Effect of the His Residue on the Cyclization of b Ions
The MS^n spectra of the [M + H]^+ and b"5 peaks derived from the peptides HAAAAA, AHAAAA, AAHAAA, AAAHAA, and AAAAHA have been measured, as have the spectra of the b"4 ions derived from the first four peptides. The MS^2 spectra of the [M + H]^+ ions show a substantial series of b"n ions with enhanced cleavage at the amide bond C-terminal to His and substantial cleavage at the amide bond N-terminal to His (when there are at least two residues N-terminal to the His residue). There is compelling experimental and theoretical evidence for formation of nondirect sequence ions via cyclization/reopening chemistry in the CID spectra of the b ions when the His residue is near the C-terminus. The experimental evidence is less clear for ions when the His residue is near the N-terminus, although this may be due to the use of multiple alanine residues in the peptide making identifying scrambled peaks more difficult. The product ion mass spectra of the b"4 and b"5 ions from these isomeric peptides with cyclically permuted amino acid sequences are similar, but also show clear differences. This indicates less active cyclization/reopening followed by fragmentation of common structures for b"n ions containing His than for sequences of solely aliphatic residues. Despite more energetically favorable cyclization barriers for the b"5 structures, the b"4 ions experimental data show more clear evidence of cyclization and sequence scrambling before fragmentation. For both b"4 and b"5 the energetically most favored structure is a macrocyclic isomer protonated at the His side chain.
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