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

Dorit Shemesh, Wolfgang Domcke

Research output: Contribution to journalArticlepeer-review

12 Scopus citations

Abstract

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 (γD, γL) 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 γL conformers may have shorter excited-state lifetimes (and thus higher photostability) than the γD 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 γL- conformers have shorter lifetimes (and thus higher photostability) than γD-conformers (see picture).

Original languageEnglish
Pages (from-to)1833-1840
Number of pages8
JournalChemPhysChem
Volume12
Issue number10
DOIs
StatePublished - 11 Jul 2011

Keywords

  • ab initio calculations
  • charge transfer
  • conical intersections
  • peptides
  • photochemistry

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