title: On the near UV photophysics of a phenylalanine residue: conformation-dependent ππ* state deactivation revealed by laser spectroscopy of isolated neutral dipeptides.
abstract: The primary step of the near UV photophysics of a phenylalanine residue is investigated in one- and two-color pump-probe R2PI nanosecond experiments carried out on specific conformers of the Ac-Gly-Phe-NH2 molecule and related neutral compounds isolated in a supersonic expansion. Compared to toluene, whose ππ* state photophysics is dominated by intersystem crossing with a lifetime of ∼80 ns at the origin, the first ππ* state of Phe in the peptide environment is systematically found to be shorter-lived. The lifetime at the origin of transition is found to be significantly shortened in the presence of a primary amide (-CONH2) group (20-60 ns, depending on the conformer considered), demonstrating the existence of an additional non-radiative relaxation channel related to this chemical group. The quenching effect induced by the peptide environment is still more remarkable beyond the origin of the ππ* state, since vibronic bands of one of the 4 conformers observed (the 27-ribbon conformation) become barely detectable in the ns R2PI experiment, suggesting a significant conformer-selective lifetime shortening (below 100 ps). These results on dipeptides, which extend previous investigations on shorter Phe-containing molecules (N-Ac-Phe-NH2 and N-Ac-Phe-NH-Me), confirm the existence of conformer-dependent non-radiative deactivation processes, whose characteristic timescales range from tens of ns down to hundreds of ps or below. This dynamics is assigned to two distinct mechanisms: a first one, consistent with an excitation energy transfer from the optically active ππ* state to low-lying amide nπ* excited states accessed through conical intersections, especially in the presence of a C-terminal primary amide group (-CONH2); a second one, responsible for the short lifetimes in 2(7) ribbon structures, would be more specifically triggered by phenyl ring vibrational excitations. Implications in terms of spectroscopic probing of Phe in a peptide environment, especially in the presence of a quenching amide group, are discussed.
authors.affiliation: CEA, IRAMIS, Laboratoire Interactions, Dynamique et Lasers, CEA Saclay, Bât 522, 91191 Gif-sur-Yvette, France. michel.mons@cea.fr.
authors.forename: Yohan
authors.initials: Y
authors.lastname: Loquais
authors.forename: Eric
authors.initials: E
authors.lastname: Gloaguen
authors.forename: Mohammad
authors.initials: M
authors.lastname: Alauddin
authors.forename: Valérie
authors.initials: V
authors.lastname: Brenner
authors.forename: Benjamin
authors.initials: B
authors.lastname: Tardivel
authors.forename: Michel
authors.initials: M
authors.lastname: Mons
mesh_terms: D004151:Dipeptides / Q000737:chemistry / Q000302:isolation & purification*; D007834:Lasers; D008968:Molecular Conformation; D010649:Phenylalanine / Q000737:chemistry*; D055668:Photochemical Processes; D011789:Quantum Theory; D013056:Spectrophotometry, Ultraviolet; D014466:Ultraviolet Rays
chemical_list: D004151:Dipeptides; D010649:Phenylalanine