MASS-SPECTROMETRIC ANALYSIS OF OXIDIZED TRYPTOPHAN

Oxindolylalanine and oxindolylalanine-containing peptides were prepare d by treatment of tryptophan and tryptophan-containing peptides with m ixtures of dimethyl sulfoxide and hydrochloric acid in acetic acid (DM SO-HCl-HAc). The reaction between tryptophan and DMSO-HCl-HAc was moni tored by fast atom bombardment mass spectrometry (FAB-MS) and the prop osed chlorotryptophan intermediate in the reaction was observed. Almos t complete conversion of tryptophan to oxindolylalanine was obtained i n reaction mixtures containing 3.75 M HCl when the reaction was perfor med in an open tube. A higher HCl concentration (5.5 M) and a closed r eaction tube promoted the formation of by-products, such as dioxindoly lalanine and 3-chlorooxindolylalanine. Extensive hydrolysis C-terminal of tryptophan was observed when tryptophan-containing peptides were t reated with DMSO-HCl-HAc containing 5.5 M HCl, during which the trypto phan residue was modified to dioxindolylalanine lactone, Hydrolysis wa s not observed in mixtures containing 3.75 M HCl. The presence of oxin dolylalanine in peptides could be demonstrated by characteristic peaks in FAB collision-induced dissociation tandem mass spectra. (C) 1998 J ohn Wiley & Sons, Ltd.