Infrared Photodissociation Spectroscopy of Electrosprayed Ions in a Fourier Transform Mass Spectrometer

Abstract

Previous gas-phase methods for infrared photodissociation spectroscopy (IRPD) require sample volatility. Our method instead uses electrospray ionization to introduce even large nonvolatile molecules into a Fourier transform mass spectrometer, where extended (>15 s) ion storage makes possible high sensitivity spectral measurements with an OPO laser over a range of 3050−3800 cm^-1. The spectra of 22 gaseous proton-bound amino acid complexes are generally correlated with the H-stretching frequencies established for O−H and N−H functional groups in solution. For theoretical structure predictions of the Gly₂H⁺ and N-acylated Asp₂H⁺ dimers, IRPD spectra clearly differentiate between the predicted lowest energy conformers. In contrast to solution, in the gas phase the glycine zwitterion is ∼20 kcal/mol less stable than the neutral; however, glycine is clearly zwitterionic in the gaseous GlyLysH⁺ dimer. The level of theory is inadequate for the larger Lys₂H⁺ dimer, as all low energy predicted structures have free carboxyl O−H groups, in contrast to the IR spectrum. IRPD appears to be a promising new technique for providing unique information on a broad range of biomolecular and other gaseous ions, especially on noncovalent bonding involving O−H and N−H groups.