Low-energy collision induced dissociation of protonated peptides. Importance of an oxazolone formation for a peptide bond cleavage

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Abstract
One of the fragmentation mechanisms of protonated peptides assumes formation of an oxazolone both protonated as a b ion or neutral as a counterpart of a y ion. This assumption is based on structures of these species derived from their respective fragmentation. Here, the question of an oxazolone formation is approached from the point of view of the reactant ion, a protonated peptide. A series of model peptides with the general structure Acyl–AlaPro–NH2 was synthesized, where the Acyl represents acyl groups with a range of nucleophilicities. The nucleophilicity of these acyl groups was approximated by proton affinity (PA) of the corresponding acyl N-methyl amides, calculated by ab initio methods. A linear correlation of y1 ion formation with PA of the acyl group is found that provides evidence for the oxazolone formation during fragmentation of protonated peptides. Three other dipeptides were synthesized to probe the mechanistic aspects of peptide ion fragmentations.

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