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

Abstract

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 th..