Determination and stereochemistry of proteinogenic and non-proteinogenic amino acids in Saudi Arabian date fruits

Whereas an abundance of literature is available on the occurrence of common proteinogenic amino acids (AAs) in edible fruits of the date palm (Phoenix dactylifera L.), recent reports on non-proteinogenic (non-coded) AAs and amino components are scarce. With emphasis on these components we have analyzed total hydrolysates of twelve cultivars of date fruits using automated ion-exchange chromatography, HPLC employing a fluorescent aminoquinolyl label, and GC-MS of total hydrolysates using the chiral stationary phases Chirasil(A (R))-L-Val and Lipodex(A (R)) E. Besides common proteinogenic AAs, relatively large amounts of the following non-proteinogenic amino acids were detected: (2S,5R)-5-hydroxypipecolic acid (1.4-4.0 g/kg dry matter, DM), 1-aminocyclopropane-1-carboxylic acid (1.3-2.6 g/kg DM), gamma-amino-n-butyric acid (0.5-1.2 g/kg DM), (2S,4R)-4-hydroxyproline (130-230 mg/kg DM), l-pipecolic acid (40-140 mg/kg DM), and 2-aminoethanol (40-160 mg/kg DM) as well as low or trace amounts (< 70 mg/kg DM) of l-ornithine, 5-hydroxylysine, beta-alanine, and in some samples (< 20 mg/kg DM) of (S)-beta-aminoisobutyric acid and (< 10 mg/kg DM) l-allo-isoleucine. In one date fruit, traces of alpha-aminoadipic acid could be determined. Enantiomeric analysis of 6 M DCl/D2O hydrolysates of AAs using chiral capillary gas chromatography-mass spectrometry revealed the presence of very low amounts of d-Ala, d-Asp, d-Glu, d-Ser and d-Phe (1.2-0.4 %, relative to the corresponding l-enantiomers), besides traces (0.2-1 %) of other d-AAs. The possible relevance of non-proteinogenic amino acids in date fruits is briefly addressed.