How the site of ionisation influences side-chain fragmentation in histidine radical cation

Based on MPWB1K/6-31++G(d,p) calculations, we have performed a charge and spin density analysis for the most representative structures of Hisδ(+) and Hisε(+) radical cations. This analysis shows that depending on the site where ionisation takes place, fragmentation of the side-chain can occur via R+ elimination to give the m/z 81 peak in the 70 eV EI-MS spectrum, or via the most favoured RH + elimination to give the most intense m/z 82 peak. The mechanism for RH + elimination involves a proton-transfer from the amino group to the free Nδ1 of the imidazole ring.