Simulations of the absorption and fluorescence of indole in aqueous solution and at a nonpolar/polar interface

Theoretical results are presented on the absorption and fluorescence of indole in aqueous solution as well as at the air/water surface. We use a combined quantum chemical statistical mechanical model with explicit solvent. An approximate ab initio complete active space self-consistent field description of the indole molecule is used, coupled to a discrete polarizable water medium. From the bulk simulations, strong support is found for the interchange mechanism, which explains the unusual solvent shift of the fluorescence of indole or tryptophan in a polar surrounding by a solvent induced switch of the fluorescing state. Two mechanisms are given to explain the different shifts for indole at the interface. First, a dielectric depletion effect, which is expected from the reduction of the amount of polar media. Second, an interface-specific effect, which derives from the stronger hydrogen bond formation at the surface. The latter effect acts to increase the shift for both absorption and emission at the surface as compared to the bulk. From these results, the intrinsic probe photophysics of tryptophan in proteins is discussed in terms of the properties of the protein/solvent interface and the orientation of the amino acid.