In vitro quantification of hydrolysis-induced racemization of amino acid enantiomers in environmental samples using deuterium labeling and electron-impact ionization mass spectrometry.
D-amino acids indicate aging, bacterial origin, and pathogenic properties of peptides in the environment, but the reliable assessment of D-enantiomers must account for a yet unknown formation during hydrolyses. Here, we introduce a method for the in vitro determination of the hydrolysis-induced racemization (HIR) of amino acids in environmental samples. It involves hydrolyses with hydro- and deuteriochloric acid (6 M, 12 h, 105 degrees C), desalting, and selective detection of chiral mass fragments of amino acid-N-pentafluoropropionyl derivatives. D-Amino acids formed in 2HCl incorporated deuterium into their C(alpha) position. This resulted in a relative signal loss of the nondeuterated fragment compared with the 1HCl hydrolysate. Mathematically evaluating the relative target signal intensities of both hydrolysates allowed the quantification of the proportion of D-amino acids formed during sample processing. Side-chain incorporations of deuterium were no limitations for this method as they could be estimated from that of the respective L-enantiomers. In soil and litter samples, between 0 (D-glutamic acid) and 85% (D-alloisoleucine) of the detected D-amino acids were formed upon hydrolysis (standard error, 5-11%). For a given amino acid, the HIR varied by a factor of 2-10 between samples, thereby confirming that HIR must be individually assessed for samples from different environments.
Institute of Soil Science and Soil Geography, University of Bayreuth, Germany. wulf.amelung@tu-berlin.de
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Amelung
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Brodowski
D000596:Amino Acids / Q000032:analysis*; D003903:Deuterium; D004787:Environmental Pollution / Q000032:analysis*; D008401:Gas Chromatography-Mass Spectrometry; D006868:Hydrolysis; D007553:Isotope Labeling; D012989:Soil Pollutants / Q000032:analysis*; D013237:Stereoisomerism
D000596:Amino Acids; D012989:Soil Pollutants; D003903:Deuterium