Fluorogenic reagents for detection of glycoconjugates alpha-ketoacids and diketones

Abstract: Compounds containing α-dicarbonyl groups are rendered capable of fluorescent emission by conjugation with carboxylic acid-terminated orth-diaminoaryl reagents. The carboxylic acid groups serve as ionizable moieties which facilitate the conjugation reaction, and are retained in the conjugation product to enable, or enhance the ability of, the conjugated compounds to be separated in chromatographic and electro-migration separation processes, notably electrophoresis. Ortho-dinitro derivatives of the reagents are also disclosed, these derivatives offering the advantage of stability as well as the ability to be used directly in the reaction in the presence of a reducing agent.

Description: This invention relates to fluorescence labeling techniques and to materials and methods for the detection of α-dicarbonyl compounds. Sialic acids, such as N-acetylneuraminic acid (NANA), are α-ketoacids which occur as non-reducing terminal residues in the carbohydrate chains of glycoproteins and glycolipids. Abnormal levels of NANA have been detected in the serum and urine of cancer patients and of persons who are afflicted with certain congenital metabolic diseases. The early detection and quantitation of NANA thus offers a means of diagnosing various disease states and of providing early signals for the initiation of therapy. The detection and measurement of sialic acid-terminated oligosaccharides has long presented a problem in biochemical investigations and routine analyses. The lack of detectable moieties which distinguish these molecules from other oligosaccharides has hindered developments in this area of glycobiology. Techniques for detection and quantitation of these molecules at early stages of their proliferation have been hampered by numerous problems, including lack of specificity of a derivatization reagent for α-keto acids or aldehydes, low sensitivity of the particular analytical technique employed, a lack of reagents which can be employed with highly polar analytes, and the instability of the derivatization reagents. A spectrophotometric method of sialic acid determination using thiobarbituric acid has been employed, but this method lacks both selectivity and sensitivity. See, L. Warren, J. Biol. Chem . 234 :1971(1959) and D. Aminoff, Virology 7 :355 (1959). Assays of NANA have also been performed as disclosed in the following publications: R. Schauer, Methods Enzymol. 50C :64 (1978), using gas chromatography (GC); I. Mononen, et al ., FEBS Lett. 59 :190(1975), using gas chromatography-mass spectrometry (GC-MS); A.K. Shukla, et al ., J Chromatogr . 244 :81 (1982) and H.K.B. Silver, et al ., J. Chromatogr . 224 :381(1981), using high-performance liquid chromatography (HPLC) with UV detection; and S. Hara, et al ., J. Chromatogr. 377 :111-119(1986), using HPLC with fluorescence detection. Both GC and HPLC with UV detection lack the sensitivity to detect NANA at picomole levels or below. Sensitivity is enhanced using GC-MS, but the cumbersome procedure for sample preparation limits the usefulness of this technique in rapid, diagnostic assays. The more recent method using HPLC with fluorescence detection involves the formation of a fluorescent quinoxaline from 1,2-diamino-4,5-dimethoxybenzene and NANA This technique does not however permit the determination of highly polar analytes or provide for optimal reaction product behavior in electrophoresis and high-sensitivity fluorimetry. An additional problem is that the shelf life of the reagent employed is very limited. Also of interest in this context are EP-A-0273388 which describes various sialic acid derivatives having an active carbonyl group; WO 91/00511 which describes detection and assay methods using chemiluminescent markers; and US-A-3,950,322 which describes the fluorogenic detection of neuraminidase. The present invention seeks to provide stable fluorogenic reagents that make feasible performance of high-sensitivity analyses of numerous biologically important compounds. In one aspect, the present invention concerns fluorogenic reagents which contain two nitro groups in adjacent positions on an aromatic ring, the ring also bearing one or more other substituents which collectively carry two carboxyl groups. These dinitro compounds are stable and permit the in situ generation of the less stable diamino derivatives, which in turn react with α-ketoacids or α-diketones to form fluorescent quinoxalines. Incorporation of carboxyl groups into the fluorogenic reagent provides the added benefit of facilitating the reaction with a highly polar analyte. A further benefit of the incorporation of carboxyl groups is that it enables, or enhances the ability of, the fluorescently tagged compounds to be separated by capillary electrophoresis and other forms of chromatography. In another aspect, the present invention concerns the use of laser-induced fluorescence to detect and measure the product species. By appropriate selection of a fluorogenic reagent within the scope of this invention for reaction with an analyte of interest, one achieves a reaction product which is excitable to an optimal degree by lasers dedicated to certain wavelengths, such as a helium/cadmium laser or an argon/ion laser. For α-ketoacid analytes, the sensitivity of detection by this method extends below femtomole levels (10 -15 mole levels) . For example, the method may be used for solutions having concentrations of 10 -7 M to 10 -8 M, with volumes injected being on the order of 10 -9 L. The compounds of the invention are further useful for the detection and quantitation of widely-occurring groups of neutral sugars possessing reducing ends, other naturally-occurring α-ketoacids and neurotoxic diketones. In the case of the neutral sugars, the reducing ends are oxidized to α-keto moieties prior to derivatization. According to the invention, therefore, there is provided a compound having the formula in which: R is -alkylene, -O-alkylene, alkylene or in which "alkylene" is straight chained or branched, has 1 to 10 carbon atoms and may contain carbon-carbon double or triple bonds; m is an integer of 1 to 4; n is an integer of 1 to 4; and the product m x n is at least 2. According to the invention, there is provided also a compound having the formula in which R, m and n have the meanings specified above. The invention provides additionally a method of converting an α-dicarbonyl containing analyte into a form which is detectable by fluorescence, the method comprising reacting the analyte with a diamino compound as described above, and further a method for detecting the presence of at least one α-dicarbonyl containing analyte in a liquid sample, the method comprising: (a) treating the sample with a diamino compound as described above to form fluorescently tagged derivatives of the α-dicarbonyl-containing analyte, the derivatives each containing at least two carboxyl groups; (b) passing the liquid sample so treated through a chromatographic separation medium to chromatographically separate components of the liquid sample from each other; and (c) detecting the fluorescently tagged derivatives among the components so separated, by fluorescence emission. The term "fluorescently tagged" refers to a sample analyte which has been chemically altered to contain a moiety which is fluorescent when irradiated by electromagnetic radiation at the appropriate wavelengths. The term "α-dicarbonyl-containing analyte" refers to a molecule which is to be analyzed and which contains two carbonyl groups which share a common bond. The two carbonyl groups may be in the form of an α-diketone, α-keto aldehyde or an α-keto acid. Similarly, the term "terminal α-dicarbonyl-containing analyte" refers to molecules as just described in which the two carbonyl groups make up either an α-keto aldehyde or an α-keto acid. These are often found at the end of a carbohydrate residue or glycoprotein. Examples of these analytes are N-acetyl neuraminic acid and α-ketoglutaric acid. In its broadest sense, therefore, the present invention resides in the synthesis and use of novel fluorogenic reagents for the high-sensitivity determination of biologically important compounds. As indicated above, these reagents contain ionizable groups incorporated in their structures. The benefits of these groups are twofold. First, the α-dicarbonyl compounds which are to be determined are often highly polar components of an aqueous mixture. The use of reagents for fluorescence tagging with ionizable groups results in a homogeneous reaction which permits quantitative detection of the α-dicarbonyl species in the analyte. Second, the retention of the ionizable groups in the quinoxaline products permits these products to be easily separated from impurities through capillary electrophoresis or high-performance liquid chromatography in buffered aqueous solvent systems. More particularly, the fluorogenic reagents of the present invention comprise ortho-dinitro and ortho-diamino compounds, represented by Formulae I and II, respectively. In both Formula I and Formula II, a "R" represents a radical bonded to the phenyl through one or two single bonds, and is alkylene, ―O―alkylene, alkylene or The indices m and n are each integers ranging from 1 to 4, and the product m × n is at least 2. Certain features are preferred. For example, alkyl groups within the definition of R are preferably C 1 -C 6 alkylene and most preferably C 1 -C 4 alkylene. A particularly preferred subclass is that in which R is either ―O―(C 1 -C 4 alkylene) or In still further preferred embodiments, m and n are each integers of 1 or 2 such that the product m × n is 2. To use a fluorogenic reagent within the scope of this invention for the determination of a α-dicarbonyl compound, the reagent is reacted with the compound to form a fluorescent quinoxaline, which is detectable by laser-induced fluorescence detection and other fluorometric techniques, at levels as low as the femtomole range and lower. One or more such quinoxalines thus formed are then capable of being chromatographically separated from any impurities present in the sample and irradiated with electromagnetic radiation for analysis for fluorescence emission. Reagents are in the form of orth-dinitro species may be used to form the quinoxalines. This is accomplished by first converting the ortho-dinitro species to the corresponding ortho-diamino species which are then reacted with the α-dicarbonyl containing analyte. This is conveniently done in a single reaction mixture without the isolation of the ortho-diamino intermediates. The quinoxalines, thus formed, are separated from other polar species, irradiated and analyzed for fluorescence emission. In a preferred embodiment, the method of the invention involves detecting the presence of at least one α-dicarbonyl containing analyte, which is present a liquid sample at concentrations below about 10 -6 M, by contacting the analyte with the diamino form of the preferred fluorogenic reagents above. The diamino form of the reagents may be generated immediately prior to use by the reduction of the dinitro form using any of the wide variety of conventional reducing agents well known among those skilled in the art. Examples of suitable reducing agents are NaBH 4 /CuSO 4 , H 2 /Pd(C), and ammonium formate/Pd(C). The liquid sample containing the newly formed fluorescently tagged species, which is a quinoxaline, is subjected to analytical separation which may be capillary electrophoresis, high-performance liquid chromatography, slab gel electrophoresis, thin-layer chromatography, or any of the wide variety of known separation methods suitable for materials of this type. The separated species are then irradiated with a laser, which selectively excites the analytes of interest in an optimal manner, and the species are then analyzed for fluorescence emission. In a further preferred embodiment, at least one of the α-dicarbonyl species is an α-ketoacid or an α-keto aldehyde and at least one of these species is present in the liquid sample at a level in the range of 10 -18 mole. The diamino fluorogenic reagent is formed immediately prior to use from the reduction of the dinitro precursor with NaBH 4 /CuSO 4 and is used in solution. The fluorescently tagged species which is formed is either a quinoxaline or a 2-hydroxyquinoxaline and is separated on capillary electrophoresis. Irradiation of the fluorescent species is carried out with a laser dedicated to a wavelength at which the species are selectively excitable, and emission is monitored using a fluorescence detector. The invention is particularly useful in the detection of terminal α-keto acids present in a sample at an amount of less than about 10 -12 mole. In the practice of this invention, the sample is reacted with a solution of one of the diamino fluorogenic reagents of the invention. The resulting mixture is then subjected to capillary electrophoresis, irradiated with a helium/cadmium laser and monitored for fluorescence emission. Additionally, the fluorogenic reagents used are those which have been freshly prepared from dinitro precursors by reduction with NaBH 4 /CuSO 4 . In all embodiments of the invention, conditions such as temperature, pressure, choice of solvent and concentration are not critical and may vary widely. Preferred ranges for temperature are from about 0°C to about 100°C, with about 15°C to about 45°C being optimal. The preferred pressure for all aspects of the invention is atmospheric pressure, although those skilled in the art will recognize that some aspects, for example long-term storage of fluorogenic reagents, may be carried out at reduced pressure ( i.e ., storage in a vacuum dessicator). The preferred solvents are generally aqueous mixtures of common organic solvents, for example, water/methanol, water/acetonitrile and water/dimethylformamide. Chromatographic techniques used in the practice of the invention are conventional techniques well known among those skilled in the art. The following examples are offered by way of illustration. Examples 1 and 2 illustrate the two steps required for the synthesis of 4,5-dinitrocatechol-O,O-diacetic acid. Example 4 illustrates a method by which the compounds of the present invention may be used for the detection and quantitation of α-dicarbonyl containing analytes. Examples 5-8 illustrate the benefits and advantages of the invention. Figs 1, 2, 3 and 4 of the accompanying drawings are detector traces of electropherograms of derivatized α-ketoacids, both singly and in mixtures, prepared according to the invention. EXAMPLE 1 Synthesis of 4-Nitrocatechol-O,O-diacetic Acid Catechol-O,O-diacetic acid (829 mg, 3.67 mmol from Lancaster Synthesis) was added gradually to 4.2 mL or 80% nitric acid which was cooled in an ice-salt bath. The mixture was stirred at below 0°C for 2.5 h and 0-10°C for 1 h. The resulting homogeneous mixture was poured onto 25g ice and the pasty precipitate was isolated by filtration, washed with water, dissolved in THF and dried over magnesium sulfate. Removal of solvent provided the desired product (623 mg, 2.30 mmol, 63% yield). EXAMPLE 2 Synthesis of 4,5-Dinitrocatechol-O,O-diacetic Acid To 6.0 mL of 80% nitric acid, chilled in an ice-salt bath, was added in one portion 4-nitrocatechol-O,O-diacetic acid (623 mg, 2.30 mmol). The mixture was stirred 3 hr, warming gradually to 2°C, poured onto ice and filtered. The residue was washed with water and dried to provide 4,5-dinitrocatechol-O,O-diacetic acid (356 mg, 1.13 mmol, 49% yield). EXAMPLE 3 Synthesis of Step 1: 2,1,3-Benzoselenodiazole-5-carboxylic Hydrazide The synthesis of 2,1,3-benzoselenodiazole-5-carboxylic hydrazide may be accomplished by first treating 3,4-diaminobenzoic acid with selenium dioxide to form 2,1,3-benzoselenodiazole-5-carboxylic acid. Treatment of this product with methanol and sulfuric acid provides the ester which is subsequently converted to its acid hydrazide by treatment with hydrazine. Step 2: Synthesis of 3,6-Dimethoxysalicylic Aldehyde Treatment of 2,5-dimethoxybenzaldehyde with m -chloroperbenzoic acid provides, after workup, 2,5-dimethoxypbenol. Protection of the phenol can be accomplished by formation of a THP ether using dihydropyran and acid. Formylation of the resultant compound can be carried out using butyllithium and DMF. Subsequent acidification liberates the phenol to yield 3,6-dimethoxysalicylic acid. Step 3: Synthesis of 1,4-Bis(carboxymethoxy)-7,8-diaminoanthracene Synthesis of 1,4-Bis(carboxymethoxy)-7,8-diaminoanthracene may be accomplished by first combining 3,6-dimethoxysalicylic aldehyde with 2,1,3-benzoselenodiazole-5-carboxylic hydrazide. The resulting Schiff base can be treated with lead tetraacetate to provide 5-(2',5'-dimethoxy-6'-formylbenzoyl)-2,1,3-benzoselenodiazole. Oxidation of the formyl substituent can be accomplished using pyridinium dichromate. Subsequent reduction of the ketone may be carried out using Wolf-Kishner conditions (N 2 H 4 ·H 2 O/base). Removal of selenium to generate 6-(3',4'-diaminobenzyl)-2,5-dimethoxybenzoic acid is accomplished with borane dimethyl sulfide complex. Polyphosphoric acid cyclization produces the tricyclic nucleus which can be converted to 7,8-diamino-1,4-dimethoxyanthracene. Removal of the methyl groups with borane tribromide and subsequent alkylation with 2 equivalents of chloroacetic acid provides 1,4-Bis(carboxymethoxy)-7,8-diaminoanthracene. EXAMPLE 4 Fluorescent Tagging of Analytes The fluorescent tagging of a α-dicarbonyl-containing analyte with the compounds of this invention may be achieved with either the diamino compounds or the dinitro compounds. When a diamino compound is used, the reaction should be conducted immediately following preparation. The dinitro precursors may be stored indefinitely and reduced to their diamino forms just prior to their contact with the analyte mixture. A typical reduction of a dinitro compound is as follows. The dinitro compound (1 mg) is dissolved in 10 µL of methanol and treated with 10 µL of a 2 M aqueous CuSO 4 solution. Sodium borohydride (5 µL of a 5 M solution) is added followed by additional portions of 10 µL of 2 M CuSO 4 and 5 µL of 5 M NaBH 4 . After 10 min at ambient temperature, methanol (40 µL) is added and the mixture is centrifuged. The supernatant containing the diamino compound is drawn off and used directly. Derivatization of an α-ketoacid is accomplished by treatment of the α-ketoacid (500 µg) with 10 µL of the diamino compound solution described above. Addition of 10 µL of 0.5 N HCl provides a fluorescent derivative which may be analyzed using capillary electrophoresis together with laser-induced fluorescence detection. EXAMPLE 5 Comparison of Two Fluorogenic Reagents The dinitro compounds of the present invention have the added benefit of stability which leads to less interference during analysis than is observed with the use of the diamino compounds. FIG. 1 compares the results obtained using the 1,2-diamino-4,5-dimethoxybenzene described by Hara, et al. , J. Chromatogr . 377 :111-119 (1986) with those obtained using 4,5-dinitrocatechol-O,O-diacetic acid of the present invention. Derivatization of NANA from human α-1 acid glycoprotein with either 1,2-diamino-4,5-dimethoxybenzene or 4,5-dinitrocatechol-O,O-diacetic acid (which is reduced to the diamine in situ ) provides the quinoxalines shown beside their respective electropherograms. The electropherograms were generated using laser-induced fluorescence detection following elution on a capillary column (50µm × 60cm, 14 kv, 20µA) with a buffer of 20mM borate and 20mM phosphate at pH 9.5. EXAMPLE 6 Analysis of Polygalacturonic Acid Mixtures The method of the present invention may also be used for the analysis of complex mixtures of oligosaccharides. FIG. 2 illustrates the capillary electrophoresis separation of fluorescently tagged polygalacturonic acids. EXAMPLE 7 Analysis of Two α-Ketoacids Contaminating α-ketoacids are readily separated and detected using the method of the present invention, as shown in FIG. 3. Capillary electrophoresis was run using a coated capillary column of 50 cm length with a 50 µm inside diameter, a buffer of 200 mM borate at pH 7.5 and 20 kv, 6 µA (voltage reversed). Fluorescence was detected at 452 nm following eluant irradiation with a He/Cd laser (325 nm). The electropherograms of quinoxalines derived from either α-ketoglutarate or NANA with diaminocatechol diacetic acid have been superimposed to demonstrate the difference in their respective migration times despite the similarity of their functional groups. EXAMPLE 8 Determination of the Level of Detection Using Laser-Induced Fluorescence Using the method of the present invention, quinoxaline derivatives formed may be detected at below femtomole levels as shown in FIG. 4 . The quinoxaline derivative formed by the treatment of NANA with 4,5-diaminocatechol-O,O-diacetic acid was subjected to He/Cd laser fluorescence detection following capillary electrophoresis on a coated capillary column using a buffer of 200 mM borate at pH 7.5 and 20 kv, 6µA (voltage reversed). The quinoxaline was detected at levels of 0.27 fmol (two orders of magnitude below those observed by Hara, et al . in similar experiments).

Claims: A compound having the formula in which: R is -alkylene, -O-alkylene, alkylene or in which "alkylene" is straight chained or branched, has 1 to 10 carbon atoms and when containing two or more carbon atoms may contain carbon-carbon double or triple bonds; m is an integer of 1 to 4; n is an integer of 1 to 4; and the product m x n is at least 2. The compound of the formula A compound having the formula in which the R, m and n have the meanings specified in claim 1. The compound of the formula A method of converting an α-dicarbonyl-containing analyte into a form which is detectable by fluorescence, the method comprising reacting the analyte with a compound according to claim 3 or claim 4 A method for detecting the presence of at least one α-dicarbonyl-containing analyte in a liquid sample, the method comprising: (a) treating the sample with a compound according to claim 3 or claim 4 to form fluorescently tagged derivatives of the α-dicarbonyl-containing analyte, the derivatives each containing at least two carboxyl groups; (b) passing the liquid sample so treated through a chromatographic separation medium to chromatographically separate components of the liquid sample from each other; and (c) detecting the fluorescently tagged derivatives among the components so separated, by fluorescence emission.

Inventors: NOVOTNY MILOS V, SHIROTA OSAMU, WIESLER DONALD, NOVOTNY, MILOS V., SHIROTA, OSAMU, WIESLER, DONALD
Applicants: INDIANA UNIVERSITY FOUNDATION
Application year: 1993
Main classification: C07C205
IPC codes: C07C217, C07C45, G01N33, C07D317, C07C47, C07C205, G01N21, G01N30