Chromatographic determination of L- and D-amino acids in plants

Quantities of free L- and D-amino acids (L- and D-AAs) in plants (leaves of coniferous and decidious trees, fleshy fruits, leaf blades of fodder grasses, and seeds and seedlings of edible legumes) were determined. Amino acid (AA) enantiomers were converted into diastereomers using pre-column derivatization with o-phthaldialdehyde together with N-isobutyryl-L(or D)-cysteine followed by separation of the resulting fluorescent isoindol derivatives on an octadecylsilyl stationary phase using high-performance liquid chromatography. Relative amounts of D-AAs were also determined by enantioselective gas chromatography-mass spectrometry on Chirasil-L-Val. Free D-AAs acids in the range of about 0.2% up to 8% relative to the corresponding L-AAs acids were found in plants. D-Asp, D-Asn, D-Glu, D-Gln, D-Ser and D-Ala could be detected in most of the plants, and D-Pro, D-Val, D-Leu and D-Lys in certain plants. As D-AAs were detected in gymnosperms as well as mono- and dicotyledonous angiosperms of major plant families it is concluded that free D-AAs in the low percentage range are principle constituents of plants.