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Several cationized forms of bradykinin (BK) were generated in the gas phase using matrix assisted laser desorption ionization (MALDI). Accurate collision cross sections were obtained using the ion chromatography method. The species studied include (BK + H) + , (BK + Na) + , and (BK -H + 2Na) + . It was found that all three species had very similar cross sections of 245 ( 3 Å 2 , and these cross sections were independent of temperature from 300 to 600 K. It could be concluded from these data that BK wraps itself around the charge center(s) in a globular shape whose time average size changes little up to 600 K. The arrival time distributions of all three systems were narrow, only slightly broader than expected for a single species indicating cationized BK exists in only a few low-energy conformers at low temperature. A detailed analysis of the data was done using molecular mechanics/dynamics of the AMBER 4.0 suite of programs. The calculations were in excellent agreement with experiment in that scatter plots indicated cross sections of 100 member structural sets of (BK + H) + , (BK + Na) + , and (BK -H + 2Na) + were very similar. Further, very extensive dynamics studies over the range 200 to 600 K indicated the lowest energy conformers exhibited cross sections independent of temperature in agreement with experiment and supported the indication that only a few conformers are involved. The absolute magnitudes of the AMBER generated 0 K structures were ∼10% smaller than experiment. The discrepancy decreased to ∼5% when the systems were thermally averaged at 300 K. Selected 0 K conformers of (BK + H) + were calculated using AM1 and PM3 from AMBER starting structures. It was found that the 0 K cross sections increased by ∼5% over the AMBER structures providing better agreement with experiment. The extensive conformer sets generated in the scatter plots were analyzed to see which parts of BK preferred to bind to the charge sites. As expected the binding was global, but each isomer or system had different preferred binding sites. We looked for a preference of BK forming a β-turn in the Ser 6 -Pro 7 -Phe 8 -Arg 9 sequence since such a feature had been proposed in solution NMR studies. We found little evidence for β-turns in our 500 conformers of variously cationized BK in the gas phase.

Infrared multiple-photon dissociation (IR-MPD) spectroscopy has been applied to singly-charged complexes involving the transition metals Ag + and Zn 2+ with the aromatic amino acid phenylalanine. These studies are complemented by DFT calculations. For [Phe+Ag] + the calculations favor a tridentate charge solvation N/O/ring structure. The experimental spectrum strongly supports this as the predominant binding geometry and, in particular, rules out a significant presence of the salt-bridge conformation. Zn 2+ forms a deprotonated dimer complex with Phe, [Zn+Phe2-H] + , in which the +2 oxidation state serves as a useful biomimetic model for zinc protein sites. A number of low-energy conformations were located, of which the lowest-energy conformer predicted by the calculations involves a Phe ligand deprotonated on the carboxylic acid, while the other Phe ligand is in the tridentate charge solvation conformation. The calculated IR spectrum of this conformer gives a close fit to the experimental spectrum, strongly supporting this as the predominant binding geometry. This most stable calculated complex is characterized by N/ O/ring metal chelation with a tetrahedral-type coordination core of Zn 2+ to N and O of both ligands. Another similar tightly chelated structure shows a square-planar-type coordination core, but this structure is computed to be less stable and gives a less satisfactory match to the experimental spectrum. This preference for the tetrahedral geometry of the Lewis-basic atomic ligands parallels the common Zn(II) coordination geometry in proteins. The number of clearly identifiable peaks resolved in the IR-MPD spectra as well as the much-improved matches between the observed spectra and the DFT-calculated spectra of the most stable geometries compared to previous studies are noteworthy for systems of this size and complexity. These results demonstrate that IR spectroscopy of transition metal-amino acid complexes in combination with DFT calculations is a very powerful structural tool, readily applicable to biomimetic systems that model, for example, the reaction centers of proteins in the solvent-free environment. In addition, we present a novel ion-capturing method for Fourier transform ion cyclotron resonance mass spectrometry which removes the necessity of a buffer gas pulse, while allowing ion trapping at moderate voltages with apparently reduced collisional excitation of the ions.

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