“Building Block Picture” of the Electronic Structure of Aqueous Cysteine Derived from Resonant Inelastic Soft X-ray Scattering

Abstract

The electronic structure of the amino acid L-cysteine in an aqueous environment was studied using resonant inelastic soft X-ray scattering (RIXS) in a 2D map representation and analyzed in the framework of a "building block" approach. The element selectivity of RIXS allows a local investigation of the electronic structure of the three functional groups of cysteine, namely, the carboxyl, amino, and thiol groups, by measuring at the O K, N K, and S L2,3 edges, respectively. Variation of the pH value allows an investigation of molecules with protonated and deprotonated functional groups, which can then be compared with simple reference molecules that represent the isolated functional groups. We find that such building blocks can provide an excellent description of X-ray emission spectroscopy (XES) and RIXS spectra, but only if all nearest-neighbor atoms are included. This finding is analogous to the building block principle commonly used in X-ray absorption spectroscopy. The building blocks show a distinct spectral character (fingerprint) and allow a comprehensive interpretation of the cysteine spectra. This simple approach opens the path to investigate the electronic structure of more complex biological molecules in aqueous solutions using XES and RIXS.

Local electronic structure of the peptide bond probed by resonant inelastic soft X-ray scattering

“…17 In comparison, only a few publications using XES and RIXS to study amino acids can be found. 9,[25][26][27][28][29] These also include a library of proteinogenic amino acids at all relevant absorption edges 29 and allow for a ''building block picture''. 28 However, to our knowledge, peptides have not been studied with these techniques at all.…”

“…9,[25][26][27][28][29] These also include a library of proteinogenic amino acids at all relevant absorption edges 29 and allow for a ''building block picture''. 28 However, to our knowledge, peptides have not been studied with these techniques at all. And while XAS probes the unoccupied electronic states, XES and RIXS also give insights into the occupied valence states, including those directly responsible for the peptide bond.…”

“…[26][27][28][29] In an early XES study on amino acids, glycine adsorbed on a Cu(110) surface was probed, 25 while later studies focus on amino acids in bulk films/powders 27,29 and aqueous solutions at different pH values. 9,26,28 Gråsjö et al 9 show a strong influence of protonation/deprotonation at the amino and carboxyl groups. In a later study, 26 we could furthermore show that the N K spectrum of the molecule with protonated amino group is strongly affected by proton dynamics on the time scale of the X-ray emission process.…”

Interface Electronic Structures of the L-Cysteine on Noble Metal Surfaces Studied by Ultraviolet Photoelectron Spectroscopy

“…In aqueous solution, the zwitterionic form is stabilized by hydrogen bonding with water molecules. However, it has been suggested that different chemical states of L-cysteine adsorbed on metallic substrates using aqueous solution are strongly correlated to the pH of the solution during adsorption [38][39][40][41]. Thus, investigations of cysteine-metal bonding using aqueous solutions have revealed complex scenarios.…”

VUV Photoelectron Spectroscopy of Cysteine Aqueous Aerosols: A Microscopic View of Its Nucleophilicity at Varying pH Conditions

“…Kamada et al have investigated the ultraviolet photoelectron spectroscopy of solid Cys films, suggesting that the electronic state closest to the valence band maximum should be associated with sulfur based on the photon energy dependence of the ionization cross sections of each constituting atom of Cys. 35 The only two available experimental studies on the electronic property of Cys in the solution phase focused on the core-level structure of Cys, including the sulfur K-edge X-ray absorption near-edge structure spectra of Cys by Risberg et al, 36 and more recently the resonant inelastic soft X-ray scattering study by Meyer et al 37 who probed the oxygen K-, nitrogen K-, and sulfur L 2,3 -edges of Cys at varying pH conditions. These two X-ray based studies agreeably pointed out the significance of the local chemical environment of Cys in affecting their core-level electronic structure.…”