Ion mobility-mass spectrometry (IM-MS) data is interpreted as evidence that gas-phase bradykinin fragment 1-5 (BK1-5, RPPGF) [M ϩ H] ϩ 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 ϩ H] ϩ 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 ␤-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. tudies of peptide and protein structure play a major role in modern molecular biophysics and drive development of new, more specific and sensitive techniques. Gas-phase techniques for sequencing peptides and proteins are well established, and significant progress has been made in developing similar techniques for characterization of secondary (2°) and tertiary (3°) structures of gas-phase peptides/proteins [1][2][3][4][5]. Such studies increase our understanding of intra-molecular interactions and drive development of sequencing algorithms, but the relevance of gas-phase structure, structure in the absence of solvent [6], to solution-phase structure is still an open issue. The most important role of solvent is charge stabilization, and in the gas phase, charge stabilization occurs by intramolecular interactions [7]. Rapid progress is being made in the development of techniques that allow selective solvation (in terms of number of solvent molecules) of gas-phase peptide/protein ions [8], and these studies may serve to further our understanding of how solvent affects structure and function. In addition, gas-phase studies could enhance our understanding of intra-molecular interactions (i.e., hydrogen bonding, van der Waals interactions and hydrophobic forces), as well as how such interactions influence conformations in the solution phase or low dielectric environments (i.e., gas phase, membranes, and/or multimeric complexes).Bradykinin (amino acid sequence RPPGFSPR) has been extensively studied using both experimental and computational techniques. Bradykinin is an excellent model peptide for structural studies because the presence of arginine (R) at both the N-and C-terminus can potent...

The rates of reaction, Arrhenius preexponential factors, and activation energies of cyclopropane and difluoromethoxydifluoromethane, CHF 2 OCHF 2 (HFOC-134), reacting with hydroxyl radical were measured by a relative method. Using ethane as the reference, eight measurements in the range from 25 to 186 °C yielded an Arrhenius fit of k ) (6.82 ( 1.5) × 10 -12 e -(1335(90)/T cm 3 molecule -1 s -1 for the reaction of cyclopropane with hydroxyl radical. Measurements were repeated using CH 3 CHF 2 (FC-152a) instead of ethane as the reference. Both measurements agreed within 3%. Nine measurements in the range from 0 to 191 °C were made of HFOC-134 using CF 3 CHF 2 (FC-125) as the reference. The result was k ) (1.29 ( 0.5) × 10 -12 e -(1895(100)/T cm 3 molecule -1 s -1 for HFOC-134. Preexponential factors for these two reactions are shown to be consistent with values predicted from a wide range of observed values for OH abstraction reactions. The rate constant results are compared to previously published values for these same compounds obtained by both relative and absolute methods.

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