Influence of cysteine to cysteic acid oxidation on the collision-activated decomposition of protonated peptides: Evidence for intraionic interactions.

Oxidation of cysteine residues to cysteic acids in C-terminal arginine-eontaining peptides (such as those derived by tryptic digestion of proteins) strongly promotes the formation of multiple members of the Y- series of fragment ions following low energy collision-activated decomposition (CAD) of the protonated peptides, Removal of the arginine residue abolishes the effect, which is also attenuated by conversion of the arginine to dimethylpyrim-idylornithine. The data indicate the importance of an intraionic interaction between the cysteic acid and arginine side-chains. Low energy CAD of peptides which include cysteic acid and histidine residues, also provides evidence for intraionic interactions. It is proposed that these findings are consistent with the general hypothesis that an increased heterogeneity (with respect to location of charge) of the protonated peptide precursor ion population is beneficial to the generation of a high yield of product ions via several charge-directed, low energy fragmentation pathways. Furthermore, these data emphasize the significance of gas-phase conformations of protonated peptides in determining fragmentation pathways.