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

The capped dipeptides Ac-L-Phe-Xxx-NH₂, 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 ¹ππ* 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. Exciting experiments: The excited-state electron/proton transfer along the hydrogen bond of γ-turns of several chiral dipeptides (Ac-L-Phe-Xxx-NH₂, Xxx=L-Ala, D-Ala, Aib, where Aib=aminoisobutyric acid) is explored with the algebraic diagrammatic construction method. The results reveal an efficient radiationless deactivation pathway and suggest that γ_L- conformers have shorter lifetimes (and thus higher photostability) than γ_D-conformers (see picture).