Mass spectrometric analysis of oxidized tryptophan

Oxindolylalanine and oxindolylalanine-containing peptides were prepared by treatment of tryptophan and tryptophan-containing peptides with mixtures of dimethyl sulfoxide and hydrochloric acid in acetic acid (DMSO–HCl–HAc). The reaction between tryptophan and DMSO–HCl–HAc was monitored by fast atom bombardment mass spectrometry (FAB-MS) and the proposed chlorotryptophan intermediate in the reaction was observed. Almost complete conversion of tryptophan to oxindolylalanine was obtained in reaction mixtures containing 3.75 M HCl when the reaction was performed in an open tube. A higher HCl concentration (5.5 M) and a closed reaction tube promoted the formation of by-products, such as dioxindolylalanine and 3-chlorooxindolylalanine. Extensive hydrolysis C-terminal of tryptophan was observed when tryptophan-containing peptides were treated with DMSO–HCl–HAc containing 5.5 M HCl, during which the tryptophan residue was modified to dioxindolylalanine lactone. Hydrolysis was not observed in mixtures containing 3.75 M HCl. The presence of oxindolylalanine in peptides could be demonstrated by characteristic peaks in FAB collision-induced dissociation tandem mass spectra.