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

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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®-L-Val and Lipodex® 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), γ-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, β-alanine, and in some samples (<20 mg/kg DM) of (S)-β-aminoisobutyric acid and (<10 mg/kg DM) l-allo-isoleucine. In one date fruit, traces of α-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.

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Gas chromatography mass spectrometry


Molecular mass (weight)


High performance liquid chromatography


Selected ion monitoring


6-Aminoquinoyl-carbamyl-N-hydroxysuccinimidyl carbamate




Isobaric tag for relative and absolute quantitation




Deuterium chloride in deuterium oxide


Trifluoroacetic acid anhydride








Dimethylpolysiloxane functionalized with l-Val-tert.butylamide

Lipodex® E:


α-aminoadipic acid:

α-Aaa or a-AAA

β-Ala or b-Ala:


Aba or a-AB:

α-Amino-n-butyric acid

β-Aba or β-Aib:

β-Aminoisobutyric acid


1-Aminocyclopropane-1-carboxylic acid








γ-Amino-n-butyric acid

Hyl or Hy-Lys:



(2S,4R)-4-hydroxyproline (trans-4-hydroxy-l-proline)


Norleucine (internal standard)




Pipecolic acid


(2S,5R)-5-hydroxypipecolic acid (trans-5-hydroxypipecolic acid)


















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This project was supported by NSTIP strategic technologies program number 11-AGR 1600-2 of the Kingdom of Saudi Arabia. HB acknowledges his position as a Visiting Professor and Scientific Consultant at KSU. Special thanks are addressed to Katja Dettmer, University of Regensburg, and Małgorzata Jaworska, National Medicines Institute, Warsaw, Jürgen Gerhardt, C.A.T. Tübingen, and Frank Gutjahr, Chromatographie Balingen, for valuable discussions and assistance in parts of the work, in particular disproving or confirming the presence of tentatively assigned amino compounds in selected date samples.

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Correspondence to Hans Brückner.

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Ali, H.S.M., Alhaj, O.A., Al-Khalifa, A.S. et al. Determination and stereochemistry of proteinogenic and non-proteinogenic amino acids in Saudi Arabian date fruits. Amino Acids 46, 2241–2257 (2014).

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