LIQUID-CHROMATOGRAPHIC DETERMINATION OF AMINO-ACID ENANTIOMERS BY DERIVATIZATION WITH O-PHTHALDIALDEHYDE AND CHIRAL THIOLS - APPLICATIONS WITH REFERENCE TO FOOD-SCIENCE

Using a fully automated liquid chromatograph, D- and L-alpha-amino acids (D- and L-AA) were determined in foods and beverages by precolumn derivatization with o-phthaldialdehyde (OPA) combined with the chiral thiol N-isobutyryl-L-cysteine (IBLC) or its enantiomer N-isobutyryl-D-cysteine (IBDC) with the structure (CH3)(2)CHCONHCH(CH2SH)COOH. The resulting diastereomeric isoindole derivatives were resolved on an octadecylsilyl stationary phase using a linear gradient formed from sodium acetate buffer (pH 5.95) and methanol-acetonitrile. For the detection of the isoindoles, their fluorescence at 445 nm when excited at a wavelength of 230 nm was used. The suitability of the LC method was demonstrated by the detection of free D-AA as native constituents of grape juice, their determination in alcoholic fermented beverages (beer and wine), fermented dairy products (coumiss, dietary whey drink, hard cheese), lactic acid fermented cabbage juice, a commercial yeast extract and honey. D-AA were detected in all foodstuffs. The results provide further proof that D-AA are common in microbially fermented foods and that microorganisms (bacteria, yeasts) are major, but not exclusive, sources of free D-AA occurring in foodstuffs.