Negative ion fragmentations of deprotonated peptides containing post-translational modifications. An unusual cyclisation/rearrangement involving phosphotyrosine; a joint experimental and theoretical study

The characteristic fragmentations of a pTyr group in the negative ion electrospray mass spectrum of the [M–H]− anion of a peptide or protein involve the formation of PO (m/z 79) and the corresponding [(M-H)−–HPO3]− species. In some tetrapeptides where pTyr is the third residue, these characteristic anion fragmentations are accompanied by ions corresponding to H2PO and [(M-H)−–H3PO4]− (these are fragmentations normally indicating the presence of pSer or pThr). These product ions are formed by rearrangement processes which involve initial nucleophilic attack of a C-terminal -CO [or -C(NH)O−] group at the phosphorus of the Tyr side chain [an SN2(P) reaction]. The rearrangement reactions have been studied by ab initio calculations at the HF/6-31+G(d)//AM1 level of theory. The study suggests the possibility of two processes following the initial SN2(P) reaction. In the rearrangement (involving a C-terminal carboxylate anion) with the lower energy reaction profile, the formation of the H2PO and [(M-H)−–H3PO4]− anions is endothermic by 180 and 318 kJ mol−1, respectively, with a maximum barrier (to a transition state) of 229 kJ mol−1. The energy required to form H2PO by this rearrangement process is (i) more than that necessary to effect the characteristic formation of PO from pTyr, but (ii) comparable with that required to effect the characteristic α, β and γ backbone cleavages of peptide negative ions. Copyright © 2009 John Wiley & Sons, Ltd