Role of excited-state hydrogen detachment and hydrogen-transfer processes for the excited-state deactivation of an aromatic dipeptide: N-acetyl tryptophan methyl amide

The excited-state electronic potential-energy surfaces of the three conformers of the capped dipeptide N-acetyl tryptophan methyl amide (NATMA), for which UV and IR spectra have been reported by Dian et al. [J. Chem. Phys., 2003, 118, 2696], have been explored with ab initio electronic-structure methods. The results provide insight into the nonadiabatic electronic coupling mechanisms which are responsible for the pronounced and conformer-specific perturbations of the spectra, such as broad and congested UV spectra as well as the deletion of certain fundamentals in the IR spectrum of the S₁ state. It is shown that the photophysical dynamics of NATMA is governed by at least five excited singlet electronic states: the two spectroscopic ¹L_b and ¹L_a states and the dissociative ¹πσ* state of the indole chromophore, as well as a locally-excited state and a charge-transfer state of the peptide backbone. For the conformer NATMA C, which exhibits a γ-turn of the backbone, a potentially very efficient excited-state deactivation mechanism to the electronic ground state via three conical intersections has been revealed. The results confirm the important role of hydrogen bonds for rapid excited-state deactivation of peptides, which enhances their photostability.