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Side-chain involvement in the fragmentation reactions of the protonated methyl esters of histidine and its peptides

2001; Elsevier BV; Volume: 209; Issue: 2-3 Linguagem: Inglês

10.1016/s1387-3806(01)00443-2

1873-2798

Jason M. Farrugia, Thomas Taverner, Richard A. J. O’Hair,

Advanced Proteomics Techniques and Applications

The gas phase fragmentation reactions of [M+H]+ ions of the methyl esters of histidine and histidine containing di- and tripeptides were examined by electrospray ionization (ESI) multistage mass spectrometry (MSn) experiments using a quadrupole ion trap mass spectrometer. The MS/MS spectra tend to be dominated by bn sequence ions, whose structures were probed via MS3 experiments and ab initio calculations at the MP2(FC)/6-31G∗//HF6-31G∗ level of theory (for bn ions where n = 1 and 2). The ab initio calculations suggest a structure for the b1 ion that is stabilized by the formation of a bicyclic ring via involvement of the side-chain imidazole ring. In contrast, MS3 experiments reveal that the b2 ion derived from the sequences HG-Y and GH-Y yield identical spectra to the MS/MS spectrum of the protonated diketopiperazine of (GH). These experimental results are consistent with ab initio calculations that reveal the side-chain protonated diketopiperazine of (GH) to be thermodynamically favored over all other b2 isomeric structures. Thus, the histidine side chain appears to exert both a direct and an indirect (through base catalysis) role in the formation of bn sequence ions from protonated peptides.