title: Gaseous bradykinin and its singly, doubly, and triply protonated forms: a first-principles study.
abstract: The conformers of gaseous bradykinin, BK, (Arg(1)-Pro(2)-Pro(3)-Gly(4)-Phe(5)-Ser(6)-Pro(7)-Phe(8)-Arg(9)) and its protonated forms, [BK + H](+), [BK + 2H](2+), and [BK + 3H](3+), were examined theoretically using a combination of the Merck molecular force field, Hartree-Fock, and density functional theory. Neutral BK, [BK + H](+), and [BK + 2H](2+) exist in zwitterionic forms that are stabilized by internal solvation and have compact structures; [BK + 3H](3+) differs by the absence of a salt bridge and adopts an elongated form. The common structural feature in all four BK species is a beta-turn in the Ser(6)-Pro(7)-Phe(8)-Arg(9) sequence. The gas-phase basicity of [BK + H](+) estimated from the calculated protonation energy is in accord with published experimental basicity; population-weighted collision cross-sections of the three ionic forms are in agreement with experimental cross-sections in the literature.
authors.affiliation: Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto, Ontario M3J 1P3, Canada.
authors.forename: Christopher F
authors.initials: CF
authors.lastname: Rodriquez
authors.forename: Galina
authors.initials: G
authors.lastname: Orlova
authors.forename: Yuzhu
authors.initials: Y
authors.lastname: Guo
authors.forename: Xiaomao
authors.initials: X
authors.lastname: Li
authors.forename: Chi-Kit
authors.initials: CK
authors.lastname: Siu
authors.forename: Alan C
authors.initials: AC
authors.lastname: Hopkinson
authors.forename: K W Michael
authors.initials: KW
authors.lastname: Siu
mesh_terms: D001120:Arginine / Q000737:chemistry; D001920:Bradykinin / Q000737:chemistry*; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003198:Computer Simulation; D005740:Gases; D019791:Guanidine / Q000737:chemistry; D006859:Hydrogen / Q000737:chemistry; D006860:Hydrogen Bonding; D008958:Models, Molecular; D009010:Monte Carlo Method; D009584:Nitrogen / Q000737:chemistry; D010100:Oxygen / Q000737:chemistry; D011522:Protons
chemical_list: D005740:Gases; D011522:Protons; D006859:Hydrogen; D001120:Arginine; D019791:Guanidine; D009584:Nitrogen; D010100:Oxygen; D001920:Bradykinin