From Gas to Solution:\nThe Changing Neutral Structure\nof Proline upon Solvation

Liquid-jet photoelectron spectroscopy (LJ-PES) and electronic-structure\ntheory were employed to investigate the chemical and structural properties\nof the amino acid l-proline in aqueous solution for its three\nionized states (protonated, zwitterionic, and deprotonated). This\nis the first PES study of this amino acid in its biologically relevant\nenvironment. Proline’s structure in the aqueous phase under\nneutral conditions is zwitterionic, distinctly different from the\nnonionic neutral form in the gas phase. By analyzing the carbon 1s\nand nitrogen 1s core levels as well as the valence spectra of aqueous-phase\nproline, we found that the electronic structure is dominated by the\nprotonation state of each constituent molecular site (the carboxyl\nand amine groups) with small yet noticeable interference across the\nmolecule. The site-specific nature of the core-level spectra enables\nthe probing of individual molecular constituents. The valence photoelectron\nspectra are more difficult to interpret because of the overlapping\nsignals of proline with the solvent and pH-adjusting agents (HCl and\nNaOH). Yet, we are able to reveal subtle effects of specific (hydrogen-bonding)\ninteraction with the solvent on the electronic structure. We also\ndemonstrate that the relevant conformational space is much smaller\nfor aqueous-phase proline than for its gas-phase analogue. This study\nsuggests that caution must be taken when comparing photoelectron spectra\nfor gaseous- and aqueous-phase molecules, particularly if those molecules\nare readily protonated/deprotonated in solution.