Data set pertaining to the manuscript "From Gas to Solution: The Changing Neutral Structure of Proline Upon Solvation", submitted for peer review. In this work, Liquid-jet photoelectron spectroscopy (LJ-PES) and electronic-structure theory were employed to investigate the chemical and structural properties of the amino acid L-proline in aqueous solution for its three ionized states (protonated, zwitterionic, deprotonated). Experimental data were recorded by photoemission spectroscopy from a liquid jet source using synchrotron radiation. The data set documents the experimentally recorded spectra, including the proline photoelectron spectra and spectra of the zero energy cut-off, that were used to calibrate the binding energy scale. Files with extension .h5 are hdf5-files structured according to the NeXus standard, see theNeXus Data Format definition (v2024.02), https://manual.nexusformat.org/index.htmlNXmpes expansion for FAIRmat data (v.2024.07), https://fairmat-nfdi.github.io/nexus_definitions/classes/contributed_definitions/NXmpes.htmlNXmpes_liquid expansion to NXmpes (v.2024.07), https://fairmat-nfdi.github.io/nexus_definitions/mpes-liquid/classes/contributed_definitions/NXmpes_liquid.htmlNeXus data files can be opened with any software capable of opening hdf5-structured files. The following viewers are adapted to the specifics of the NeXus data format:* nexpy (distributed with python)* https://h5web.panosc.eu/h5wasm (web-based NeXus viewer maintained by the European Photon and Neutron Open Science Cloud-consortium) In each NeXus file-entry, two types of spectra are shown:1. Sweep-averaged spectra, integrated over the non-dispersive coordinate of our detector ('data'). 2. As-measured data ('raw'). If you use these data for your scientific work we kindly ask you to send us an electronic version or the citation of your work. Contact: Uwe Hergenhahn, uhe@fhi.mpg.de .

Acknowledgements and funding information: We acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. Parts of this research were carried out at PETRA III, and we would like to thank Moritz Hoesch and his team for assistance in using beamline P04. Beamtime was allocated for proposal II-20210015. F.T. acknowledges funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - Project 509471550, Emmy Noether Programme. F.T. and B.W. acknowledge support by the MaxWater initiative of the Max-Planck-Gesellschaft. B. W. acknowledges funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and investigation programme (grant agreement No. 883759). S.T. acknowledges support from ISHIZUE 2024 of Kyoto University.