Fragmentation mechanisms of oxidized peptides elucidated by SID, RRKM modeling, and molecular dynamics

The gas-phase fragmentation reactions of singly charged angiotensin II (AngII, DR(+)VYIHPF) and the ozonolysis products AngII+O (DR(+)VY*IHPF), AngII+3O (DR(+)VYIH*PF), and AngII+4O (DR(+)VY*IH*PF) were studied using SID FT-ICR mass spectrometry, RRKM modeling, and molecular dynamics. Oxidation of Tyr (AngII+O) leads to a low-energy charge-remote selective fragmentation channel resulting in the b(4)+O fragment ion. Modification of His (AngII+3O and AngII+4O) leads to a series of new selective dissociation channels. For AngII+3O and AngII+4O, the formation of [MH+3O](+)-45 and [MH+3O](+)-71 are driven by charge-remote processes while it is suggested that b(5) and [MH+3O](+)-88 fragments are a result of charge-directed reactions. Energy-resolved SID experiments and RRKM modeling provide threshold energies and activation entropies for the lowest energy fragmentation channel for each of the parent ions. Fragmentation of the ozonolysis products was found to be controlled by entropic effects. Mechanisms are proposed for each of the new dissociation pathways based on the energies and entropies of activation and parent ion conformations sampled using molecular dynamics.