Abstract
The enantiomeric separations of D,L-amino acids derivatized with fluorogenic reagents, 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F), 4-(N,N-dimethylaminosulphonyl)-7-fluoro-2,1,3-benzoxadiazole (DBD-F) and 4-aminosulphonyl-7-fluoro-2,1,3-benzoxadiazole (ABD-F) by high-performance liquid chromatography (HPLC) on various Pirkle type chiral stationary phases (CSPs, Sumichiral OA series) with citric acid in methanol as a mobile phase were studied. Since the least retention and no separation was observed for the derivatives of racemic phenylalanine methyl-ester, -amide and a drug without an alpha-carboxyl group, the carboxylic acid group of the amino acid derivatives seemed to contribute to the enantioselective fixation of the derivatives through hydrogen bonding on the N-acyl-amino acid amide moiety of the CSP. The enantioselective retention of the derivatives was attained through the (S) or (R) configuration of valine, phenylglycine, naphthylglycine, naphthylethylamine or the tert-leucine moiety in the CSP. The 2,1,3-benzoxadiazole (benzofurazan) moiety in the derivatives helps the effective fixation of the derivatives through a pi-pi interaction with an aromatic moiety such as a 3,5-dinitrophenyl or naphthyl group in the Pirkle type chiral stationary phases. D-Amino acids in biological samples were easily determined utilizing the present derivatization with NBD-F, enantiomeric separation and fluorometric detection (530 nm em/470 nm ex) following deproteinization of biological samples (serum or brain homogenate) with methanol and centrifugation. The applications of the method were clearly demonstrated by the following results; D-Ala was detected in sera of healthy volunteers at a level of 0.48-3.10 microM. D-Lys was found in the serum of a patient with myeloma and requiring renal dialysis, and D-Ser was found in rat and bovine cerebrum. Peak identification was performed by use of different types of stationary phases especially those bearing the opposite configuration to that of the chiral centre.
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