Charge-separation reactions of doubly-protonated peptides: effect of peptide chain length.

The collision induced dissociation of doubly-protonated (Ala)(x)His (x = 5, 6, 7, 8, 10) peptides have been studied. The major fragmentation reactions observed are symmetrical amide bond cleavages to give the complementary b(m) and y(N-m) ions, where N is the total number of residues in the peptide. Minor asymmetric cleavage to give doubly-protonated y ions also is observed, involving cleavage near the N-terminus. The shorter peptides (x = 5, 6, 7) show major cleavage of the second amide bond to yield b2 and y(N-2) ions, while (Ala)10His shows major symmetrical cleavage at the fourth and fifth amide bonds. (Ala)8His appears to be a transitional peptide in showing substantial symmetrical cleavage at the second, fourth, and fifth amide bonds. The results are in general agreement with the bifurcating nature of charge separation noted by Zubarev (J. Am. Soc. Mass Spectrom.2008, 19, 1755-1763) from a statistical analysis of a large body of doubly-protonated tryptic peptide CID mass spectra. It is shown that the b2 ion derived from doubly-protonated (Ala)5His has a protonated oxazolone structure.