title: Effect of the chirality of residues and γ-turns on the electronic excitation spectra, excited-state reaction paths and conical intersections of capped phenylalanine-alanine dipeptides.
abstract: The capped dipeptides Ac-L-Phe-Xxx-NH(2) , Xxx=L-Ala, D-Ala, Aib, where Aib (aminoisobutyric acid) is a non-chiral amino acid, have been investigated by means of UV/IR double-resonance spectroscopy in supersonic jets and density functional theory calculations by Gloaguen et al. [Phys. Chem. Chem. Phys. 2007, 9, 4491]. The UV and IR spectra of five different species were observed and their structures assigned by comparison with calculated vibrational frequencies in the NH-stretching region. The peptides with two chiral residues can form homochiral or heterochiral species. In addition, γ-turns exist as two helical forms (γ(D), γ(L)) of opposite handedness. Herein, we explore the excited-state potential-energy surfaces of these dipeptides with ab initio calculations. Vertical and adiabatic excitation energies, excited-state reaction paths and conical intersections are characterized with the ADC(2) propagator method. It is shown that electron/proton transfer along the hydrogen bond of the γ-turn gives rise to efficient radiationless deactivation of the (1)ππ* state of the chromophore via several conical intersections. While the homo/hetero chirality of the residues appears to have a negligible effect on the photophysical dynamics, we found evidence that the γ(L) conformers may have shorter excited-state lifetimes (and thus higher photostability) than the γ(D) conformers.
authors.affiliation: Fritz Haber Center for Molecular Dynamics, The Hebrew University, Jerusalem 91904, Israel. doritsh@fh.huji.ac.il
authors.forename: Dorit
authors.initials: D
authors.lastname: Shemesh
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Domcke
mesh_terms: D000409:Alanine / Q000737:chemistry; D004150:Dipeptidases / Q000737:chemistry*; D004583:Electrons; D006860:Hydrogen Bonding; D010649:Phenylalanine / Q000737:chemistry; D013237:Stereoisomerism; D013816:Thermodynamics
chemical_list: D010649:Phenylalanine; D004150:Dipeptidases; C038937:dipeptidase; D000409:Alanine