title: Doubly charged protonated a ions derived from small peptides.
abstract: Protonated a(2) and a(3) (therefore doubly charged) ions in which both charges lie on the peptide backbone are formed in collision-induced dissociations of [La(III)(peptide)(CH(3)CN)(m)](3+) complexes. Abundant (a(3)+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 (a(2)+H)(2+) and (a(3)+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 a(3) ion derived from PPP to be 167.6 kcal mol(-1), 2.6 kcal mol(-1) higher than that of water. The protonated a(2) ions of diglycine and diproline and a(3) 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.
authors.affiliation: Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto, Canada M3J 1P3.
authors.forename: Irine S
authors.initials: IS
authors.lastname: Saminathan
authors.forename: Junfang
authors.initials: J
authors.lastname: Zhao
authors.forename: K W Michael
authors.initials: KW
authors.lastname: Siu
authors.forename: Alan C
authors.initials: AC
authors.lastname: Hopkinson
mesh_terms: D000595:Amino Acid Sequence; D056831:Coordination Complexes / Q000737:chemistry; D007477:Ions / Q000737:chemistry*; D007811:Lanthanum / Q000737:chemistry; D010455:Peptides / Q000737:chemistry*; D011522:Protons; D053719:Tandem Mass Spectrometry
chemical_list: D056831:Coordination Complexes; D007477:Ions; D010455:Peptides; D011522:Protons; D007811:Lanthanum