Separation and determination of the tryptophan enantiomers
Diastereoisomers of L- and D-tryptophan were formed with a chiral reagent 1-thio-β-D-glucose tetraacetate and o-phthaldialdehyde and they 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 an adequate agent for hydrolyzing proteins made the derivatization with o-phthaldialdehyde and 1-thio-β-D-glucose tetraacetate impossible, contrary the reaction completed in the presence of p-toluenesulfonic acid, but the oxidative losses during hydrolysis is significant. During boiling, the racemization of tryptophan can be detected above pH=9 after 12 hours, but the rate of conversion was lower than expected (<1%). The concentration decrease of L-tryptophan after 24 h was 2−5% depending on pH. Besides racemization other reactions e.g. oxidative deterioration may played a role in the loss of L- tryptophan.