| Abstract
| - Previous gas-phase methods for infrared photodissociation spectroscopy (IRPD) require samplevolatility. Our method instead uses electrospray ionization to introduce even large nonvolatile moleculesinto a Fourier transform mass spectrometer, where extended (>15 s) ion storage makes possible highsensitivity spectral measurements with an OPO laser over a range of 3050−3800 cm-1. The spectra of 22gaseous proton-bound amino acid complexes are generally correlated with the H-stretching frequenciesestablished for O−H and N−H functional groups in solution. For theoretical structure predictions of theGly2H+ and N-acylated Asp2H+ dimers, IRPD spectra clearly differentiate between the predicted lowestenergy conformers. In contrast to solution, in the gas phase the glycine zwitterion is ∼20 kcal/mol lessstable than the neutral; however, glycine is clearly zwitterionic in the gaseous GlyLysH+ dimer. The levelof theory is inadequate for the larger Lys2H+ dimer, as all low energy predicted structures have free carboxylO−H groups, in contrast to the IR spectrum. IRPD appears to be a promising new technique for providingunique information on a broad range of biomolecular and other gaseous ions, especially on noncovalentbonding involving O−H and N−H groups.
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