On the Origins of Core−Electron Chemical Shifts of Small Biomolecules in Aqueous Solution: Insights from Photoemission and ab Initio Calculations of Glycine_aq

Abstract

The local electronic structure of glycine in neutral, basic, and acidic aqueous solution is studied experimentally by X-ray photoelectron spectroscopy and theoretically by molecular dynamics simulations accompanied by first-principle electronic structure and spectrum calculations. Measured and computed nitrogen and carbon 1s binding energies are assigned to different local atomic environments, which are shown to be sensitive to the protonation/deprotonation of the amino and carboxyl functional groups at different pH values. We report the first accurate computation of core-level chemical shifts of an aqueous solute in various protonation states and explicitly show how the distributions of photoelectron binding energies (core-level peak widths) are related to the details of the hydrogen bond configurations, i.e. the geometries of the water solvation shell and the associated electronic screening. The comparison between the experiments and calculations further enables the separation of protonation-induced (covalent) and solvent-induced (electrostatic) screening contributions to the chemical shifts in the aqueous phase. The present core-level line shape analysis facilitates an accurate interpretation of photoelectron spectra from larger biomolecular solutes than glycine.

Alcohols at the aqueous surface: chain length and isomer effects

“…The lower binding energies of the alcohols in solution are primarily due to polarization screening of the charged C 1s core-hole final state by the water molecules. 34 A concentration-dependent study was performed on several different alcohols (1-butanol, tert-butanol, 1-pentanol, 3-pentanol, 1-hexanol and 3-hexanol), in order to study variations in their adsorption behavior at the liquid-vapor interface with changes in molecular structure and individual concentration. All acquired spectra were evaluated regarding the total area A tot of the liquid phase C 1s photoelectron (PE) signal (A tot = A(C C ) + A(C OH )) and the PE intensity ratio R between the two liquid phase C 1s peak areas (R = A(C C ) / A(C OH )).…”

RIXS investigations of liquids, solutions, and liquid/solid interfaces

“…Experiments on amino acids are a natural starting point, since these molecules are comparably small, but still very important as the building blocks of most biologically relevant macromolecules. While amino acids have been studied intensively with soft X-ray techniques in the solid state [49][50][51][52][53][54][55][56][57][58] and the gas phase [54,[59][60][61][62], only a small number of experiments have been performed in solution, most of them studying a liquid jet with XAS or photoelectron spectroscopy [9,[63][64][65]. Beside the results presented here (see also [35]), only one other study using X-ray emission spectroscopy of glycine solutions has been published to date [9].…”

Surface-Altered Protonation Studied by Photoelectron Spectroscopy and Reactive Dynamics Simulations

“…This shift agrees well with that of 2.51 eV obtained for glycine. 19 As shown in Figure 2, the photoemission spectra have been collected at different photon energies. The dependence of the relative intensities of the two peaks (protonated versus nonprotonated species) from the photon energy is evidently very strong in both spectra.…”