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, and deprotonated). This is the first PES study of this amino acid in its biologically relevant environment. Proline's structure in the aqueous phase under neutral conditions is zwitterionic, distinctly different from the nonionic neutral form in the gas phase. By analyzing the carbon 1s and nitrogen 1s core levels as well as the valence spectra of aqueous-phase proline, we found that the electronic structure is dominated by the protonation state of each constituent molecular site (the carboxyl and amine groups) with small yet noticeable interference across the molecule. The site-specific nature of the core-level spectra enables the probing of individual molecular constituents. The valence photoelectron spectra are more difficult to interpret because of the overlapping signals of proline with the solvent and pH-adjusting agents (HCl and NaOH). Yet, we are able to reveal subtle effects of specific (hydrogen-bonding) interaction with the solvent on the electronic structure. We also demonstrate that the relevant conformational space is much smaller for aqueous-phase proline than for its gas-phase analogue. This study suggests that caution must be taken when comparing photoelectron spectra for gaseous- and aqueous-phase molecules, particularly if those molecules are readily protonated/deprotonated in solution.

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