Analysis of the Racemization of Tryptophan

Abstract

Diastereoisomers of L- and D-tryptophan were formed with a chiral reagent, 1-thio-β-D-glucose tetraacetate and o-phthaldialdehyde, and were separated from the derivatives of the other amino acids that occur in food proteins on an achiral column by high performance liquid chromatography. Mercaptoethanesulfonic acid that is used as an agent for hydrolyzing proteins made the derivatization with o-phthaldialdehyde and 1-thio-β-D-glucose tetraacetate impossible. On the other hand the reaction was completed in the presence of p-toluenesulfonic acid, but the oxidative losses during hydrolysis were significant. During boiling, the racemization of tryptophan can be detected after 12 h above pH 9, but the rate of conversion was lower than expected (<1%). The decrease of L-tryptophan after 24 h was 2–5% depending on pH. Besides racemization, other reactions, e.g. oxidative deterioration may play a role in the loss of L-tryptophan.