cholate. The void volume band was collected and after 3 days at 4° was centrifuged to remove a small amount of blue precipitate and chromatographed on 18 ml of insoluble potato starch made up in PBS. The fast-moving blue band showed no non-covalent dye when spotted on silica and eluted with acidic dimethoxyethane. After concentration by ultrafiltration to 1.2ml, UV analysis in PBS/BCD with DECAS and sheep digoxin as standards showed the product to have λmax at 650 and a dye: protein ratio of 2.2 . EXAMPLE 5 Assay for Digoxin Buffers: PBS PBS/BCD PBS/10% BGG PBS/BGG/Triton PBS/SDS Reagents: Sheep antidigoxin-dye conjugate prepared as in Example 4 Magnetic particle-digoxigenin-aminodextran reagent prepared as in Example 3 diluted with an equal volume of PBS/BGG-Triton (M-Dig) Digoxin calibrators in PBS/BGG-Triton at the following concentrations: 1.0x10⁴ng/ml 3.16x10³ng/ml 1.0x10³ng/ml 3.16x10²ng/ml 1.0x10²ng/ml 0 ng/ml Protocol: To 10µl of PBS/BGG-Triton® was added 25µl of Ab-Dye 50µl of calibrator was added. A 20 min incubation (capped) followed. 100µl of M-Dig was added. A 2-hr incubation (vertical rotor) followed. Tubes were centrifuged and supernatants decanted. 200µl of PBS/BGG-Triton® was added with 1-2 pulses of sonication. Tubes were centrifuged 1 min and supernatants decanted. 200µl of PBS/BGG-Triton® was added with 1-2 pulses of sonication. Tubes were centrifuged 1 min and supernatants decanted. 200µl of PBS/BGG-Triton® was added with 1-2 pulses of sonication. Tubes were centrifuged 1 min and supernatant decanted. 50µl of PBS/SDS was added with 20-30 pulses of sonication. Tubes wer e warmed for 20 sec under hot tap water. 150µl of PBS/BCD was added Tubes were centrifuged 1 min. 190µl of the supernatant was diluted with 750µl of PBS/BCD Solutions were read on a fiber optic laser fluorescence reader (λex = 633 nm) Results are summarized in Table 1. Digoxin (ng/ml) Fluorescence (units) 1.0x10⁴ 368 3.16x10³ 427 1.0x10³ 488 3.16x10² 645 1x10² 692 0 708 The above results demonstrate that a rapid, accurate, sensitive assay for digoxin can be carried out in accordance with the teaching of the present invention. An increase in concentration of digoxin of two orders of magnitude (1X10² to 1X10⁴ ng/ml) resulted in a substantial decrease in the observed fluorescence from 692 to 368. Furthermore, the above assay readily distinguished between digoxin concentrations of 1X10² ng/ml and 3.16X10² ng/ml. EXAMPLE 6 Preparation of 1,3-bis[4-(N-ethyl-carboxymethylamino) -2-hydroxyphenyl]-2,4-dihydroxycyclobutenediylium dihydroxide, (DCAS) and its bis hydrazide To a solution of 161g of 3-acetamidophenol in 800ml of tetrahydrofuran was added dropwise over 1-2 hr 275 ml of a 1.0M solution of borane-methyl sulfide in dichloromethane. The reaction was heated to reflux 4-5 hr and then poured slowly into 1 1 of methanol. The solvent was evaporated to a heavy syrup which deposited crystals on cooling and scratching. The solid was collected by filtration and redisolved in methanol. Evaporation and crystallization as above gave a 74% yield of 3-(ethylamino) phenol. Methyl bromoacetate (10.7g, 70mmole), 3-(ethylamino) phenol from above (4.8g, 35mmole), and ethyl diisopropyl amine (4.45g, 35mmole) were combined in 25 ml of toluene. The mixture was heated at 100°C for 30 min. and then evaporated under vacuum and chromatographed on a silica gel column using 5% (v/v) methanol/methylene chloride. The combined fractions were washed with saturated sodium bicarbonate and neutralized with 0.1N HCl. The combined fractions were dried over Na₂SO₄ and evaporated to give 7.24g of N-ethyl-N-3-hydroxyphenyl glycine methyl ester. The above product (0.78g, 3.73mmole) was combined with squaric acid (D.19g, 1.67mmole) in a 10 ml round bottom flask. n-Butyl alcohol (1 ml) and 4 ml of benzene were added and the mixture was refluxed overnight. The solvent was removed by rotary evaporation. The dried product from above (50mg) was dissolved in 10 ml of DMF and 1 ml of anhydrous hydrazine was added. The mixture was stirred at room temperature overnight. The reaction mixture was diluted with 50 ml of water and dilute HCl was added to give a precipitate, which was collected and washed with water and dried on a suction filter. EXAMPLE 7 Preparation of Squarate Dye-Digoxin Conjugate To a vigorously stirred solution of 390 mg (0.5 mmol) of digoxin dissolved in 5 ml of N,N-dimethylformamide maintained at 0°C was added dropwise 214 mg (1.0 mmol) of sodium periodate dissolved in 5 ml of deionized water. The reaction was stirred at 0° for four hours; then an additional 107 mg (0.5 mmol) of solid sodium periodate was added. The reaction was allowed to warm gradually to room temperature with stirring for a further 16 hours. The reaction was diluted with 10 ml of water and stirred for 20 minutes. Then 10 ml of saturated sodium chloride was added and the reaction was extracted with 3x15 ml of ethyl acetate. The ethyl acetate phases were combined, washed with 6x15 ml of water, 1x15 ml of saturated sodium chloride, and dried over sodium sulfate. Filtration of the drying agent and removal of ethyl acetate by rotary evaporation yielded 246 mg (31% yield) of digoxin dialdehyde, which by thin-layer chromatography on silica gel (chloroform - methanol (9:1); Rf 0.43) was greater than 95% pure. The product was used in subsequent reactions without further purification. To 25mg (0.05 mmol) of DCAS bis-hydrazide prepared as described in Example 6 and 78 mg (0.10 mmol) of digoxin dialdehyde prepared as above was added 2 ml of dry dimethylformamide and 17 µl (0.10 mmol) of N,N-diisopropylethylamine. The reaction was stirred at room temperature under nitrogen for 48 hours, during which time a new product formed. By thin layer chromatography, using chloroform-methanol (9:1, v/v) as developing solvent; this product had R f 0.45 and was blue-green in color. The reaction mixture was evaporated to dryness by rotary evaporation and purified by preparative thin layer chromatography on two 20x20 cm silica gel type GF preparative plates, using chloroform-methanol (9:1, v/v) as developing solvent. The band corresponding to the product at R f 0.45 was scraped off the plate and eluted from the silica gel with methanol. Evaporation of methanol yielded a dark greenish-blue solid product. EXAMPLE 8 Preparation of Sheep Anti-Digoxin-Gallocyanine Conjugate To a stirred solution of 16mg of sheep anti-digoxin in 10ml of pH8.0 phosphate buffered saline at 0° was added dropwise 0.1ml of a 0.1M solution of gallocyanine NHS ester (prepared by a procedure similar to that of Exampe 4 for the preparation of the NHS ester of DECAS) in dry DMF over a five minute period. After stirring at 0-4° for 18 hours, 0.1 ml of 4M hydroxylamine hydrochloride was added all at once and stirring was continued for thirty minutes at 0-4°. The blue solution was then purified by gel filtration, dialysis, and centrifugation to give 7.6ml of 1.44mg/ml blue antibody solution. The solution contained an average of 2.6 moles gallocyanine per mole anti-digoxin. The absorption maxima of the conjugate were at 278nm and 625 nm, and the relative quantum yield of the conjugate was determined to be 0.02. EXAMPLE 9 Competitive Assay An assay buffer and three reagent stock solutions in the assay buffer were prepared: Assay Buffer: pH 7.4 10mM phosphate buffered saline which is 10mM in β-cyclodextrin and 0.1% in rabbit serum albumin. Squarate Dye-Digoxin Conjugate: 1.1x10⁻⁷M in squarate dye and 2.2x10⁻⁷M in digoxin. Digoxin: 1000 ng/ml. Sheep Anti-digoxin-Gallocyanine Conjugate: 13.4mg/ml, or 8.4x10⁻⁵M in antibody. Assay Protocol: To plastic assay cups was added 10µl of Squarate Dye-Digoxin Conjugate stock solution, 12 µl of Antibody-Gallocyanine stock solution, digoxin stock solution of volumes 0, 1, 2, 5, 10, 20, 50, 75, and 100 µl, and assay buffer sufficient to bring the total assay volume to 200µl. The assay solutions were incubated at room temperature for one hour. Then, 800µl of assay buffer was added and the fluorescence was measured on a fiber optic fluorometer, using a He-Ne laser at 632.8nm as the excitation source and measuring the emission at 660±10nm. The results appear in Table 2. Digoxin (ng/ml) Fluorescence (kHz) 0 17.96 ± 1.4 1 18.33 ± 0.39 2 19.50 ± 0.27 5 27.48 ± 0.22 10 29.06 ± 0.39 20 31.10 ± 0.31 50 30.70 ± 0.45 75 30.81 ± 0.18 100 30.81 ± 0.15 The above results demonstrate that a rapid, accurate, senstive assay for digoxin can be carried out in accordance with the teaching of the present invention. An increase in concentration of digoxin of two orders of magnitude (1 to 100 ng/ml) resulted in asubstantial increase in the observed fluorescence from 18.33 to 31.23 kHz. Furthermore, the above assay readily distinguished between low levels of digoxin concentrations such as, for example, 1 and 2 ng/ml.

Claims: Claims for the following Contracting States : BE, CH, DE, FR, GB, IT, LI, LU, NL, SE A compound for use in an assay for determining an analyte in a sample, said compound being a conjugate of a dye and a member of a specific binding pair (sbp) consisting of ligand and its complementary receptor, characterized in that a squarate dye is employed as the dye in said conjugate. A compound according to Claim 1, of the formula - wherein: D and Y are the same or different and each comprises a chain of from 1 to 6 moieties selected from -CH= and -N=, said chain having alternating single and double bonds and terminating in a functionality selected from the group consisting of O, S, and disubstituted nitrogen, wherein the sum of the number of said moieties and said terminal functionality in both D and Y is an even number, said chain, or part thereof including the terminal functionality, being aliphatic or part of one or more alicyclic or aromatic rings, or a combination thereof, said chain or rings, or functionalities having zero, one, or more substituents comprising from 1 to 30 atoms other than hydrogen, which atoms are selected from carbon, oxygen, nitrogen, sulfur, halogen of atomic numbers 9 to 53, arsenic, silicon, selenium and phosphorus wherein said substituents may be taken together to form one or more rings which may be alicylic or aromatic, wherein at least one of D or Y is bonded to A; A is a group comprising at least one member of a specific binding pair (sbp) consisting of ligand and its complementary receptor; n is a number on the average of 1 up to the molecular weight of A divided by 100; and q is 1 or 2. A compound according to Claim 2, selected from and wherein: A₃ is a member of a specific binding pair; n₃ is a number on the average of 1 up to the molecular weight of A₃ divided by 5000; t is 1 or 2; q₃ is 1 or 2; R₃ is independently selected from substituents having 1-10 carbon atoms, 0-6 oxygen atoms, and 0-3 nitrogen atoms, and may be taken together to form an alicylic or aromatic ring; R₄ is independently selected from hydroxyl, hydrogen, phosphate, phosphonate, sulfonate, nitro, halogen, amino and R₃; and L replaces any hydrogen of R₃ and is a bond or a linking group to A₃. A compound of Claim 3 wherein at least one R₃ on nitrogen has a hydrogen replaced by L. A compound of Claim 4 wherein every remaining R₃ on nitrogen is lower alkyl of 1 to 5 carbons. A compound according to anyone of the Claims 3 to 5 wherein t is 1 and R₄ is hydroxyl. A compound according to anyone of the Claims 3 to 6 wherein L is selected from non-oxo-carbonyl, thio, carbamoyl, thiocarbamoyl, and amino. A compound of anyone of the Claims 1 to 7, wherein said squarate dye has an absorption maximum greater than 600nm. An assay for an analyte in a sample suspected of containing said analyte, which analyte is a member of a specific binding pair (sbp member), said method involving an sbp member conjugated to a dye, characterized in that a squarate dye is employed as the conjugated dye. An assay according to Claim 9, which assay comprises - combining in an assay medium the sample, the sbp member squarate dye compound, and a third sbp member, wherein two of the three sbp members are complementary to the remaining member, and determining the effect of said sample on the fluorescence of the compound as related to the presence or amount of analyte in said sample. An assay of Claim 10 wherein said third sbp member is conjugated to a compound capable of quenching the fluorescence of the squarate dye compound when said third sbp member is bound to said squarate dye compound. An assay for a material of interest in a sample suspected of containing the material of interest wherein a fluorescent compound is employed to generate a signal in relation to the presence or amount of the material of interest in said sample and an energy source for excitation of said fluorescent compound is also employed, characterized in that a squarate dye conjugated to sbp is employed as the fluorescent compound and a helium/neon laser as the energy source. An assay of Claim 12 wherein said squarate dye is water compatible. An assay of Claim 12 or 13, wherein said squarate dye is not incorporated into a cell. A kit for use in determining the presence or amount of an analyte in a sample suspected of containing said analyte, said analyte being a member of a specific binding pair (sbp), said kit comprising in packaged combination - a first compound comprising a squarate dye conjugated to a second sbp member, a second compound comprising a third sbp member wherein two of the three sbp members are complementary to the remaining sbp member, and ancillary materials as required. A process for preparing the compound of Claim 1, which process comprises conjugating the sbp member with the squarate dye. Claims for the following Contracting State : AT A process which comprises the production of a compound for use in an assay for determining an analyte in a sample, said compound being a conjugate of a squarate dye and a member of a specific binding pair (sbp) consisting of ligand and its complementary receptor. A process according to claim 1 wherein the compound has the formula - wherein: D and Y are the same or different and each comprises a chain of from 1 to 6 moieties selected from -CH= and -N=, said chain having alternating single and double bonds and terminating in a functionality selected from the group consisting of O, S, and disubstituted nitrogen, wherein the sum of the number of said moieties and said terminal functionality in both D and Y is an even number, said chain, or part thereof including the terminal functionality, being aliphatic or part of one or more alicyclic or aromatic rings, or a combination thereof, said chain or rings, or functionalities having zero, one, or more substituents comprising from 1 to 30 atoms other than hydrogen, which atoms are selected from carbon, oxygen, nitrogen, sulfur, halogen of atomic numbers 9 to 53, arsenic, silicon, selenium and phosphorus wherein said substituents may be taken together to form one or more rings which may be alicylic or aromatic, wherein at least one of D or Y is bonded to A; A is a group comprising at least one member of a specific binding pair (sbp) consisting of ligand and its complementary receptor; n is a number on the average of 1 up to the molecular weight of A divided by 100; and q is 1 or 2. A process according to claim 2 wherein the compound is selected from and wherein: A₃ is a member of a specific binding pair; n₃ is a number on the average of 1 up to the molecular weight of A₃ divided by 5000; t is 1 or 2; q₃ is 1 or 2; R₃ is independently selected from substituents having 1-10 carbon atoms, 0-6 oxygen atoms, and 0-3 nitrogen atoms, and may be taken together to form an alicylic or aromatic ring; R₄ is independently selected from hydroxyl, hydrogen, phosphate, phosphonate, sulfonate, nitro, halogen, amino and R₃; and L replaces any hydrogen of R₃ and is a bond or a linking group to A₃. A process according to claim 3 wherein at least one R₃ on nitrogen has a hydrogen replaced by L. A process according to claim 4 wherein every remaining R₃ on nitrogen is lower alkyl of 1 to 5 carbons. A process according to anyone of the Claims 3 to 5 wherein t is 1 and R₄ is hydroxyl. A process according to anyone of the Claims 3 to 6 wherein L is selected from non-oxo-carbonyl, thio, carbamoyl, thiocarbamoyl, and amino. A process according to any one of the Claims 1 to 7, wherein said squarate dye has an absorption maximum greater than 600nm. An assay for an analyte in a sample suspected of containing said analyte, which analyte is a member of a specific binding pair (sbp member), said method involving an sbp member conjugated to a dye, characterized in that a squarate dye is employed as the conjugated dye. An assay according to Claim 9, which assay comprises - combining in an assay medium the sample, the sbp member squarate dye compound, and a third sbp member, wherein two of the three sbp members are complementary to the remaining member, and determining the effect of said sample on the fluorescence of the compound as related to the presence or amount of analyte in said sample. An assay of Claim 10 wherein said third sbp member is conjugated to a compound capable of quenching the fluorescence of the squarate dye compound when said third sbp member is bound to said squarate dye compound. An assay for a material of interest in a sample suspected of containing the material of interest wherein a fluorescent compound is employed to generate a signal in relation to the presence or amount of the material of interest in said sample and an energy source for excitation of said fluorescent compound is also employed, characterized in that a squarate dye conjugated sbp is employed as the fluorescent compound and a helium/neon laser as the energy source. An assay of Claim 12 wherein said squarate dye is water compatible. An assay of Claim 12 or 13, wherein said squarate dye is not incorporated into a cell. A kit for use in determining the presence or amount of an analyte in a sample suspected of containing said analyte, said analyte being a member of a specific binding pair (sbp), said kit comprising in packaged combination - a first compound comprising a squarate dye conjugated to a second sbp member, a second compound comprising a third sbp member wherein two of the three sbp members are complementary to the remaining sbp member, and ancillary materials as required.

Inventors: BERGER, JR. DONALD E., TARNOWSKI, THOMAS L., ULLMAN, EDWIN F.
Applicants: SYNTEX (U.S.A.) INC.
Application year: 1986
Main classification: G01N33
IPC codes: G01N33, G01N21, C09B23, C09B57