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.

On the near UV photophysics of a phenylalanine residue: conformation-dependent ππ* state deactivation revealed by laser spectroscopy of isolated neutral dipeptides

“…The clear occurrence of a fast deactivation process capable of overcoming, in specific conformations, those naturally occurring in the isolated chromophore, i.e., ISC and fluorescence, qualitatively supports the recent findings of excited state quantum chemistry studies, [1][2][3][4][5]48 Finally the competition between the presently evidenced fast dynamical channel and ISC or radiative relaxation should lead to various deactivation pathways and subsequent population of excited states, depending on the conformation considered. This point might be of some importance for the spectroscopy of peptide ions, 10,11 in which absorption is monitored through the observation of specific fragmentation channels.…”

Isolated Neutral Peptides

“…Attention has been drawn to the field by pioneering excited-state calculations in the past decade conducted on peptides [177,[202][203][204][205][206][207][208][209], following an approach already successful for showing the role of πσ* excited states in the deactivation of aromatic compounds, such as phenol or indole [210]. These authors emphasised the possible role of several excited states, namely a locally excited (LE) state and an intrabackbone charge transfer (CT) state to convert efficiently the photodeposited electronic energy into vibrational energy.…”

Neutral Peptides in the Gas Phase: Conformation and Aggregation Issues

“…Early works analyzed the electronic absorption spectrum or the dispersed fluorescence before other observables were used to document the first electronic excited states such as their vibrational spectra or their lifetimes . Further experimental and theoretical works on the excited states of neutral peptides followed ,− and are mentioned throughout this review, but more details can be found in another review dedicated to photophysical aspects …”

“…Their conformation and isomer distribution is then expected to be out of equilibrium, and kinetic aspects must be taken into account in the conformational analysis. 219 Several types of calculations in the electronic excited states were developed in order to interpret IR spectra of S 1 states, 75,179,333 rationalize the relative position of S 0 → S 1 transitions of each conformer 180,181,183,365 and their Franck− Condon activity, 179 or explain S 1 lifetime variations with the conformation, 181−183 revealing sometimes ultrafast processes responsible for experimentally missing conformers when using R2PI techniques. 177,178 When excited and ground state properties must be compared, e.g., for excitation energies or shifts between S 0 and S 1 IR spectra, the approximate secondorder coupled cluster method CC2 366 is mostly preferred for capped amino acids by comparison with multireference methods.…”