Specific versus Nonspecific Solvent Interactions of a Biomolecule in Water

Solvent interactions, particularly hydration, are vital in chemical and biochemical systems. Model systems unveil microscopic details of such interactions. We uncover a specific hydrogen-bonding motif of the biomolecular building block indole (C$_8$H$_7$N), tryptophan's chromophore, in water: a strong localized $\text{N-H}\cdots\text{OH}_2$ hydrogen bond, alongside unstructured solvent interactions. This insight is revealed from a combined experimental and theoretical analysis of indole's electronic structure in aqueous solution. We have recorded the complete X-ray photoemission and Auger spectrum of aqueous-phase indole, quantitatively explaining all peaks through \emph{ab initio} modeling. The efficient and accurate technique for modeling valence and core photoemission spectra involves the maximum-overlap method and the non-equilibrium polarizable-continuum model. A two-hole electron-population analysis quantitatively describes the Auger spectra. Core-electron binding energies for nitrogen and carbon highlight the specific interaction with a hydrogen-bonded water molecule at the N-H group and otherwise nonspecific solvent interactions.