| Abstract
| - The conformers of gaseous bradykinin, BK, (Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg) and its protonatedforms, [BK + H]+, [BK + 2H]2+, and [BK + 3H]3+, were examined theoretically using a combination of theMerck 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 structuralfeature in all four BK species is a β-turn in the Ser6-Pro7-Phe8-Arg9 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 experimentalcross-sections in the literature.
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