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Strecker Degradation Products of Aspartic and Glutamic Acids and Their Amides

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Strecker Degradation Products of Aspartic and Glutamic Acids and Their Amides Czech J. Food Sci. Vol. 19, No. 2: 41–45 Strecker Degradation Products of Aspartic and Glutamic Acids and their Amides JAN RÖSSNER, JAN VELÍEK, FRANTIEK PUDIL and JIØÍ DAVÍDEK Institute of Chemical Technology Department of Food Chemistry and Analysis, Prague, Czech Republic Abstract RÖSSNER J., VELÍEK J., PUDIL F., DAVÍDEK J. (2001): Strecker degradation products of aspartic and glutamic acids and their amides. Czech J. Food Sci., 19: 4145. Aspartic and glutamic acids, asparagine and glutamine were oxidised with either potassium peroxodisulphate or glyoxal. Non- volatile products were derivatised and analysed by GC/FID and GC/MS. Volatile reaction products were isolated and analysed by the same methods. It was found that the degradation reactions of amino acids are complex. Amino acids are principally degraded via the corresponding α-keto acids to Strecker aldehydes (aspartic acid to oxalacetic and 3-oxopropionic acids and glutamic acid to α-ketoglutaric and 4-oxobutyric acids), which are unstable and decomposed by decarboxylation to the corresponding alde- hydes. Aspartic acid also eliminates ammonia and yields fumaric acid whereas glutamic acid gives rise to an imine, pyroglutamic acid. A recombination of free radicals leads to dicarboxylic acids (succinic acid from aspartic acid, succinic, glutaric and adipic acids from glutamic acid). The major volatile products (besides the aldehydes) are lower carboxylic acids (acetic acid from aspartic acid and propionic acid acid from glutamic acid) that can at least partly arise by radical reactions. In both quality and quantity terms, a higher amount of degradation products arises by oxidation of amino acids by peroxodisulphate. Keywords: Strecker degradation; Strecker aldehydes; amino acids; glyoxal; sodium peroxodisulphate; aspartic acid; glutamic acid; asparagine; glutamine; radicals The reaction of an α-amino acid with an oxidation re- asparagine (Asn) and glutamine (Gln). The Strecker deg- agent to give carbon dioxide and an aldehyde containing radation of Asp theoretically leads to 3-oxopropionic acid one carbon atom less is known as Strecker degradation (malonic semialdehyde). This Strecker aldehyde has been (SCHÖNBERG et al. 1948). A number of chemical reagents identified as a product of Asp oxidation with N-bromoac- have been recognised as having the power to cause such etamide (BISHNOI & BANERJI 1985; REDDY et al. 1990), oxidative decarboxylation of amino acids. The reactions chloramine T (GOWDA & RAO 1987) and potassium per- between amino acids and α-dicarbonyl compounds are of oxodisulphate (SRIVASTAVA & MATHUR 1982). On the special importance. They involve transaminations and other hand, the action of methylglyoxal on Asp resulted yield carbon dioxide, aldehyde and aminocarbonyls. The in the formation of acetaldehyde, which arises by decar- aldehydes formed, often called Strecker aldehydes, can boxylation of 3-oxopropionic acid (SCHÖNBERG & MOU- act as food odourants per se. The aminocarbonyls formed BACHER 1952). Analogously, the Strecker degradation of can yield pyrazine derivatives known as important fla- Glu leads to 4-oxobutyric acid (succinic semialdehyde), vour-active constituents of many processed foods. Mech- which was found to be a product of Glu oxidation by isat- anisms of these reactions have been recently described in (indole-2,3-dione) (SCHÖNBERG & MOUBACHER (BELITZ & GROSCH 1999; ADAMIEC et al. 2001a, b). 1952), sodium hypochlorite (FRIEDMAN & MORGULIS When amino acids with functional groups in the side 1936) or sodium hypobromite (FOX & BULLOCK 1951), chain are involved in the Strecker degradation, even more N-bromoacetamide (REDDY et al. 1990) and potassium complex reactions are possible. Such amino acids are as- peroxodisulphate (SRIVASTAVA & MATHUR 1982). partic (Asp) and glutamic (Glu) acids and their amides This work was supported by the research project CEZ: J19/98:223300004 granted by the Ministry of Education, Youth and Sports of the Czech Republic. 41 Vol. 19, No. 2: 41–45 Czech J. Food Sci. It is well known that Asp also behaves as a β-amino od. The aqueous phase was re-extracted with one 50 ml a acid. It can eliminate ammonia and yield fumaric acid two 25 ml portions of diethyl ether and the combined ex- (BELITZ & GROSCH 1999). Similarly, Glu behaves as a tracts were treated as described above. An aliquot (usual- γ-amino acid as it readily forms 5-oxopyrrolidine carb- ly 1 ml) of the aqueous phase was evaporated in a vacuum oxylic (pyroglutamic) acid upon heating. Under roasting evaporator, the residue was derivatised with diazomethane conditions where radical reaction mechanisms can pre- solution in diethyl ether, concentrated to 100 µl and anal- dominate, Asp and Asn yield various pyrrole-2,5-diones ysed by GC/FID and GC/MS. (WILKEN & BALTES 1990). Asp gives rise to maleimide Oxoacids: Using another aliquot of the evaporated aque- (pyrrole-2,5-dione) as the major component. A pathway ous phase, 10 mg of hydroxylamine.HCl and 1 ml of dry for the formation of this compound may be maleic acid pyridine were added and the mixture was let to stand for formed from Asp by the loss of ammonia, which subse- 10 min at room temperature. Then 0.1 ml of hexamethyl- quently reacts with carboxyl groups of dicarboxylic acid, disilazane and 0.1 ml of trimethylchlorosilane were add- yielding the imide. Substituted pyrrole-2,5-diones (3- ed and 1 µl of the solution was analysed by GC/FID and methyl and 3,4-dimethyl) were minor products. Asn yields GC/MS after 5 min of standing at room temperature. predominantly 3-methyl- and 3,4-dimethyl-pyrrole-2,5-di- Gas Chromatographic (GC/FID) and Gas Chromato- ones, whereas the concentration of succinimide (pyrroli- graphic/Mass Spectrometric (GC/MS) Analysis: A dine-2,5-dione) is low. It is suggested that pyrrole-2,5- Hewlett-Packard (H/P) Model 4890A gas chromatograph diones arise by a radical methyl group transfer to either equipped with a flame ionization detector and a fused maleimide or succinimide. capillary column (HP-Inowax, 30 m × 0.25 mm i.d., film This study was undertaken as a part of the investigation thickness: 0.25 µm) was used in this study. The GC oven of beer changes due to oxidation during pasteurization was temperature programmed from 60 to 220°C at a rate and storage which is followed by a decomposition of of 5°C/min, the injector and detector temperatures were amino acids and other compounds. Glutamin is a very held at 220 and 250°C, respectively. The carrier gas (N2) labile amino acid. It was even used as an indicator of beer flow rate was 1 ml/min. The sample (1 µl) was injected staling (HILL et al. 1998). The aim was to identify the using a split ratio of 1:10. Duplicate analyses of samples compounds arising during Strecker degradation of Asp were done. and Glu, their amides Asn and Gln, and to show the im- GC retention indices (relative retention index, R.I.) were portance of radical reactions in the pathways leading to determined internally with a series of n-alkanes (VA N DEN these products. DOOL & KRATZ 1963). The GC conditions were the same as described above. MATERIAL AND METHODS A H/P Model G1800A apparatus equipped with the same column operating under conditions described above Chemicals: Aspartic acid (L-isomer), glyoxalhydrate was used for GC/MS analysis. Carrier gas (He) flow rate trimer and 4-oxobutanoic acid (Sigma Chemical Compa- was 0.7 ml/min. Mass spectra were obtained by EI ion- ny, St. Louis, USA), L-asparagine monohydrate, L-glu- ization at 70 eV. The ion source temperature was main- tamine (Aldrich, Steinheim, Germany), L-glutamic, acetic, tained at 250°C. The NIST/EPA/NIK 75k Mass Spectral propionic, butanoic, 2-methylbutanoic, pentanoic (Lachema, Database (Hewlett-Packard) enabled the tentative identi- Brno, Czech Republic), 2-oxobutyric acid and oxalacetic fication of analysed compounds. acid (Merck, Darmstadt, Germany) were commercial prod- ucts. Potassium peroxodisulphate (K2S2O8), hydroxyl- RESULTS AND DISCUSSION amine.HCl and other compounds were obtained from Lachema (Brno, Czech Republic). Diazomethane solu- The oxidative decarboxylation of amino acids in this tion in diethyl ether was prepared from p-toluenesulpho- study was induced by a free radical initiator, potassium nyl-N-methylnitrosamide (Aldrich, Steinheim, Germany). peroxodisulphate, or achieved by glyoxal, a representa- Solvent grade diethyl ether, pyridine, hexamethyldisila- tive of α-dicarbonyl compounds (ADAMIEC et al. 2001a, zane and trimethylchlorosilane were purchased from b). Under the reaction conditions employed, the decom- Merck (Darmstadt, Germany). posed amount of Asp ranged from 53 to 64% and that of Oxidative Decarboxylation of Amino Acids: Amino Asn from 40 to 46%. Similar results were achieved with acid (5 mmol) and potassium peroxodisulphate or glyox- Glu (52–67%) and its amide Gln (37–62%) (Table 1). al (5 mmol) were dissolved in 500 ml water and the mix- Major pathways of Asp transformation are outlined in ture was heated in the Likens-Nickerson apparatus and Fig. 1. The intermediate oxalacetic acid can yield either extracted with 100 ml of diethyl ether for 1 h. The sol- pyruvic or 3-oxopropionic acid (the Strecker aldehyde) vent was dried over anhydrous sodium sulphate, concen- by decarboxylation and fumaric acid by the elimination trated to 500 µl using a Snyder column and a gentle stream of ammonia. Decarboxylation of pyruvic acid gives ace- of nitrogen and analysed by GC/FID and GC/MS meth- taldehyde. Acetaldehyde was identified as a product of 42 Czech J. Food Sci. Vol. 19, No. 2: 41–45 Table 1. Decomposition of Asp, Asn, Glu and Gln by potassium peroxodisulphate or glyoxal Concentration [mmol/l]a) System 1 2 Average Std. dev. Decomposed amount [%] Asp/K2S2O8 4.7 4.6 4.7 0.1 53 Asp/glyoxal 3.6 3.5 3.6 0.1 64 Asn/K S O 6.0 6.0 6.0 0.0 40 2 2 8 Asn/glyoxal 5.4 5.3 5.4 0.1 46 Glu/K S O 4.8 4.7 4.8 0.1 52 2 2 8 Glu/glyoxal 3.2 3.3 3.3 0.3 67 Gln/K2S2O8 3.8 3.7 3.8 0.1 62 Gln/glyoxal 6.3 6.2 6.3 0.1 37 a) The starting concentration of all amino acids was 10.0 mmol/l Asp, oxalacetic and pyruvic acids but not quantified.
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