Spectroscopy of protonated peptides assisted by infrared multiple photon excitation.

We report here a new technique for spectroscopic studies of protonated, gas-phase biomolecules and demonstrate its utility by measuring highly resolved electronic and infrared spectra of peptides of up to 17 amino acids. After UV excitation of an aromatic chromophore of a protonated peptide, a CO(2) laser further excites the molecules, increasing their vibrational energy and hence their dissociation rate, allowing detection of the UV excitation by monitoring the resulting photofragments. We show that addition of the CO(2) laser excitation increases the fragmentation yield on the time scale of our experiments by as much as 2 orders of magnitude, significantly enhancing the sensitivity of UV photofragment spectroscopy. We also demonstrate that this approach can be applied in an IR-UV double-resonance scheme, allowing measurement of conformer-specific infrared spectra of protonated peptides.