Resonant Inelastic Soft X-ray Scattering and X-ray Emission Spectroscopy of Solid Proline and Proline Solutions

Abstract

The amino group of proline is part of a pyrrolidine ring, which makes it unique among the proteinogenic amino acids. To unravel its full electronic structure, proline in solid state and aqueous solution is investigated using X-ray emission spectroscopy and resonant inelastic soft X-ray scattering. By controlling the pH value of the solution, proline is studied in its cationic, zwitterionic, and anionic configurations. The spectra are analyzed within a "building-block principle" by comparing with suitable reference molecules, i.e., acetic acid, cysteine, and pyrrolidine, as well as with spectral calculations based on density functional theory. We find that the electronic structure of the carboxyl group of proline is very similar to that of other amino acids as well as acetic acid. In contrast, the electronic structure of the amino group is significantly different and strongly influenced by the ring structure of proline.

Solvent Effects and pH Dependence of the X-ray Absorption Spectra of Proline from Electrostatic Embedding Quantum Mechanics/Molecular Mechanics and Mixed-Reference Spin-Flip Time-dependent Density-Functional Theory

“…The interaction of zwitterionic proline with water is well documented, indicating that the carboxylate group accepts less than two hydrogen bonds and the amine hydrogen atoms donate less than one hydrogen bond to the surrounding water molecules . Proline has a rich XAS structure for the oxygen, nitrogen, and carbon K-edges, , and the corresponding spectra depend on the pH. , Indeed, at neutral pH, proline exists in a zwitterionic form, in which the carboxylic group is deprotonated and the amino group is protonated. At very acidic pH (<2), the carboxylic group is protonated, the proline being in a cationic form, whereas at very basic pH (>11) the amino group is deprotonated and the proline is in anionic form.…”

“…18 Proline has a rich XAS structure for the oxygen, nitrogen, and carbon K-edges, 7,19 and the corresponding spectra depend on the pH. 6,20 Indeed, at neutral pH, proline exists in a zwitterionic form, in which the carboxylic group is deprotonated and the amino group is protonated. At very acidic pH (<2), the carboxylic group is protonated, the proline being in a cationic form, whereas at very basic pH (>11) the amino group is deprotonated and the proline is in anionic form.…”