title: Fragmentation mechanism of UV-excited peptides in the gas phase.
abstract: We present evidence that following near-UV excitation, protonated tyrosine- or phenylalanine-containing peptides undergo intersystem crossing to produce a triplet species. This pathway competes with direct dissociation from the excited electronic state and with dissociation from the electronic ground state subsequent to internal conversion. We employ UV-IR double-resonance photofragment spectroscopy to record conformer-specific vibrational spectra of cold peptides pre-excited to their S1 electronic state. The absorption of tunable IR light by these electronically excited peptides leads to a drastic increase in fragmentation, selectively enhancing the loss of neutral phenylalanine or tyrosine side-chain, which are not the lowest dissociation channels in the ground electronic state. The recorded IR spectra evolve upon increasing the time delay between the UV and IR pulses, reflecting the dynamics of the intersystem crossing on a timescale of ~80 ns and <10 ns for phenylalanine- and tyrosine-containing peptides, respectively. Once in the triplet state, phenylalanine-containing peptides may live for more than 100 ms, unless they absorb IR photons and undergo dissociation by the loss of an aromatic side-chain. We discuss the mechanism of this fragmentation channel and its possible implications for photofragment spectroscopy and peptide photostability.
authors.affiliation: Laboratoire de Chimie Physique Moléculaire, École Polytechnique Fédérale de Lausanne, EPFL SB ISIC LCPM, Station 6, CH-1015 Lausanne, Switzerland.
authors.forename: Aleksandra V
authors.initials: AV
authors.lastname: Zabuga
authors.affiliation: Laboratoire de Chimie Physique Moléculaire, École Polytechnique Fédérale de Lausanne, EPFL SB ISIC LCPM, Station 6, CH-1015 Lausanne, Switzerland.
authors.forename: Michael Z
authors.initials: MZ
authors.lastname: Kamrath
authors.affiliation: Laboratoire de Chimie Physique Moléculaire, École Polytechnique Fédérale de Lausanne, EPFL SB ISIC LCPM, Station 6, CH-1015 Lausanne, Switzerland.
authors.forename: Oleg V
authors.initials: OV
authors.lastname: Boyarkin
authors.affiliation: Laboratoire de Chimie Physique Moléculaire, École Polytechnique Fédérale de Lausanne, EPFL SB ISIC LCPM, Station 6, CH-1015 Lausanne, Switzerland.
authors.forename: Thomas R
authors.initials: TR
authors.lastname: Rizzo
mesh_terms: D005740:Gases / Q000737:chemistry*; D007700:Kinetics; D008956:Models, Chemical; D010455:Peptides / Q000737:chemistry*; D010649:Phenylalanine; D055668:Photochemical Processes; D011522:Protons; D011789:Quantum Theory; D014443:Tyrosine; D014466:Ultraviolet Rays
chemical_list: D005740:Gases; D010455:Peptides; D011522:Protons; D014443:Tyrosine; D010649:Phenylalanine