Doubly charged protonated a ions derived from small peptides

Abstract

Protonated a₂ and a₃ (therefore doubly charged) ions in which both charges lie on the peptide backbone are formed in collision-induced dissociations of [La^III(peptide)(CH₃CN)_m]³⁺ complexes. Abundant (a₃+H)²⁺ ions are formed from triproline (PPP) and peptides with a proline residue at the N-terminus; these peptides are the most effective in producing ions of the type (a₂+H)²⁺ and (a₃+H)²⁺. A systematic study of the effect of the location of the proline residue and other residues of aliphatic amino acids on the generation of protonated a ions is reported. Density functional theory calculations at B3LYP/6-311++G(d,p) gave the proton affinity of the a₃ ion derived from PPP to be 167.6 kcal mol⁻¹, 2.6 kcal mol⁻¹ higher than that of water. The protonated a₂ ions of diglycine and diproline and a₃ ions of triglycine have lower proton affinities and are only observed in lower abundances, possibly due to proton transfer to water in ion–molecule reactions.