Ab initio study of the electronic singlet excited-state properties of tryptophan in the gas phase: The role of alanyl side-chain conformations

Abstract

The complex electronic structure of gas phase tryptophan is investigated by ab initio methods and time-dependent density functional theory (TD-DFT), as a function of the conformations of the alanyl side chain. Energies, oscillator strengths, and dipole moments of the four lowest lying excited states are determined from multi-configurational (CASSCF/MS-CASPT2) calculations using a rather large basis set. While the second excited state always carries the largest oscillator strength for all conformers investigated in the present work, we find that the permanent dipole moment is strongly conformer dependent. TD-DFT results are barely useful for the proper characterization of the excited states.