Hydrolysis-induced racemization of amino acids

Acid hydrolysis-induced racemization compromises accurate determination of enantiomeric amino acid compositions. In the present study, the extent of amino acid racemization during a vapor (6 M HCl, 32.5 min, 150 degrees C) and liquid-phase hydrolysis (6 M HCl, 20 h, 110 degrees C) was assessed, and the influence on the accuracy of enantiomer measurements was investigated. The extent of hydrolysis-induced racemization was controlled by the molecular structure of the amino acid polymer and hydrolysis conditions. Vapor-phase hydrolysis showed significantly higher racemization compared to liquid-phase hydrolysis. Without correction, hydrolysis-induced racemization resulted in overestimation of D-enantiomers and could lead to erroneous conclusions about the occurrence of D-enantiomers in natural samples. The range of racemization was determined with proteins and free amino acids, and an averaged racemization value is proposed to account for enantiomer exchange during hydrolysis. In addition, a microwave-assisted vapor-phase hydrolysis is described that allows preparation of 40 samples in about 3 h with very low blanks.