The structure of gas-phase bradykinin fragment 1-5 (RPPGF) ions: An ion mobility spectrometry and H/D exchange ion-molecule reaction chemistry study

Ion mobility-mass spectrometry (IM-MS) data is interpreted as evidence that gas-phase bradykinin fragment 1-5 (BK1-5, RPPGF) [M + H](+) ions exist as three distinct structural forms, and the relative abundances of the structural forms depend on the solvent used to prepare the matrix-assisted laser desorption ionization (MALDI) samples. Samples prepared from organic rich solvents (90 % methanol/10 % water) yield ions having an ion mobility arrival-time distribution (ATD) that is dominated by a single peak; conversely, samples prepared using mostly aqueous solvents (10 % methanol/90 % water) yield an ATD composed of three distinct peaks. The BK1-5 [M + H](+) ions were also studied by gas-phase hydrogen/ deuterium (H/D) exchange ion-molecule reactions and this data supports our interpretation of the IM-MS data. Plausible structures for BK1-5 ions were generated by molecular dynamics (MD). Candidate MD-generated structures correlated to measured cross-sections suggest a compact conformer containing a beta-turn whereas a more extended, open form does not contain such an interaction. This study illustrates the importance of intra-molecular interactions in the stabilization of the gas-phase ions, and these results clearly illustrate that solution-phase parameters (i.e., MALDI sample preparation) greatly influence the structures of gas-phase ions. (c) 2005 American Society for Mass Spectrometry.