Solvent interactions and specifically hydration are of utmost importance in chemical and biochemical systems. Model systems enable us to unravel the microscopic details of these interactions. Here, we clearly unraveled a specific hydrogen-bonding motif of the prototypical biomolecular building block indole (C8H7N), the chromophore of tryptophan, in water: The system exhibits a strong localized N-H...OH2 hydrogen bond, but otherwise unstructured interactions of the molecule with the solvent. This surprising segmentation of the solvent interaction was obtained from a combined experimental and theoretical investigation of the electronic structure of indole in aqueous solution. We recorded the complete x-ray photoemission and Auger spectrum of aqueous-phase indole and quantitatively explained all peaks with the aid of extensive ab initio modeling. The combination of the maximum-overlap method with the non-equilibrium polarizable-continuum model was demonstrated as an efficient and accurate technique for a modeling of both the valence and core photoemission spectra. A two-hole electron-population analysis shows a quantitative theoretical description of Auger spectra. Especially the core-electron binding energies for nitrogen and carbon demonstrated the distinct specific interaction of the one hydrogen-bound water molecule with the N-H group and the otherwise unspecific solvent interactions. The valence photoemission data provided the reorganization energy of aqueous-phase indole associated with its ionization, which we could directly connect to is electrochemical redox potential.
Comment: 13 pages, 5 figures