Microsolvation effects on the excited state dynamics of protonated tryptophan

To better understand the complex photophysics of the amino acid tryptophan, which is widely used as a probe of protein structure and dynamics, we have measured electronic spectra of protonated, gas-phase tryptophan solvated with a controlled number of water molecules and cooled to ~10K. We observe that even at this temperature, the bare molecule exhibits a broad electronic spectrum, implying ultra-fast, non-radiative decay of the excited state. Surprisingly, the addition of two water molecules sufficiently lengthens the excited state lifetime that we obtain a fully vibrationally resolved electronic spectrum. First-principles based simulations of the excited state dynamics clearly demonstrate how interactions with water destabilize the photodissociative states and increase the excited state lifetime.