External electric field effect on the lowest excited states of indole: ab initio and molecular dynamics study

The external electric field effect on the lowest excited states of indole was studied with the aid of second-order Møller–Plesset perturbation theory based on the complete-active-space self-consistent-field wave function (CASMP2) and the time-dependent density functional theory (TDDFT) methods. The order of magnitude of the electric field experienced by indole in water and by the indole chromophore of tryptophan within a protein in aqueous environment was estimated using molecular dynamics simulations with the Amber force field. It has been shown that, at 300 K, the magnitude of the field is fluctuating significantly up to 5 × 10−3 a.u. The CASMP2 and TDDFT energy of the lowest ππ* singlet state (Lb) shows only a relatively small variation within the limit of the applied field, but the next ππ* singlet state (La) and the lowest πσ* singlet state of Rydberg character are strongly influenced by the field, and for |E|≅5 × 10−3 a.u. either the strongly emitting La(ππ*) state or the essentially “dark” πσ* state (depending on the orientation of the electric field vector) becomes the lowest excited singlet state of the system. Since the lifetime of the emitting singlet state is governed by the ππ*/πσ* crossing, as demonstrated in many experiments in clusters, this local field effect provides an attractive mechanistic picture for understanding the variations of the tryptophan fluorescence lifetime in proteins.