Electronic Structure of 5-Hydroxyindole: From Gas Phase to Explicit Solvation

We have investigated the absorption and emission spectrum of 5-hydroxyindole in the gas phase and in various solvents. 5-Hydroxyindole is the fluorophore of the non-natural amino acid 5-hydroxytryptophan, which has attracted recent interest as a novel intrinsic probe for protein structure, dynamics, and function. Gas-phase and implicit solvent calculations were performed with multiconfigurational perturbation theory (CASPT2). An explicit solvent model was calculated using a decoupled quantum mechanics/molecular mechanics approach, utilizing recent advances in time-dependent density functional theory. The importance of hydrogen bonding is shown by comparing the implicit solvent model calculations with the explicit solvent calculations and experimental results. In line with other indole systems, the order of the L-1 state peaks in 5-hydroxindole is L-1(b) at lower energy than L-1(a), with the emitting state being L-1(a).