A mass spectrometric and molecular orbital study of H2O loss from protonated tryptophan and oxidized tryptophan derivatives.

Protonated N-acetyltryptophan, oxindolylalanine (a mono-oxidized derivative of tryptophan), and N-acetyloxindolylalanine, as well as several di- and tripeptide derivatives containing oxindolylalanine, undergo a range of fragmentation reactions in the gas phase, including the loss of water. In order to elucidate the sites of water loss within these ions, and to determine the mechanisms associated with these processes, we have conducted a series of experiments employing multistage tandem mass spectrometry (MS/MS and MS(3)) in a quadrupole ion trap mass spectrometer, regiospecific structural labeling, and independent solution-phase syntheses of proposed product ion structures, coupled with the use of molecular orbital calculations at the B3LYP/6-31G* level of theory. We demonstrate that the loss of H(2)O from the amide carbonyl group of protonated N-acetyltryptophan O-methyl ester occurs via a "side-chain-backbone" neighboring group reaction to yield a protonated carboline derivative. In contrast, the loss of water from the O-methyl ester of protonated oxindolylalanine results in the formation of a tricyclic structure by "backbone-side-chain" nucleophilic attack from the amino nitrogen to the C2 position of the indole ring. The O-methyl ester of protonated N-acetyloxindolylalanine was found to dissociate via the loss of water from both possible sites, i.e. from the side-chain indolyl oxygen and the backbone amide carbonyl group. An estimate of the relative preference for water loss from each site was obtained from the abundances of product ions formed from MS(3) analysis of regiospecifically labeled derivatives of N-acetyloxindolylalanine, and from the results of molecular orbital calculations. These studies indicate the absence of a characteristic 'signature' ion or neutral loss for peptides containing oxindolylalanine residues under low-energy ion trap CID conditions. Copyright 2004 John Wiley & Sons, Ltd.