title: Unraveling the mechanisms of nonradiative deactivation in model peptides following photoexcitation of a phenylalanine residue.
abstract: The mechanisms of nonradiative deactivation of a phenylalanine residue after near-UV photoexcitation have been investigated in an isolated peptide chain model (N-acetylphenylalaninylamide, NAPA) both experimentally and theoretically. Lifetime measurements at the origin of the first ππ* state of jet-cooled NAPA molecules have shown that (i) among the three most stable conformers of the molecule, the folded conformer NAPA B is ∼50-times shorter lived than the extended major conformer NAPA A and (ii) this lifetime is virtually insensitive to deuteration at the NH(2) and NH sites. Concurrent time-dependent density functional theory (TDDFT) based nonadiabatic dynamics simulations in the full dimensionality, carried out for the NAPA B conformer, provided direct insights on novel classes of ultrafast deactivation mechanisms, proceeding through several conical intersections and leading in fine to the ground state. These mechanisms are found to be triggered either (i) by a stretch of the N(Phe)H bond, which leads to an H-transfer to the ring, or (ii) by specific backbone amide distortions. The potential energy surfaces of the NAPA conformers along these critical pathways have been characterized more accurately using the coupled cluster doubles (CC2) method and shown to exhibit barriers that can be overcome with moderate excess energies. These results analyzed in the light of the experimental findings enabled us to assign the short lifetime of NAPA B conformer to a number of easily accessible exit channels from the initial ππ* surface, most importantly the one involving a transfer of electronic excitation to an nπ* surface, induced by distortions of the backbone peptide bond.
authors.affiliation: Division of Physical Chemistry, Ruđer Bošković Institute, Bijenička cesta 54, HR-10002 Zagreb, Croatia.
authors.forename: Momir
authors.initials: M
authors.lastname: Mališ
authors.forename: Yohan
authors.initials: Y
authors.lastname: Loquais
authors.forename: Eric
authors.initials: E
authors.lastname: Gloaguen
authors.forename: Himansu S
authors.initials: HS
authors.lastname: Biswal
authors.forename: François
authors.initials: F
authors.lastname: Piuzzi
authors.forename: Benjamin
authors.initials: B
authors.lastname: Tardivel
authors.forename: Valérie
authors.initials: V
authors.lastname: Brenner
authors.forename: Michel
authors.initials: M
authors.lastname: Broquier
authors.forename: Christophe
authors.initials: C
authors.lastname: Jouvet
authors.forename: Michel
authors.initials: M
authors.lastname: Mons
authors.forename: Nađa
authors.initials: N
authors.lastname: Došlić
authors.forename: Ivan
authors.initials: I
authors.lastname: Ljubić
mesh_terms: D008958:Models, Molecular; D056004:Molecular Dynamics Simulation; D010455:Peptides / Q000737:chemistry*; D010649:Phenylalanine / Q000737:chemistry*; D055668:Photochemical Processes; D013056:Spectrophotometry, Ultraviolet
chemical_list: D010455:Peptides; D010649:Phenylalanine