Solvent interactions, particularly hydration, are vital in chemical and biochemical systems. Model systems reveal microscopic details of such interactions. We uncover a specific hydrogen-bonding motif of the biomolecular building block indole (C8H7N), tryptophan's chromophore, in water: a strong localized N-H···OH2 hydrogen bond, alongside unstructured solvent interactions. This insight is revealed from a combined experimental and theoretical analysis of the electronic structure of indole in aqueous solution. We recorded the complete X-ray photoemission and Auger spectrum of aqueous-phase indole, quantitatively explaining all peaks through ab initio modeling. The efficient and accurate technique for modeling valence and core photoemission spectra involves the maximum-overlap method and the nonequilibrium 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.

Center for Free Electron Laser Science Deutsches Elektronen Synchrotron DESY Notkestraße 85 22607 Hamburg Germany

Center for Ultrafast Imaging Universität Hamburg Luruper Chaussee 149 22761 Hamburg Germany

Department of Physical Chemistry University of Chemistry and Technology Technická 5 16628 Prague Czech Republic

Department of Physics Universität Hamburg Luruper Chaussee 149 22761 Hamburg Germany

Institut für Kernphysik Goethe Universität Frankfurt Max von Laue Straße 1 60438 Frankfurt am Main Germany

Institute of Atomic and Molecular Physics Jilin University 130012 Changchun China

Molecular Physics Fritz Haber Institut der Max Planck Gesellschaft Faradayweg 4 6 14195 Berlin Germany

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From Gas to Solution: The Changing Neutral Structure of Proline upon Solvation

Liquid-jet photoemission spectroscopy as a structural tool: site-specific acid-base chemistry of vitamin C