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 CO2 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 CO2 laser excitation increases the fragmentation yield on the timescale of our experiments by as much as two orders of magnitude, significantly enhancing the sentivity 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.