Electronic spectrum of tryptophan-phenylalanine. A correlated ab initio and time-dependent density functional theory study.

The theoretical electronic spectrum of the tryptophan-phenylalanine bichromophoric dipeptide was obtained for one of the lowest-energy conformer by various high-level computational methods such as complete active space with second order perturbation theory, second-order approximate coupled-cluster theory, and time-dependent density functional theory. The results show that the first excited state is located on the tryptophan residue and called L(b) state in the amino-acid. The second and third excited states correspond respectively to the L(a) state of Trp and the excited state in the Phe residue. Time-dependent density functional methods appeared to be not efficient to calculate the excited states of such a peptide (except the first one) due to the inclusion of charge transfer states.