CYANINE DYES WITH HIGH-ABSORBANCE CROSS SECTION AS DONOR CHROMOPHORES IN ENERGY TRANSFER LABELS

Abstract: Cyanine dyes are used as the donor fluorophore in energy transfer labels in which light energy is absorbed by a donor fluorophore and transferred to an acceptor fluorophore which responds to the transfer by emitting fluorescent light for detection. The cyanine dyes impart an unusually high sensitivity to the labels thereby improving their usefulness in a wide variety of biochemical procedures, particularly nucleic acid sequencing, nucleic acid fragment sizing, and related procedures.

Description: CYANINE DYES WITH fflGH-ABSORBANCE CROSS SECTION AS DONOR CHROMOPHORES IN ENERGY TRANSFER LABELS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention resides in the field of energy transfer fluorescent labels. 2. Description of the Prior Art Energy transfer between pairs of covalently linked fluorescent dyes is well known. The advantage of energy transfer is that the combined dyes produce a larger Stokes shift than either of the individual dyes, and this enhances each of the various procedures in which these covalently linked dyes are used as fluorescent labels by producing a clearer and more accurate readout. Included among these procedures are DNA sequencing, gene mapping studies and other nucleic acid fragment analyses, but the technology applies in general to any procedure in which a species of interest, or several distinct species, are labeled for purposes of detection, identification, or separation. A recent development in the use of energy transfer dye pairs is the use of sets of energy transfer labels where each member of the set has a common donor fluorophore while the acceptor fluorophore differs from one member to the next within the set. Sets of this type are useful in the multiplexing of samples to separate, identify and detect two or more components simultaneously in a single procedure. One procedure in which these sets of labels are particularly useful is the Sanger dideoxy chain-termination method of DNA sequencing, but sets of this type are generally useful in any procedure where fluorescent labels are used to differentiate between components. By virtue of the common donor fluorophore, all labels in the set are irradiated at a single wavelength while fluorescent emissions are detected at distinct wavelengths that permit differentiation between the components. These sets and their uses are disclosed in International Patent Application Publication No. WO 95/21266, published August 10, 1995 (Regents of the University of California, Mathies et al , and Wang, Y., et al. , Anal. Chem. 67: 1997-1203 (1995). Selection of the particular donor and acceptor fluorophores to be used in synthesizing the labels and compiling the various members of a set is done with certain considerations. Included among these considerations are the wavelengths of the lasers available for irradiation, and the extinction coefficients, quantum yields, emission spectra, and photostability of the dyes. Unfortunately, donor/ acceptor pairs used heretofore are not optimal in all of these properties, and this limits the sensitivity of the labels and their range of use. The best donor/acceptor pairs would be those in which the donor has superior photostability and a high extinction coefficient at the wavelength of available lasers, and in which the rate of energy transfer from the excited donor to the acceptor well exceeds the rates of the other processes that compete for the excited state of the donor, i.e. , the natural fluorescence rate of the donor as well as the various radiationless processes that deplete the excited state of the donors. SUMMARY OF THE INVENTION It has now been discovered that a class of dyes, the cyanine dyes, not heretofore used as donors in energy transfer labels can indeed he used for this purpose, and when thus used, impart an unusually high sensitivity to the labels. As donor dyes in the present invention, these dyes are useful in combination with a wide variety of acceptor fluorophores to result in fluorescence emission intensities that are significantly higher than those obtained from energy transfer pairs formed from other donor and acceptor fluorophores. Use of these donor dyes improves the performance and usefulness of individual energy transfer labels as well as the performance and usefulness of sets of two or more labels of different emission spectra used in combination. The invention is therefore applicable to fluorescence labeling in general, as well as to the various processes in which fluorescence labeling is used. Energy transfer dye pairs containing these fluorophores are thus useful as labeled probes for identification and detection of nucleic acid fragments, and as labeled primers for nucleic acid sequencing. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 depicts the structures of four energy transfer primers in accordance wid this invention. FIG. 2 is an outline of a synthetic scheme for the introduction of acceptor and donor fluorophores to an oligonucleotide chain, forming one of the four primers shown in FIG. 1. FIGS. 3a through 3d show the absorption and fluorescence emission spectra of four energy transfer labels in accordance with this invention, comparing them in each case with the emission spectra of the acceptor fluorophores alone. Each of the labels has the same donor fluorophore. The acceptor fluorophore in FIG. 3a is FAM; the acceptor fluorophore in FIG. 3b is R6G (compared with JOE rather than R6G alone); the acceptor fluorophore in FIG. 3c is TAMRA; and the acceptor fluorophore in FIG. 3d is ROX. In each case, the absorption spectrum of the label is the thick continuous line, the emission spectrum of the label is the dashed line, and the emission spectrum of the acceptor fluorophore alone is the thin continuous line. FIG. 4 shows the fluorescence emission spectra of the four labels of FIGS. 3a through 3d, superimposed to show the separation in peak maxima. FIG. 5a compares the absorbance spectrum of a donor fluorophore within the present invention with that of a donor fluorophore of the prior art. FIG. 5b compares the emission spectra of the same two donor fluorophores. FIG. 5c compares the absorbance spectra of a donor-acceptor label within the present invention with that of a donor-acceptor label of the prior art. FIG. 5d compares the emission spectra of the same two donor- acceptor labels. FIGS. 6a, 6b, 6c, and 6d show the emission spectra of four different energy transfer primers, each with five different spacings between the dyes along the oligonucleotide backbone, the five spectra superimposed in each case to show the differences in the fluorescence emission intensity due to variations in the spacing. FIG. 7a shows the absorption spectrum of a single energy transfer label within the scope of the invention. FIG. 7b shows the emission spectrum of this label superimposed over the emission spectra of three other labels in a four-label set. FIG. 8a shows the electrophoretic migration times of the four labels of FIGS. 3a through 3d and 4. FIGS. 8b, 8c, 8d, and 8e compare the electrophoretic migration times of each of the four labels individually with analogous labels of the prior art that have the same acceptor fluorophores but a different donor fluorophore. DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS Cyanine dyes useful in this invention as donor fluorophores are those having a quantum yield within the range of about 0.01 to about 0.25. A preferred range for the quantum yield is from about 0.05 to about 0.15. Preferred cyanine dyes also have a molar extinction coefficient of greater than 60,000 -vT'cm "1 , and a preferred range for the molar extinction coefficient is from about 100,000 M 'cm" 1 to about 250,000 M 'cm 1 . Further characteristics of preferred cyanine dyes are dieir absorption maxima, whose preferred range is from about 400 nm to about 900 nm, while the most preferred are those with absorption maxima ranging from about 480 nm to about 550 nm. Particularly preferred cyanine dyes are those that, when bonded to the backbone, have the formula In this formula: X is O or S; Y is O or S; R 1 is either H, halo, or C,-C 6 alkyl; R 2 is either H, halo, or C r C 6 alkyl; R 3 is C r C 6 alkyl; and m is zero to 3. Within this formula, certain embodiments are preferred. Preferred X and Y, for example, are O. Preferred groups for R 1 are H and C r C 3 alkyl, with H and methyl more preferred, and methyl the most preferred. Similarly, preferred groups for R 2 are H and C[-C 3 alkyl, with H and methyl more preferred, and methyl the most preferred. For R 3 , preferred groups are C,-C 3 alkyl, with ethyl as the most preferred. Preferred values for m are 1, 2, and 3, with 1 the most preferred. Specific cyanine dyes of particular interest are 3-(e- carboxypenty l)-3 ' -ethy 1-5 , 5 ' -dimethy loxacarbocyanine and 3-(e-carboxypentyl)-3 ' - ethyloxacarbocyanine, and of these the most preferred is 3-(e-carboxypentyI)-3'-ethyl-5,5'- dimethy loxacarbocyanine . When two or more different energy pairs or labels are used as a set for simultaneous excitation and detection of distinct components in a mixture, the cyanine donor fluorophores can be the same or different, but if different are preferably selected such that all cyanine dyes are efficiently excited at a common wavelength, preferably a single light source of narrow bandwidth, and particularly a laser source. The absorption maxima of the cyanine dyes are preferably within about 20 nm of each other, more preferably within about 10 nm, and still more preferably within about 5 nm. In the most preferred embodiments, the same cyanine dye will be used as the donor fluorophore for each pair or label. The acceptor fluorophore will be selected on the basis of its ability to receive energy from the donor fluorophore by energy transfer and to emit light energy from me energy thus received. The acceptor fluorophores will generally be dyes other than the cyanine dyes used as donor fluorophores, since the acceptor fluorophores will preferably have quantum yields of about 0.3 or greater, and more preferably widiin the range of about 0.4 to about 0.9. In terms of their molar extinction coefficients, preferred acceptor fluorophores are d ose whose value is 30,000 M^cm '1 or higher, and more preferably within the range of about 40,000 to about 250,000 M'On '1 . The emission maxima for preferred acceptor fluorophores are at wavelengths in the range of about 450 nm to about 1,000 nm, and most preferably within the range of about 500 nm to about 700 nm. Any of a wide variety of fluorescent dyes can be used as the acceptor fluorophore. Examples of types of fluorescent dyes are xanthene dyes, cyanine dyes, coumarins, benzimide dyes, phenarithridine dyes, ethidium dyes, acridine dyes, carbazole dyes, phenoxazine dyes, porphyrin dyes, and quinoline dyes. Subclasses of xanthene dyes are fluoresceins and rhodamines. An example of a coumarin is umbelliferone. An example of a benzimide dye is Hoechst 33258. An example of a phenanthridine dye is Texas Red. Xanthene dyes, and particularly fluorescein and rhodamine dyes, are of particular interest. Specific examples are 6-carboxyfluorescein (commonly known by the abbreviations FAM and F), 6-carboxy-4\5'-d_ch_oro-2\7'-dimemoxyfluorescein (JOE or J), N,N,N\N'- tetramethyl-6-carboxyrhodamine (TAMRA or T), 6-carboxy-X-rhodamine (ROX or R), 5- carboxyrhodamine-6G (R6G 5 or G 5 ), 6-carboxyrhodamine-6G (R6G 6 or G 6 ), and rhodamine 110. When two or more different energy pairs or labels are used as a set, each will contain a different acceptor fluorophore, each acceptor fluorophore emitting light that is detectably different from the other acceptor fluorophores in the set, thereby enabling the user to distinguish between the various labels in the set. In preferred embodiments of the usage of a set of labels, the individual labels in the set will emit at emission maxima separated by at least 10 nm, more preferably at least 15 nm, and most preferably at least 20 nm. In addition to those embodiments in which the energy transfer relationship passes directly from the cyanine dye donor fluorophore to the acceptor fluorophore whose emitted energy is detected, this invention also contemplates energy transfer combinations in which energy is transferred from the cyanine dye donor fluorophore to two or more fluorophores in succession in a cascading manner, with emission detected from the final fluorophore in the cascade. Thus, labels of the present invention can include intervening fluorophores that will receive energy from the donor fluorophore and pass the energy thus received to further fluorophore(s) until it reaches the fluorophore from which fluorescence is emitted and detected. These combinations of three or more fluorophores in a single label offer the potential of an extended Stokes shift, i.e. , a greater difference between the wavelength at which absorption occurs and the wavelength at which emission is detected. For example, excitation in the visible range with emission in the infra-red range, for example below 1,000 nm or preferably below 900 nm, can be obtained with the extended Stokes shift obtained by using three or more fluorophores. Detection in the infra-red range has the advantage that it is not subject to interference from Raman or Rayleigh scattering resulting from the excitation light. Regardless of the number of fluorophores present on a single label, the fluorophores will be joined by a backbone or supporting structure that establishes the distance between fluorophores between which energy is transferred. The transfer of light energy between fluorophores varies as 1/R 6 , i.e. , as.thε inverse of the sixth power of the R, where R is the distance between the two fluorophores. With this relationship as a guide, and the need to avoid self-quenching resulting from too small a spacing between adjacent fluorophores, the distance used between fluorophores can vary considerably. In general, the distance providing d e most efficient energy transfer in accordance with the well-known Forster mechanism will be the preferred distance. The distance that provides the greatest efficiency for any particular label can further vary with the particular molecular structure of the backbone. A variety of molecular structures can be used as the backbone. Examples of various classes of backbone structures are (a) nucleic acids, including bodi RNA and DNA, (b) modified nucleic acids, for example those where oxygen atoms are replaced by sulfur, carbon, or nitrogen atoms, and those where phosphate groups are replaced by sulfate groups, carboxylate groups, or N-(2-aminoethyl)glycine, (c) polypeptides, (d) polysaccharides, and (e) groups that can be joined in stepwise manner, examples of which are di- functional groups such as haloamines. Polymeric backbones are preferred. For many applications, a class of particular interest as the backbone is that of nucleic acids or oligonucleotides. The bases within the oligonucleotides include those that are naturally occurring as well synthetic bases, plus bases that have been modified to facilitate the attachment of the fluorophores; naturally occurring bases (including modified analogues) are currently of greatest interest. Oligonucleotide backbones contemplated in mis invention are polymerized chains of monomeric units selected from purine and pyrimidine mononucleotides, hybridizing analogues of these mononucleotides, and all other structures listed in (a) and (b) above. Within this class, DNA and RNA are particularly preferred. The length of the oligonucleotide can vary considerably and is not critical to d is invention. In most cases, best results will be obtained with oligonucleotides of 5 to 30, preferably 5 to 20, nucleotides in length. Applying the above considerations regarding the spacing of fluorophores on the backbone, the donor and acceptor fluorophores, or adjacent fluorophores in a fluorophore cascade of three or more, are preferably bonded to appropriately modified nucleotides mat are separated by 1 to 29 intervening nucleotides, more preferably 1 to 15 intervening nucleotides, and most preferably 4 to 12 intervening nucleotides. In addition to die energy transfer effect, the spacing between adjacent fluorophores will affect die mobility of the label, which will be of importance in separation processes where sets of labels are used for d e separation and identification of two or more components in a mixture. The separation of the mixtures into components in these processes is based on the differences in die mobilities (for example, in electrophoretic separations) of these components. In these situations, it is preferred that the labels themselves do not contribute to or otherwise affect the mobility differences of die mixture components that the labels are attached to. It is thus preferred that the labels themselves, when unattached to any o er species, have identical mobilities or that any differences in mobility among the labels be insignificant compared to the species being labeled. Differences in mobility between the labels themselves can be eliminated by varying the distances between the fluorophores, and d is can be done while still achieving effective energy transfer. In general, the relationship between spacing and mobility is not linear, and best results are achieved by determining me effect of spacing on mobility in an empirical manner for any given label. Mobilities of the labels themselves are readily determined by performing the separation process on the labels alone. Alternatively, mobilities can be determined by first binding the labels to a common molecule that is relevant to the separation that will ultimately be performed, such as an oligonucleotide of die appropriate size, and dien performing the separation process on the bound labels. For embodiments of the invention where individual labels contain three or more fluorophores in energy-transfer relationship rather than a pair, the added fluorophores will also affect the mobility of the labels. It is therefore preferable when sets of different labels are used that each label widiin the set have the same number of fluorophores. When it is necessary to control mobilities in sets of labels by control of the spacing or any other technique, it is preferred that the mobilities wimin a set of labels be controlled to widiin about 20% of each other, more preferably within about 10% of each other, and most preferably within about 5 % of each other. Regardless of die particular backbone structure, die fluorophores can be attached by covalent means, notably by functionalization of a selected backbone component residing at the desired location along the backbone. The fluorophore can be attached eidier to the backbone component prior to its incorporation into the backbone or after the backbone has been formed. The molecular weights of the labels (i.e., the fluorophores plus the backbone through which diey are linked) will generally be at least about 250 Dal and not more than about 20,000 Dal, although usually not more than about 10,000 Dal. The molecular weight of each fluorophore will generally be in me range of about 250 to about 1,000 Dal, and the molecular weights of donor and acceptor fluorophores will usually not differ by more than about 30% . The donor and acceptor can be attached to the backbone at internal locations on the backbone, either one can be bound at a terminal position on the backbone, or both can be bound at terminal positions. In embodiments where die backbone is an oligonucleotide, me preferred site of attachment of one fluorophore is at the 5' terminal unit of the oligonucleotide. This can be either the donor or d e acceptor, aldiough in certain cases the donor is preferred. This invention is particularly useful in processes involving nucleic acid chains, examples of such processes being DNA or RNA sequencing, the polymerase chain reaction (PCR), fragment sizing, and other procedures in which primers are employed for nucleic acid extension or ligation and in which distinctions must be made between various fragments. To apply this invention to the Sanger dideoxynucleotide sequencing technique, for example, universal primers can be employed widi a set of different energy transfer labels, all having a cyanine dye as die donor fluorophore, with a different label for each of the different dideoxy nucleotides. The method can be performed by hybridizing an oligonucleotide primer to the fragment, copying the fragment with a DNA polymerase in separate reaction vessels, each vessel containing dNTPs, one of a set of distinct energy transfer primers of this invention, and one of a set of dideoxy nucleotides, to generate die single stranded DNA sequencing fragments. The resulting mixtures from each reaction vessel are then separated in individual lanes on a gel, and d e sequence is determined from die relative locations of bands on d e gel. A large number of nucleotides, already functionalized, are available for use in the synthesis of a polynucleotide. In synthesizing labels having nucleic acids as backbones, one can establish specific sites for d e attachment of the fluorophores. Synthesizers well known among those skilled in polynucleotide synthesis can be employed and used in conventional procedures to achieve a desired sequence including eidier the fluorophores or sites for selective fluorophore attachment at selected locations along the chain. In polymerase chain reactions where different primers are used, each of die primers can be labeled in accordance widi this invention, permitting detection of me presence of the target sequence complementary to each of the different primers. Double or single stranded nucleic acid fragments can be identified and detected in this manner, by labeling each fragment by means of a polymerase chain reaction employing energy transfer primers of this invention, separating the labeled fragments according to meir mobilities, and detecting me fragments by irradiating d e labels. Identification and detection of nucleic acid fragments in a mixture by hybridization is achieved by hybridizing different fragments in me mixture to different energy transfer probes of diis invention, separating the hybridized fragments, and detecting diem by irradiation of the probes. Procedures depending on fragment labeling by ligation are performed in an analogous manner. This invention is also useful in many other applications, an example of which is identifying lectins by use of different polysaccharides. A further example is the rapid sizing of alleles, as exemplified by short tandem repeat (STR) alleles, or other sequences where a small number of base pair differences are to be detected, such as a difference of a single base pair. By using labels in accordance widi-this invention in conjunction with capillary electrophoresis, particularly capillary array electrophoresis, and employing an intercalating agent in me buffer, separations differing by one base can be achieved. This method can be used widi dsDNA, particularly dsDNA obtained using the polymerase chain reaction or the ligase chain reaction, where die labels of this invention are used as primers. The labels can serve as one or both of the primers used for the amplification, and me fluorophore pairs can be the same or different, depending on me needs of me separation. The intercalating agents can be either fluorescent or non-fluorescent, and can include such examples as thiazole orange, 9-aminoacridine, ediidium bromide. Non-fluorescent intercalating agents are preferred. Concentrations will generally be within the range of 0.1 μM to 1.0 μM. Capillary electrophoresis can be conducted in die conventional manner, using a polyacrylamide wall coating and, for example, hydroxyemylcellulose at from about 0.5% to 1 % in an appropriate running buffer. Voltages may vary from about 50 to about 150 V/cm or greater. The amount of DNA will generally be in the range of about 1 pg/μL to 1 ng/μL, although greater or lesser amounts can be used. Fragment analyses of single strand DNA can be performed in an analogous manner, using linear polyacrylamide or the like in capillary electrophoresis. For these procedures and od ers involving nucleic acid hybridization, the nucleotide sequence of d e backbone structure diat separates the fluorophores in the labels of d is invention can include the nucleotides diat hybridize to d e complementary sequence, or the hybridizing nucleotides can lie outside both fluorophores and thus be separated from the nucleotides that separate d e fluorophores. Linkage of die fluorophores to the backbone is achieved by conventional covalent binding. Linking arms suitable for diis type of purpose are well known in the art, and include a wide variety of structures. Examples are aliphatic groups, alicyclic groups, aromatic groups, heterocyclic groups, and combinations of these structures, as well as aliphatic groups interrupted by an ester group, an amide group, an amino groups, a C=O group, a C=S group, a C(O)— O group, a C(S)— O group, a C(O)— S group, a C(S)— S group, an oxygen atom, and a sulfur atom. In me case of nucleic acid backbones, linkage is preferably achieved by use of a convenient linking arm usually consisting of from about 2 to about 20, and preferably from about 4 to about 16, atoms. A preferred linking group structure is an amide-containing chain bonding directly to me heterocyclic nitrogen atom on the cyanine dye, particularly one of the formula where n is 1 to 10, and p is 1 to 10. Preferred values for n in diis formula are 1 to 6, more preferred are 4 to 6, and a value of 5 is particularly preferred. Preferred values for p in this formula are 2 to 8, more preferred are 3 to 6, and a value of 6 is particularly preferred. For nucleic acid chains, a linking group of die above formula preferably serves as the linkage between the heterocyclic nitrogen in die cyanine dye and d e phosphate group of die nucleotide. In hybridization applications, the entire nucleic acid sequence of the backbone can be complementary to the 5' primer sequence, or only to me 3' portion of the sequence, or to any portion within the target sequence. Usually, there will be at least about 4 nucleotides, more usually at least about 5 nucleotides that are complementary to the sequence to be copied. For PCR, the primers are combined with a mixture of the sequence to be amplified, Taq polymerase, and dNTPs. After amplification, the DNA can be isolated and transferred to a gel or capillary for separation. The following examples are offered for purposes of illustration and are not intended to limit die scope of d e invention. Materials and Analytical Methods 2-Med ylbenzoxazole, 2,5-dimethylbenzoxazole, iodoethane, iodoacetic acid, 3- iodopropionic acid, 6-bromohexanoic acid, acetic anhydride, 1,2-dichlorobenzene, N,N'- diphenylformamidine, and N.N'-disuccinimidyl carbonate were purchased from Aldrich Chemical Co. (Milwaukee, Wisconsin, USA) and used without further purification. All odier solvents were purchased from Fisher Scientific (Pittsburgh, Pennsylvania, USA); absolute edianol was used as purchased. Pyridine, dimethylformamide and triethylamine were stored over 4 A molecular sieves for at least one day before use. Chemicals used for d e syndiesis of oligonucleotides were purchased from Applied Biosystems Inc. (Foster City, California, USA) unless otherwise stated. Thermo Sequenase and other DΝA sequencing reagents were obtained from Amersham Life Science Inc. (Cleveland, Ohio, USA). Reactions were monitored by d in layer chromatography (TLC) (silica gel GHLF Uniplates, Analtech Inc., Newark, Delaware, USA) under shoπ and long wavelengm UV light. Column chromatography was performed on 220-440 mesh silica gel 60 from Fluka (Ronkonkoma, New York, USA). Purified products which gave single spots on TLC were characterized by their J H NMR spectra and by dieir UV-vis absorption spectra. Proton nuclear magnetic resonance spectra were recorded on a Bruker AMX spectrometer at 300 MHz. Unless otherwise specified, samples were dissolved in 5% CD 3 OD/CDCI 3 widi tetramethylsilane (TMS) as an internal reference. NMR signals are described by use of s for singlet, d for doublet, t for triplet, q for quartet, and m for multiplet and d e chemical shifts (δ) are reported in ppm. Absorption spectra were measured on a Perkin-Elmer Lambda 6 UV-visible spectrophotometer. Fluorescence emission spectra were taken on a Perkin-Elmer model MPF 44B spectrofluorimeter. Samples were prepared at a concentration of 1 x 10 '5 M in methanol or 1 xTBE buffer (89 mM Tris/89 raM boric acid/2 mM ethylenediamine tetraacetic acid (EDTA), pH 8.0) containing 7 M urea. The symmetric cyanine dyes 3,3'-die-hyloxacarbocyanine iodide (DOCCI) and 3,3'- diedιyl-5,5'-dimethyloxacarbocyanine iodide were used as standards for d e 'H NMR studies and the spectroscopic analyses. In the spectral data, MeOH designates medianol, AcOH designates acetic acid, and PhH designates benzene. Synthesis of Cyanine Dye Donors Quaternized nitrogen heterocycle derivatives 3-ethyl-2-medιylbenzoxazolium iodide and 3-ethyl-2,5-dimethylbenzoxazolium iodide were syndiesized by alkylation of 2-methylbenzoxazole and 2,5-dimethylbenzoxazole, respectively, with 5 equivalents of iodoed ane, refluxing overnight under N 2 . These reaction products were isolated as solids after precipitation with ethyl ether to give pure 3-ethyI-2-med ylbenzoxazolium iodide (91 %) and 3-ethyl-2,5-dimethylbenzoxazolium iodide (60%), respectively. The structure of 3-ethyl-2,5-dimethylbenzoxazol_um iodide was verified by spectral data as follows: Η NMR (CDClj) δ 1.70 (3H, t, J = 7.5 Hz, 3-CH 3 ), 2.59 (3H, s, 5-CH 3 ), 3.43 (3H, s, 2-CH 3 ). 4.83 (2H, q, J = 7.5 Hz, α-CH 2 ), 7.50 (1H, d, J = 8.6 Hz, 6-H), 7.70 (1H, d, J = 8.6 Hz, 7-H), 7.71 (1H, s, 4-H). Similarly, four derivatives — 3-(e-carboxypentyl)-2-methylbenzoxazolium bromide, 3-(e-carboxypentyl)-2,5-dimethylbenzoxazolium bromide, 3-(/3-carboxyethyl)- 2,5-dimethylbenzoxazolium iodide, and 3-( -carboxymemyl)-2,5-dimethylbenzoxazolium iodide ~ were formed by carboxyalkylation of 2-methylbenzoxazole and 2,5-dimethyl- benzoxazole with two equivalents of iodoacetic acid, 3-iodopropionic acid, or 6-bromohexanoic acid iri dry 1 ,2-dichlorobenzene (5 g of benzoxazole/20 mL of 1,2-dichlorobenzene) at 110-120°C for 17 hours under N 2 , followed by precipitation with CH 2 C1 2 to give the four derivatives in pure form in 45-55% yield. The structures of these four derivatives were verified as follows: 3-(e-Carboxypentyl)-2-methylbenzoxazolium bromide: 'H NMR δ 1.46-1.54 (2H, m,γ-CH 2 ), 1.60-1.70 (2H, m, δ-CH 2 ), 1.93-2.04 (2H, m, 3-CH 2 ), 2.29 (2H, t, / = 6.9 Hz, e-CH,), 3.27 (3H, s, 2-CH 3 ), 4.67 (2H, t, J = 7.5 Hz, α-CH,), 7.65-7.87 (4H, m, PhH). 3-(e-Carboxypentyl)-2,5-dimethylbenzoxazolium bromide,: 'H NMR δ 1.44-1.54 (2H, m, γ-CHj), 1.61-1.71 (2H, m, δ-CH 2 ), 1.92-2.03 (2H, m, 0-CH 2 ), 2.30 (2H, t, J = 6.9 Hz, e-CH 2 ), 2.55 (3H, s, 5-CH 3 ), 3.24 (3H, s, 2-CH 3 ), 4.63 (2H, t, J = 7.5 Hz, -CH 2 ), 7.47 (1H, d, / = 8.7 Hz, 6-H), 7.60 (1H, s, 4-H), 7.66 (1H, d, J = 8.7 Hz, 7-H). 3-(/3-Carboxyethyl)-2,5-dimethylbenzoxazolium iodide: 'H NMR δ 2.47 (3H, s, 5- CH 3 ), 3.00 (2H, t, J = 5.9 Hz, /J-CHj), 3.16 (3H, s, 2-CH 3 ), 4.73 (2H, t, J = 5.9 Hz, α-CH 2 ), 7.40 (IH, d, J = 8.7 Hz, 6-H), 7.60 (IH, d, J = 8.7 Hz, 7-H), 7.67 (IH, s, 4-H). 3-(α-CarboxymethyI)-2,5-dimethylbenzoxazolium iodide: 'H NMR δ 2.57 (3H, s, 5-CH 3 ), 3.20 (3H, s, 2-CH 3 ), 5.45 (2H, s, -CH,), 7.52 (IH, d, J = 8.7 Hz, 6-H), 7.57 (IH, s, 4-H), 7.73 (IH, d, J = 8.7 Hz, 7-H). The compounds 3-emyl-2-methylbenzoxazolium iodide or 3-ethyl-2,5-dimetiιyl- benzoxazolium iodide were reacted widi conjugated NN'-diphenylformamidine in refluxing acetic anhydride yielding intermediate acetanilide derivatives 2-(2'-acetanilidovinyl)- 3-ethylbenzoxazolium iodide or 2-(2'-acetanilidovinyl)-3-ethyl-5-methylbenzoxazolium iodide in 67-84% yield. The strucmre of die 2-(2'-acetanilidovinyl)-3-ethyl-5-methyl- benzoxazolium iodide thus produced as verified as follows: Η ΝMR (CDC1 3 ) δ 1.41 (3H, t, J = 7.4 Hz, 0-CH 3 ), 2.10 (3H, s, aceto-CH 3 ), 2.54 (3H, s, 5-CH 3 ), 4.56 (2H, q, J = 7.4 Hz, α-CH 2 ), 5.46 (IH, d, J = 13.7 Hz, bridge α-H), 7.37-7.73 (8H, m, PhH), 9.25 (IH, d, J = 13.7 Hz, bridge β-U). Each of these intermediates was dien coupled widi one equivalent of d e activated quaternary heterocycles prepared in the preceding paragraphs, in d e presence of two equivalents of triediylamine in dry ethanol (heated to reflux for 30 minutes) to form the asymmetric (carboxyalkyl)cyanine dyes whose identities and spectral verification data are given below: 3-(e-Carboxypentyl)-3'-ethyloxacarbocyanine: Η ΝMR δ 1.38-1.48 (2H, m^CH , 1.41 (3H, t, J = 7.3 Hz, /3'-CH 3 ), 1.56-1.66 (2H, m, δ-CH 2 ), 1.76-1.81 (2H, m, 0-CH 2 ), 2.14 (2H, ϊ, J = 7.0 Hz, e-CH 2 ), 4.02 (2H, t, J = 7.3 Hz, α-CH 2 ), 4.10 (2H, q, / = 7.3 Hz, '-CH 2 ), 5.85 (IH, d, J = 13.3 Hz, bridge α-H), 5.87 (IH, d, J = 13.3 Hz, bridge α'-H), 7.23-7.45 (8H, m, PhH), 8.41 (IH, t, J = 13.3 Hz, bridge β-U); \££ (MeOH) = 484 nm; λ^J (MeOH) = 500 nm. 3-(e-Carboxypentyl)-3'-ethyl-5,5'-dimethyloxacarbocyanine: l U ΝMR (CDC1 3 ) δ 1.50 (3H, t, J = 7.2 Hz, 0'-CH 3 ), 1.53-1.63 (2H, m, γ-CHz), 1.73-1.84 (2H, m, δ-CH^, 1.84-1.95 (2H, m, 0-CH 2 ), 2.40 (2H, t, J = 6.9 Hz, e-CH 2 ), 2.46 (3H, s, 5-CH 3 ), 2.47 (3H, s, 5'-CH 3 ), 4.17 (2H, t, J = 7.1 Hz, α-CH 2 ), 4.30 (2H, q, / = 7.2 Hz, '-CHz), 5.09 (IH, brs, CO 2 H), 6.27 (IH, d, J = 13.3 Hz, bridge α-H), 6.38 (IH, d, J = 13.3Hz, bridge α'-H), 7.06-7.13 (4H, m, 4-H, 4'-H, 6-H and 6'-H), 7.30-7.35 (2H, m, 7-H and 7'-H), 8.38 (IH, t, J = 13.3 Hz, bridge /3-H); λ (MeOH) = 490 nm; λ ϊ (MeOH) = 504 nm. 3-( 3-Carboxyethyl)-3'-ethyl-5,5'-dimedιyloxacarbocyanine: Η ΝMR δ 1.41 (3H, t, J = 7.2 Hz, 0'-CH 3 ), 2.40 (3H, s, 5'-CH 3 ), 2.41 (3H, s, 5-CH 3 ), 2.60 (2H, t, J = 7.4 Hz, 0-CH 2 ), 4.04 (2H, q, J = 7.2 Hz, α'-CH 2 ), 4.28 (2H, t, J = 7.4 Hz, α-CH,), 5.76 (IH, d, J = 13.3 Hz, bridge α'-H), 6.01 (IH, d, J = 13.3 Hz, bridge α-H), 7.00-7.30 (6H, m, PhH), 8.35 (IH, t, 7 = 13.3 Hz, bridge 0-H); λ^ s (MeOH) = 490 nm; λ^ (MeOH) = 504 nm. 3-(α-Carboxymemyl)-3'-ethyl-5,5'-dimethyloxacarbocyanine: 'H NMR δ 1.41 (3H, t, 7 = 7.3 Hz, ø'-CHj), 2.39 (3H, s, 5'-CH 3 ), 2.42 (3H, s, 5-CH 3 ), 4.06 (2H, q, 7 = 7.3 Hz, α'-CH 2 ), 4.55 (2H, s, α-CH_), 5.78 (IH, d, 7 = 13.3 Hz, bridge α'-H), 5.84 (IH, d, 7 = 13.3 Hz, bridge α-H), 7.00-7.30 (6H, m, PhH), 8.36 (IH, t, 7 = 13.3 Hz, bridge β-U); λ* (MeOH) = 490 nm; λ=£ (MeOH) = 503 nm. These crude dyes were purified by flash column chromatography on silica gel, eluting with CH 2 Cl 2 :MeOH:AcOH (90:10:1, by volume) to give the dyes (90-97% yield) as red powders. The symmetric cyanine dyes 3,3'-died yloxacarbocyanine iodide (DOCCI) and 3,3'- diethyl-5,5'-dimethyloxacarbocyanine iodide, that were used as standards for Η NMR studies and spectroscopic analyses, can be synthesized in quantitative yield by condensation of intermediates 3-ethyl-2,5-diπ_ethylbenzoxazolium iodide and 2-(2'-acetanilidovinyl)- 3-ethyl-5-methylbenzoxazolium iodide, or dieir analogs, following the procedure for the synthesis of asymmetric cyanine dyes. The structure of 3,3 '-died y 1-5,5 '-dimethyloxa- carbocyanine iodide was verified as follows: Η NMR (CDC1 3 ) δ 1.54 (6H, t, 7 = 7.3 Hz, 0-CH 3 and j3'-CH 3 ), 2.49 (6H, s, 5-CH 3 and 5'-CH 3 ), 4.31 (4H, q, 7 = 7.3 Hz, α-CH 2 and α'-CH 2 ), 6.78 (2H, d, 7 = 13.2 Hz, bridge α- and α'-H), 7.07 (2H, s, 4-H and 4'-H), 7.11 (2H, d, 7 = 8.3 Hz, 6-H and 6'-H), 7.33 (2H, d, 7 = 8.3 Hz, 7-H and 7'-H), 8.44 (IH, t, 7 = 13.2 Hz, bridge β-K); λ (MeOH) = 488 nm; λ,^ (MeOH) = 502 nm. Synthesis of N-hydroxysuccinimide Esters of (Carboxyalkyl)cyanine Dyes Active N-hydroxysuccinimide (ΝHS) esters of (carboxyalkyl)cyanine dyes were prepared by a modification of the method of Ogura, H., et al. , Tetrahedron Lett. 49:4745- 4746 (1979). In a typical procedure, two equivalents of anhydrous pyridine and 1.5 equivalents of N.N'-disuccinimidyl carbonate were added to a stirred solution of 3-(e-carboxypentyl)-3'-ethyloxacarbocyanine or 3-(e-carboxypentyl)-3'-ethyl- 5,5'-dimethyloxacarbocyanine in dry dimethylformamide (DMF) (100 mg of dye/2 mL of DMF) under Ν 2 . The reaction mixture was stirred at 60°C for 60 minutes. After evaporation of the solvents, the deep-red residue was purified by column chromatography (silica gel, CH 2 Cl 2 :CH 3 OH, 95:5 by volume). Pure cyanine dye active esters were recrystallized from ethyl acetate to give nearly quantitative yields of die NHS ester of 3-(e- carboxypentyl)-3'-ethyloxacarbocyanine and d e NHS ester of 3-(e-carboxypentyl)-3'-ethyl- 5,5'-dimethyloxacarbocyanine as red powders. Strucmre verification was as follows: NHS ester of 3-(e-carboxypentyl)-3'-ethyloxacarbocyanine: Η NMR (CDC1 3 ) δ 1.51 (3H, t, 7 = 7.3 Hz, /3'-CH 3 ), 1.59-1.69 (2H, m, 7 -CH 2 ), 1.78-1.89 (2H, m, δ-CH 2 ), 1.92-2.00 (2H, m, /3-CH 2 ), 2.62 (2H, t, 7 = 7.1 Hz, e-CH 2 ), 2.82 (4H, s, succinimido CH 2 CH 2 ), 4.25 (2H, t, 7 = 7.3 Hz, α-CH 2 ), 4.31 (2H, q, 7 = 7.3 Hz, α-'CH j ), 6.63 (IH, d, 7 = 13.2 Hz, bridge α-H), 6.65 (IH, d, 7 = 13.2 Hz, bridge α'-H), 7.26-7.49 (8H, m, PhH), 8.44 (IH, t, 7 = 13.2 Hz, bridge β-K); λ^ (MeOH) = 484 nm; λ^ (MeOH) = 502 nm. NHS ester of 3-(e-carboxypentyl)-3'-emyl-5,5'-dimethyloxacarbocyanine: 'H NMR (CDC1 3 ) δ 1.53 (3H, t, 7 = 7.2 Hz, /3'-CH 3 ), 1.63-1.73 (2H, m, 7-CH 2 ), 1.82-2.02 (4H, m, 0-CH, and δ-CH 2 ), 2.47 (6H, s, 5-CH 3 and 5'-CH 3 ), 2.67 (2H, t, 7 = 7.1 Hz, e-CH 2 ), 2.84 (4H, s, succinimido CH 2 CH 2 ), 4.23 (2H, t, 7 = 7.5 Hz, α-CH 2 ), 4.29 (2H, q, 7 = 7.2 Hz, α'-CH 2 ), 6.74 (IH, d, 7 = 13.2 Hz, bridge α-H), 6.75 (IH, d, 7 = 13.2 Hz, bridge α'-H), 7.08-7.11 (4H, m, 4-H, 4'-H, 6-H and 6'-H), 7.30-7.34 (2H, m, 7-H and 7'-H), 8.42 (IH, t, 7 = 13.2 Hz, bridge β-R), λ ? (MeOH) = 489 nm; λ^ (MeOH) = 505 nm. The structures of die various dyes, toged er with the wavelengths of their absorption and emission maxima are shown in the following table, where: Dye No. 1 is 3,3'-diedιyloxacarbocyanine iodide (DOCCI), Dye No. 2 is 3,3'-diethyl-5,5'-dimethyloxacarbocyanine iodide, Dye No. 3 is 3-(e-carboxypentyl)-3'-ethyloxacarbocyanine, Dye No. 4 is 3-(e-carboxypentyl)-3'-ethyl-5,5'-dimethyloxacarbocyanine, Dye No. 5 is 3-(j8-carboxyethyl)-3'-ethyl-5,5I-dimemyloxacarbocyanine, ,t Dye No. 6 is 3-(α-carboxymethyl)-3'-ethyl-5,5'-dimethyloxacarbocyanine, Dye No. 7 is the NHS ester of 3-(e-carboxypentyl)-3'-ethyloxacarbocyanine, and Dye No. 8 is d e NHS ester of 3-(e-carboxypentyl)-3 , -ethyl-5,5'-dimethyloxacarbo- cyanine. Strucmre and Spectral Properties Of Oxacarbocyanine Dyes Dye \ Abs \ Em Λ ma* Λ max No. R R' n (nm) (nm) Solvent 1 H CH 3 1 483 500 CH 3 OH 2 CH 3 CH 3 1 488 502 CH 3 OH 486 502 H 2 O 3 H COOH 5 484 500 CH 3 OH 485 504 1 XTBE, 7M urea 4 CH 3 COOH 5 490 504 CH 3 OH 491 508 1 xTBE, 7M urea 5 CH 3 GOOH 2 490 504 CH 3 OH 491 508 1 XTBE, 7M urea 6 CH 3 COOH 1 490 503 CH 3 OH 491 507 1 XTBE, 7M urea 7 H COOSu 5 484 502 CH 3 OH 485 504 1 XTBE, 7M urea 8 CH 3 COOSu 5 489 505 CH 3 OH 491 508 1 XTBE, 7M urea "Su" denotes succinimidyl Design and Synthesis of ET primers The strucmre of die 5 '-end of each of d e cyanine dye-labeled primers whose preparations are described below is shown in me following formula. In mis formula, the cyanine dye is joined to me 5'-end nucleotide guanosine through an amide linking group, a portion of which is supplied by die strucmre of die starting dye. The remaining nucleotides are indicated by the words "Primer Sequences. " The primers used were M13 (-40) universal primers containing 18 nucleotides and two primary amine linker .arms (protected by two different groups). The oligonucleotide sequence of each primer was 5'-GTTTTCCCAGT*CACGACG-3', synthesized by the phosphoramidite memod on an Applied Biosystems 392 DNA synthesizer. The protected primary amine linker arm at the 5' end was introduced in die last step of die oligonucleotide synd esis using 6-(4-monomethoxytritylamino)hexyl-(2-cyanoemyl)-(N,N~ diisopropyO-phosphoramidite (5 '-Amino-Modifier C6, Glen Research, Sterling, Virginia, USA). The symbol T* represents a modified diymidine (T*), characterized by a linker arm terminating in a primary amine, introduced by d e use of 5'-dimethoxytrityl-5-[N- (trifluoroacetylaminohexyl)-3-acrylimido] -2 ' -deoxyuridine , 3 ' -[(2-cyanoedιyl)-(N, N- diisopropyl)]-phosphoramidite (Amino-Modifier C6 dT, Glen Research). Donor-acceptor fluorophore pairs were attached at nucleotides separated by 10 intervening bases, as described below. The starting oligonucleotide was protected at d e 5' end wid a monomethoxytrityl (MMT) group, and bore d e modified diymidine T* as described above. The syndiesis was begun by coupling d e acceptor dyes to d e primary amino group on me T* in the presence of Νa 2 CO 3 /ΝaHCO 3 at pH 9 in dimemyl sulfoxide (DMSO). The acceptor dyes used were 6-carboxyfluorescein (FAM), 6-carboxy-4',5'-dichloro-2',7'-dimemoxy- fluorescein (JOE), N,N,N',N'-tetramethyl-6-carboxyrhodamine (TAMRA), 6-carboxy- X-rhodamine (ROX), 5-carboxyrhodamine-6G or 6-carboxyrhodamine-6G (R6G), and rhodamine 110. Coupling of the acceptor dyes was followed by the removal of the monomemoxytrityl (MMT) protecting group on the 5' end by treatment with trifluoroacetic acid, and dien the donor dye 3-(e-carboxypentyl)-3'-ethyl-5,5'-dimethyloxacarbocyanine (also referred to herein by the abbreviation CYA) was introduced at the 5' end of d e oligomer, again in the presence of Na 2 CO 3 /NaHCO 3 at pH 9 in DMSO. The primers are named according to d e convention D-N-A, where D is d e donor, N is d e number of bases between the donor and die acceptor, and A is d e acceptor. The donor and acceptor in diis convention are represented by single-letter symbols. For the cyanine dye 3- (e-carboxypentyl)-3'-ethyl-5,5'-dimethyloxacarbocyanine (CYA), the single-letter symbol is C. The acceptor fluorophores are likewise designated by the single-letter symbols F for FAM, G for R6G, T for TAMRA, R for ROX, and R 110 for rhodamine 110. The structures of each of the four primers are shown in FIG. 1, and the reaction scheme for C10R, as one example representing all four primers, is shown in FIG. 2. The single acceptor dye-labeled precursors and the final ET primers (C10F, C10G, C10T and C10R) were purified as previously described (Ju, J., et al. , Proc. Natl. Acad. Sci. USA 92:4347-4351 (1995)). Primers were quantified by their 260 nm absorbance after correction for the contributions to die absorbance from the dyes and d en stored in 10 mM Tris-HCl/1 mM EDTA, pH 8.0 at a final concentration of 0.2 pmol/μL. For spectroscopic analysis, primers were dissolved in 1 xTBE buffer containing 7 M urea. DNA Sequencing Cycle sequencing was performed widi M13mpl8 DNA template and Thermo Sequenase (Reeve, M.A., et al. , Nature 376:796-797 (1995)) and die fragments analyzed on a four-color capillary electrophoresis DNA sequencer. Four reactions were run, one for each dye/ddNTP combination. The reactions containing ddCTP were run widi die C10F primer, ddATP with the C10R primer, ddGTP with d e C10T primer, and ddTTP widi d e C10G primer. The following cycle sequencing protocol was used: 0.5 μL of Thermo Sequenase reaction buffer (260 mM Tris/HCl pH 9.5, 65 mM MgCl 2 ) was combined wid 1 μL of primer (0.2 pmol), 1 μL of template DNA (0.2 μg) and 3 μL of dNTPs/ddNTP (300: 1; dGTP was replaced by dITP) mix. All tubes were preheated at 95 °C for 45-60 seconds, and d en 1 μL of a freshly diluted Thermo Sequenase (1.6 units/μL) was added to each tube. Fifteen cycles of 95 °C for 30 seconds, 56°C for 20 seconds and 72°C for 2 minutes were carried out on die reaction mixture in a PTC-100 Thermal Cycler (MJ Research Inc. , Watertown, Massachusetts, USA). The four reactions were dien combined and stopped by adding 15 μL of 7.5 M ammonium acetate and 75 μL of ed anol. After 2 hours at -20°C, the precipitated DNA was collected by centrifugation at 13,100 rpm for 20 minutes and washed twice with 150 μL of ice-cold 70% edianol. The precipitated DNA was vacuum dried and redissolved in 2 μL of 98% formamide containing 1 mM EDTA. Samples were introduced into a 50-cm long (30 cm effective separation length) 6% T polyacrylamide gel-filled capillary by electrokinetically injecting for 20 seconds. Electrophoresis was performed at 200 V/cm in 1 xTBE buffer. A matrix transformation was performed on me raw data to correct for the crosstalk between the four channels to generate the four-color CE sequencing profile. EXAMPLE 1 This examples illustrates the efficient energy transfer in primers where the donor fluorophore is a cyanine dye in accordance widi me present invention. Four ET primers were synthesized according to the procedures described above: ClOF, ClOG, ClOT, and ClOR, using die convention and single-letter symbols indicated above. Additional primers were prepared in parallel fashion, corresponding to each of die four according to d e invention but differing by not including die donor fluorophore, and widi die single fluorophore at d e 5 '-end. Thus, the latter four primers were single-dye labeled primers widi the same backbone oligonucleotide and die four acceptor fluorophores ._ • as d e single dyes, widi one exception: me single-dye labeled primer used for comparison to ClOG was JOE (J) rather than R6G (G). JOE and R6G have equivalent emission spectra, however, as shown by Hung, S.-C, et al. , Anal. Biochem. 238: 165-179 (1996), the disclosure of which is incorporated herein by reference. Absorption and emission spectra for me ET primers and d e single-dye labeled primers were obtained using equimolar solutions of me primers in 1 xTBE containing 7 M urea. The emission spectra were taken with excitation at 488 nm. The results are shown in FIGS. 3a, 3b, 3c, and 3d, where die diick lines indicate d e absorption spectra of the ET primers, the dashed lines the emission spectra of the ET primers, and d e d in lines the emission spectra of the single-dye labeled primers. The absorption spectrum of each primer shows the strong absorption peak of the donor (3-(e-carboxypentyl)-3'-ethyl-5,5'-dimethyloxacarbocyanine) at 488 nm as well as that of the acceptor. In die case of ClOG, ie absorption peak of d e acceptor occurs at 532 nm due to d e R6G in ClOG. In the case of ClOT, die absorption peak of d e acceptor occurs at 560 nm due to die TAMRA. In the case of ClOR, the absorption peak of die acceptor occurs at 589 nm due to die ROX. For the emission spectra, all obtained with excitation at 488 nm, me emission maximum of ClOF is at 521 nm, that of ClOG is at 555 nm, that of ClOT is at 580 nm, and mat of ClOR is at 605 nm. The location of these maxima (dashed lines) compared to die locations of the maxima obtained wid the single-dye labeled primers (continuous thin lines) indicates that the emission spectra are dominated by the acceptor emission in each case. This indicates efficient energy transfer from the donor to die acceptor. Efficient energy transfer is further confirmed by comparing the heights of the emission maxima for the ET primers (dashed lines) widi the heights of me emission maxima for me single-dye labeled primers (continuous min lines), showing how much of me total emission by me acceptor fluorophore results from energy transfer from the donor: at die same molar concentration, the intensity at the ClOF emission maximum is 1.4 times the intensity of the FAM single-dye labeled primer; the corresponding factor for ClOG is 2.7; d e corresponding factor for ClOT is 17; and d e corresponding factor for ClOR is 24. FIG. 4 shows the emission spectra for the four ET primers, normalized and superimposed on die same wavelength scale. These spectra show that the residual emission of die common C donor fluorophore is very low, and d ey also show mat there is very little crosstalk between the primers. This contributes materially to the accuracy of base calling. The ET efficiency was calculated to be 90% for ClOG, 78% for ClOT, and 70% for ClOR. Relative quantum yields are illustrated in FIGS. 5a, 5b, 5c, and 5d. FIGS. 5a and 5b show the absorbance and emission spectra, respectively, of me individual dyes CYA (continuous lines) and FAM (dashed lines). FIG. 5a shows d at d e absorbance of CYA ,t (whose molar extinction coefficient at 488 nm is 142,000 M 'cm 1 ) is about twice diat of FAM (whose molar extinction coefficient at 488 nm is 60,000 M 'cm 1 ). FIG. 5b shows that the fluorescence emission of FAM is approximately 5 times that of CYA. Thus, the quantum yield of CYA is approximately 10% that of FAM. FIGS. 5c and 5d show die absorption and emission spectra, respectively, of the ET primers, comparing a primer within d e scope of d e invention, ClOR, with one outside die scope of die invention F10R (in which FAM serves as the donor fluorophore). The donor (488 nm) absorbance of ClOR is approximately twice that of F10R, and d e fluorescence emission is also approximately twice that of F10R. The observation that the ratio of the maximum absorbance of CYA to that of FAM (FIG. 5a) is the same as the ratio of d e fluorescence emission of ClOR to that of F10R (FIG. 5d) upon excitation at the maximum absorbance wavelengm indicates d at die rate of energy transfer (kj) is much higher ian the competing rates of fluorescence emission (k F ) or radiationless decays (k R ) of the CYA singlet state. EXAMPLE 2 This example shows how the fluorescence emission of energy transfer labels varies with the spacing between the donor and acceptor fluorophores. The four donor-acceptor pairs addressed in Example 1 were used, each wid five different spacings between donor and acceptor (i.e. , spacings of 2, 4, 6, 8, and 10 nucleotides). The emission spectrum for each label was determined at an equimolar concentration in 1 x TBE containing 7 M urea with excitation at 488 nm. The results for C2F, C4F, C6F, C8F, and ClOF are shown superimposed in FIG. 6a; C2G, C4G, C6G, C8G, and ClOG appear in FIG. 6b; C2T, C4T, C6T, C8T, and ClOT appear in FIG. 6c; and C2R, C4R, C6R, C8R, and ClOR appear in FIG. 6d. These superimposed spectra show that the acceptor fluorescence emission increases as the spacing increases. For this set of labels when used in situations where the strongest acceptor fluorescence emission is desired, a set with a spacing of 10 nucleotides is optimal. The donor fluorescence emission also varies with spacing, however, but d e donor emission with the particular donor used in these labels is absent or insignificant for spacings ranging from 2 to 6 nucleotides. Thus, energy-transfer primers using these fluorophores widi nucleotide spacings of 2 to 6 nucleotides are particularly valuable in many DNA fragment studies, where minimization of crosstalk from donor fluorescence emission is particularly valuable. EXAMPLE 3 This example illustrates a four-label set in which all labels are rhodamines, as opposed to d ree rhodamines and one fluorescein as in die preceding examples. In many applications of diis invention, a close match between the structural characteristics and net charge of all four acceptor fluorophores is desirable because diis would minimize any differences between the mobilities of the four sets of DNA sequencing fragments obtained by extension of a set of me four ET primers with distinctive emissions. The four primers used in d is experiment were the same as the 4-nucleotide spacing primers of the preceding example, except diat C4F was replaced widi C4R 110 , where R uo is 5-(and 6-)carboxyrhodamine 110, whose λ = 498 nm, e^ = 78,000 M 'cm 1 , and λ^J = 530 nm. The absorption spectrum of C4R 110 is shown in FIG. 7a, and d e normalized superimposed fluorescence emission spectra of the four-primer set are shown in FIG. 7b, all measurements determined in 1 x TBE containing 7 M urea widi excitation at 488 nm. The superimposed emission spectra of FIG. 7b show excellent separation between me fluorescence emissions of d e four primers, which provides for excellent DNA sequencing results. EXAMPLE 4 This example shows the electrophoretic mobility shifts attributable to the four CYA-containing ET primers prepared in Example 1 , and compares them to me electrophoretic mobility shifts of the four prior art ET primers differing from die CYA- containing primers by substituting FAM for the CYA. Individual primers at concentrations of 1 X 10 "9 M each were mixed togedier in 98% formamide and 1 mM EDTA, and subjected to electrophoresis in a capillary electrophoresis system, using a capillary columns 30 cm in lengm (20 cm to detection window) and 75 microns in diameter, and a running voltage of 200 V/cm. The column contained 1 % linear polyacrylamide prepared in 1 xTBE buffer widi 7 M urea. Sample injection was achieved by electrokinetic means at 5 kV for 5 seconds. Fluorescence intensities were observed at 525, 550, 580 and > 590 nm, and electropherograms were obtained at each wavelengd . In one experiment, the four CYA-containing primers were used, and me four electropherograms (one at each of d e four wavelengdis) are shown in FIG. 8a. Since die electropherograms are superimposed, each primer appears as four peaks at the same migration time but at different intensities. The migration time difference between ClOT and ClOG is 30 seconds; the migration time difference between ClOG and ClOR is 12 seconds; and d e migration time difference between ClOR and ClOF is 5 seconds. Thus, die largest relative difference in migration time for these four CIO primers is 47 seconds (ClOT to ClOF). In a second experiment, the ClOF primer was used in combination with the F10F primer, and die four electropherograms are shown superimposed in FIG. 8b. The migration time difference is 23 seconds. Similarly, ClOG and F10G are compared in FIG. 8c, showing a migration time difference of 9 seconds; ClOT and F10T are compared in FIG. 8d wim a migration time difference of 52 seconds; and ClOR and F10R are compared in FIG. 8e wid a migration time difference of 15 seconds. The largest relative difference in migration time for the four F10 primers is 91 seconds, as compared to 47 seconds for d e four CIO primers. These electropherograms also show the relative fluorescence emission intensities. The signal strength of ClOF is 0.8 times diat of F10F (FIG. 8b); the signal strength of ClOG is equal to diat of F10G (FIG. 8c); the signal strength of ClOT is 1.7 times that of F10T (FIG. 8d); and the signal strength of ClOR is 1.7 times that of F10R (FIG. 8e). EXAMPLE 5 This example illustrates the use of a four-primer set within die scope of diis invention in DNA sequencing. The four primers were those prepared in Example 1 — ClOF, ClOG, ClOT and ClOR. The DNA sequenced was single-stranded M13mpl8. Cycle sequencing was performed using Thermo Sequenase, with 0.2 pmol primer and 0.2 μg of DNA template for each base extension. The sequencing profile was generated by capillary electrophoresis on 6% T linear polyacrylamide and 1 xTBE buffer. The sequence was identified by d e color of each peak, wid ClOF appearing as green, ClOG appearing as blue, ClOT appearing as black, and ClOR appearing as red. The separations extended to 530 bases in 110 minutes with strong signals. Beyond diis point, large biased reptations appeared. The raw intensity data were transformed using a crosstalk matrix method. After applying a small mobility correction, a blind manual reading of the resulting sequence produced 500 bases wid 100% accuracy relative to d e known sequence of M13mpl8. When a parallel run was performed widi F10F, F10G, F10T and F10R as d e primers and me two profiles compared, me fluorescence emission crosstalk between die ClO-labeled DNA fragments was less man mat seen widi me corresponding FlO-labeled fragments. The foregoing is offered primarily for purposes of illustration. It will be readily apparent to diose skilled in the art that further modifications, variations and substitutions can be made in die methods, materials, and applications disclosed herein without departing from the spirit and scope of die invention.

Claims: WE CLAIM: 1. A fluorescent label comprising two fluorophores bonded to a backbone in energy transfer relationship whereby one of said fluorophores is a donor fluorophore and d e other is an acceptor fluorophore, said donor fluorophore being a cyanine dye characterized by a quanmm yield of between about 0.01 and about 0.25, and said acceptor fluorophore having a quanmm yield of at least about 0.3. 2. A fluorescent label in accordance with claim 1 in which said cyanine dye has an absorption maximum in me wavelength range of about 400 to about 900 nm. 3. A fluorescent label in accordance wid claim 1 in which said cyanine dye has an absorption maximum in the wavelength range of about 400 to about 900 nm, and said acceptor fluorophore has an emission maximum in the wavelengm range of about 450 to about 1,000 nm. 4. A fluorescent label in accordance widi claim 1 in which said cyanine dye has an absorption maximum in the wavelengm range of about 480 to about 550 nm. 5. A fluorescent label in accordance with claim 1 in which said cyanine dye has an absorption maximum in the wavelengdi range of about 480 to about 500 nm, and said acceptor fluorophore has an emission maximum in d e wavelengdi range of about 500 to about 700 nm. 6. A fluorescent label in accordance widi claim 1 in which said cyanine dye has the formula in which: X is O or S; Y is O or S; R 1 is a member selected from the group consisting of H, halo, and C r C 6 alkyl; R 2 is a member selected from the group consisting of H, halo, and C,-C 6 alkyl; R 3 is C,-C 6 alkyl; and m is zero to 3. 7. A fluorescent label in accordance wid claim 6 in which: X is O; Y is O; R 1 is a member selected from the group consisting of H and C,-C 3 alkyl; R 2 is a member selected from the group consisting of H and C r C 3 alkyl; R 3 is C r C 3 alkyl; and m is 1, 2, or 3. 8. A fluorescent label in accordance with claim 6 in which: X is O; Y is O; R 1 is H or methyl; R 2 is H or methyl; R 3 is C r C 3 alkyl; and m is 1. 9. A fluorescent label in accordance widi claim 6 in which: X is O; Y is O; R 1 is H or methyl; R 2 is H or methyl; R 3 is emyl; and m is 1. 10. A fluorescent label in accordance wid claim 6 in which: X is O; Y is O; R l is methyl; R 2 is methyl; R 3 is emyl; and m is 1. 11. A fluorescent label in accordance wid claim 1 in which said backbone is a polymerized chain of monomeric units selected from the group consisting of purine and pyrimidine mononucleotides and hybridizing analogues thereof. 12. A fluorescent label in accordance with claim 1 in which said backbone is comprised of monomeric nucleoside units linked through N-(2-aminoethyl)glycine. 13. A fluorescent label in accordance with claim 1 in which said backbone is an oligonucleotide. 14. A fluorescent label in accordance wid claim 13 in which said donor fluorophore is bonded to die 5' terminal unit of said oligonucleotide. 15. A fluorescent label in accordance with claim 13 in which said oligonucleotide is from 5 to 30 nucleotides in lengm. 16. A fluorescent label in accordance widi claim 13 in which said oligonucleotide is from 5 to 20 nucleotides in lengd . 17. A fluorescent label in accordance wi i claim 13 in which said cyanine dye and said acceptor fluorophore are bonded to nucleotides separated by 1 to 29 intervening nucleotides. 18. A fluorescent label in accordance widi claim 13 in which said cyanine dye and said acceptor fluorophore are bonded to nucleotides separated by 1 to 15 intervening nucleotides. 19. A fluorescent label in accordance widi claim 13 in which said cyanine dye and said acceptor fluorophore are bonded to nucleotides separated by 4 to 12 intervening nucleotides. 20. A fluorescent label in accordance widi claim 1 in which said acceptor fluorophore is a xandiene compound. 21. A fluorescent label in accordance wid claim 1 in which said acceptor fluorophore is a member selected from d e group consisting of fluoresceins and rhodamines. 22. A fluorescent label in accordance wid claim 1 in which said cyanine dye is 3-(e-carboxypentyl)-3'-ethyl-5,5'-dimethyloxacarbocyanine or 3-(e-carboxypentyl)- 3'-ethyl-oxacarbocyanine, and said acceptor fluorophore is a member selected from the group consisting of fluoresceins and rhodamines. 23. A fluorescent label in accordance with claim 1 in which said cyanine dye is 3-(e-carboxypentyl)-3' -ethyl-5,5 '-dimethyloxacarbocyanine or 3-(e-carboxypentyl)- 3'-ethyl-oxacarbocyanine, and said acceptor fluorophore is a member selected from d e group consisting of FAM, R6G, TAMRA, ROX, JOE, and rhodamine 110. 24. A fluorescent label in accordance with claim 1 in which said cyanine dye is bonded to said backbone by a linking group having the formula in which n is 1 to 10, and p is 1 to 10. 25. A fluorescent label in accordance widi claim 24 in which n is 1 to 6 and p is 2 to 8. 26. A fluorescent label in accordance widi claim 24 in which n is 4 to 6 and p is 3 to 6. 27. A fluorescent label in accordance widi claim 24 in which n is 5 and p is 6. 28. A fluorescent label in accordance widi claim 1 in which said cyanine dye has die formula said backbone is an oligonucleotide, and said cyanine dye is bonded to a nucleotide of said backbone by a linking group having the formula 29. In a method for sequencing a nucleic acid fragment, comprising: (i) hybridizing an oligonucleotide primer to said fragment, (ii) copying said fragment with a DNA polymerase in separate reaction vessels, each reaction vessel containing dNTPs, one of a set of distinct labeled oligonucleotide primers, all of which bind to a complementary sequence in said fragment, and one of a plurality of dideoxynucleotides, to generate single stranded DNA sequencing fragments, each labeled oligonucleotide primer comprising an oligonucleotide backbone having bonded d ereto a donor fluorophore and an acceptor fluorophore in energy transfer relationship, the donor fluorophore of each labeled oligonucleotide primer absorbing light at a wavelength that is substantially common among all labeled oligonucleotide primers and d e acceptor fluorophores emitting light energy at wavelengths that differ among labeled oligonucleotide primers in said set, and (iii) separating the resulting mixture of single-stranded DNA sequencing fragments into bands and determining the sequence by means of the order of die bands, die improvement in which the donor fluorophore of each labeled oligonucleotide primer is a cyanine dye characterized by a quanmm yield of between about 0.01 and about 0.25, and each acceptor fluorophore has a quanmm yield of at least about 0.3. 30. In a method for the identification and detection of single stranded nucleic acid fragments in a mixture of nucleic acid fragments, said method comprising: (i) hybridizing different nucleic acid fragments in said mixture to different probes, each probe comprising an oligonucleotide backbone having bonded d ereto a donor fluorophore and an acceptor fluorophore in energy transfer relationship, the donor fluorophore of each probe absorbing light at a wavelengdi d at is substantially common among all probes and d e acceptor fluorophores emitting light energy at wavelengths that differ among the different probes, (ii) separating said nucleic acid fragments thus hybridized, according to mobilities of said nucleic acid fragments, and (iii) detecting said hybridized nucleic acid fragments thus separated, by irradiation at d e absorption wavelengdi of said donor fluorophores and detection of fluorescence emitted by each of said acceptor fluorophores, the improvement in which me donor fluorophore of each label is a cyanine dye having a quanmm yield of between about 0.01 and about 0.25.. and each acceptor fluorophore has a quanmm yield of at least about 0.3. 31. In a method for die identification and detection of single or double stranded nucleic acid fragments in a mixture of nucleic acid fragments, said mediod comprising: (i) labeling each nucleic acid fragment of said mixture by means of a polymerase chain reaction employing a primer comprising an oligonucleotide backbone having bonded diereto a donor fluorophore and an acceptor fluorophore in energy transfer relationship, (ii) separating said nucleic acid fragments thus labeled, according to mobilities of said nucleic acid fragments, and (iii) detecting said nucleic acid fragments thus separated, by irradiation at die absorption wavelengdi of said donor fluorophore and detection of fluorescence emitted by said acceptor fluorophore, the improvement in which said donor fluorophore is a cyanine dye having a quanmm yield of between about 0.01 and about 0.25, and said acceptor fluorophore is a dye having a quanmm yield of at least about 0.3. 32. In a method for the identification and detection of single or double stranded nucleic acid fragments in a mixture of nucleic acid fragments, said method comprising: (i) ligating different nucleic acid fragments in said mixture to different labels, each label comprising an oligonucleotide backbone having bonded thereto a donor fluorophore and an acceptor fluorophore in energy transfer relationship, the donor fluorophore of each label absorbing light at a wavelengdi that is substantially common among all labels and the acceptor fluorophores emitting light energy at wavelengths that differ among me different labels, (ii) separating said nucleic acid fragments thus ligated, according to mobilities of said nucleic acid fragments, and (iii) detecting said ligated nucleic acid fragments thus separated, by irradiation at me absorption wavelength of said donor fluorophores and detection of fluorescence emitted by each of said acceptor fluorophores, die improvement in which the donor fluorophores of each label is a cyanine dye having a quanmm yield of between about 0.01 and about 0.25, and each acceptor fluorophore has a quanmm yield of at least about 0.3. 33. A method in accordance widi claims 29, 30, 31, or 32 in which said cyanine dye has an absorption maximum in me wavelength range of about 400 to about 900 nm. 34. A method in accordance wid claims 29, 30, 31, or 32 in which said cyanine dye has an absorption maximum in me wavelength range of about 400 to about 900 nm, and said acceptor fluorophore has an emission maximum in me wavelength range of about 450 to about 1,000 nm. 35. A method in accordance widi claims 29, 30, 31, or 32 in which said cyanine dye has an absorption maximum in d e wavelength range of about 480 to about 550 nm. 36. A method in accordance wi i claims 29, 30, 31, or 32 in which said cyanine dye has an absorption maximum in me wavelength range of about 480 to about 550 nm, and said acceptor fluorophore has an emission maximum in the wavelength range of about 500 to about 700 nm. 37. A method in accordance wid claims 29, 30, 31, or 32 in which said cyanine dye has die formula in which: X is O or S; Y is O or S; R 1 is a member selected from the group consisting of H, halo, and C r C 6 alkyl; R 2 is a member selected from the group consisting of H, halo, and C r C 6 alkyl; R 3 is C,-C 6 alkyl; and m is zero to 3. 38. A method in accordance wi claims 29, 30, 31, or 32 in which said cyanine dye has me formula in which: X is O; Y is O; R 1 is a member selected from the group consisting of H and C,-C 3 alkyl; R 2 is a member selected from me group consisting of H and C r C 3 alkyl; R 3 is C,-C 3 alkyl; and m is 1, 2, or 3. 39. A mediod in accordance widi claims 29, 30, 31, or 32 in which said cyanine dye has d e formula in which: X is O; Y is O; R' is H or methyl; R 2 is H or memyl; R 3 is C,-C 3 alkyl; and m is 1. 40. A mediod in accordance widi claims 29, 30, 31, or 32 in which said cyanine dye has d e formula in which: XisO; YisO; R 1 is H or methyl; R 2 is H or methyl; R 3 is ethyl; and m is 1. 41. A method in accordance wid claims 29, 30, 31, or 32 in which said cyanine dye has de formula in which: XisO; YisO; R 1 is methyl; R 2 is methyl; R 3 is emyl; and m is 1. 42. A mediod in accordance wid claims 29, 30, 31, or 32 in which said donor fluorophore is bonded to the 5' terminal unit of said oligonucleotide backbone. 43. A method in accordance wid claims 29, 30, 31, or 32 in which said oligonucleotide backbone is from 5 to 30 nucleotides in length. 44. A method in accordance widi claims 29, 30, 31, or 32 in which said oligonucleotide backbone is from 5 to 20 nucleotides in leng i. 45. A mediod in accordance wid claims 29, 30, 31, or 32 in which said cyanine dye and said acceptor fluorophore are bonded to nucleotides separated by 1 to 29 intervening nucleotides. 46. A method in accordance widi claims 29, 30, 31, or 32 in which said cyanine dye and said acceptor fluorophore are bonded to nucleotides separated by 1 to 15 intervening nucleotides. 47. A mediod in accordance widi claims 29, 30, 31, or 32 in which said cyanine dye and said acceptor fluorophore are bonded to nucleotides separated by 4 to 12 intervening nucleotides. 48. A method in accordance widi claims.29, 30, 31, or 32 in which said acceptor fluorophore is a xanthene compound. 49. A method in accordance wid claims 29, 30, 31, or 32 in which said acceptor fluorophore is a member selected from fluoresceins and rhodamines. 50. A memod in accordance widi claims 29, 30, 31, or 32 in which said cyanine dye is 3-(e-carboxypentyl)-3'-edιyl-5,5'-dimethyloxacarbocyanine or 3-(e-carboxypentyl)-3'-ethyl-oxacarbocyanine, and said acceptor fluorophore is a member selected from the group consisting of fluoresceins and rhodamines. 51. A method in accordance widi claims 29, 30, 31, or 32 in which said cyanine dye is 3-(e-carboxypentyl)-3'-ethyl-5,5'-dimethyloxacarbocyanine or 3-(e-carboxypentyl)-3'-ethyl-oxacarbocyanine, and said acceptor fluorophore is a member selected from the group consisting of FAM, R6G, TAMRA, ROX, JOE, and rhodamine 110. 52. A method in accordance with claims 29, 30, 31, or 32 in which said cyanine dye is bonded to said backbone by a linking group having me formula in which n is 1 to 10, and p is 1 to 10. 53. A method in accordance widi claims 29, 30, 31, or 32 in which said cyanine dye is bonded to said backbone by a linking group having the formula in which n is 1 to 6 and p is 2 to 8. 54. A mediod in accordance wid claims 29, 30, 31, or 32 in which said cyanine dye is bonded to said backbone by a linking group having die formula in which n is 4 to 6 and p is 3 to 6. 55. A method in accordance wid claims 29, 30, 31, or 32 in which said cyanine dye is bonded to said backbone by a linking group having the formula 56. A method in accordance widi claims 29, 30, 31, or 32 in which said cyanine dye has d e formula said backbone is an oligonucleotide, and said cyanine dye is bonded to a nucleotide of said backbone by a linking group having the formula (CH 2 ) 5 C=0 I NH (CH 2 ) 6

Inventors: GLAZER ALEXANDER N, HUNG SU-CHUN, JUE JINGYUE, MATHIES RICHARD A, GLAZER, ALEXANDER, N., HUNG, SU-CHUN, JUE, JINGYUE, MATHIES, RICHARD, A.
Applicants: UNIV CALIFORNIA, THE REGENTS OF THE UNIVERSITY OF CALIFORNIA
Application year: 1997
Main classification: C12Q1
IPC codes: C09B23, C09K11, C12Q1