Röntgenspektroskopie an Flüssigkeiten und Lösungen

• Applicants: Professor Dr. Friedrich Reinert; Professor Dr. Michael Zharnikov
• Subject Area: Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics ; Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: from 2011 to 2015
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 197968429

• Final Report Year: 2015

Final Report Abstract

The major goal of the project was the study of selected liquids and solutions by X-ray absorption and emission spectroscopy (XAS and XES, respectively). The major objects of interest were aqueous salt solutions as well as solutions of simple biological molecules, considered as test examples for more complex biological systems. The experiments were performed with the help of a liquid cell and an X-ray spectrometer which have been specifically designed for this purpose and are unique worldwide. This setup allows the acquisition of resonant inelastic soft x-ray scattering (RIXS) maps, which represent a combination of full-range XAS and XES data for a particular absorption edge. RIXS maps and selected XE spectra of aqueous solutions of several representative monovalent and divalent salts, viz. KCl, NaCl, KBr, CaCl2, and MgCl2, were studied. For all these systems, a reorganization of the hydrogen bond (HB) network could be clearly observed using in-situ O K-edge RIXS maps and XE spectra at suitably selected excitation energies. Most important, the latter spectra can be decomposed into three components associated with the ground state of unperturbed pure water, ultrafast dissociation, and the hydration structure of a particular salt solution. The intensity of the latter component increases with increasing salt concentration, predominantly at the expense of the dissociation component. This suggests that the coordination structure(s) around the salt ions is/are distinctly different and less favorable for dissociation than the HB configurations of pure water. The decomposition of the XE spectra of the salt solutions into specific components with clear physical meanings is an important step to their correct analysis. It allows a clear separation of dynamic effects, paving the way for deriving details of the molecular organization in complex solutions. Furthermore, we studied a variety of most important building blocks of natural peptides and proteins, viz. proteinogenic amino acids (AAs) both in solid state and in aqueous environment. We measured and compiled a comprehensive library of XE spectra for the 20 most common proteinogenic AAs at all relevant absorption edges. The analysis of these spectra demonstrated impressively the local character of XE spectroscopy, paving the way for detangling spectral contributions of individual functional group in RIXS maps and XE spectra. Such a possibility was demonstrated by the example of cysteine, measured in the solid state. For this particular AA, we were able to separate spectral contributions from three chemically non-equivalent carbon atoms as well as to derive site-specific information about the location of the all-molecule HOMO and LUMO states. Further, taking cysteine as a representative example, we tested the application of the so called "building block approach" to the analysis of RIXS maps and XE spectra of AAs in aqueous environment. Variation of the pH value allowed an investigation of cysteine with protonated and deprotonated functional groups, which was compared with several simple reference molecules that represented the isolated functional groups. We found that such building blocks can provide an excellent description of XE spectra and RIXS maps, but only if all nearest-neighbor atoms are included. This conclusion was supported by further experiments, performed on several other AAs and reference systems. The experimental studies were complemented by density functional theory (DFT) calculations for individual molecules, using the Sto-Be-deMon code. Finally, XA spectra of most important peptides were measured - as a reference library. The obtained results are of significant scientific value, serving also as a reasonable basis for further, dedicated studies of liquids and solutions by soft X-ray spectroscopies. Decomposition of XE spectra and the "building block approach" in XES can serve as powerful tools for the analysis of the experimental data.

Publications

• Nuclear dynamics and spectator effects in resonant inelastic soft x-ray scattering of gas-phase water molecules, J. Chem. Phys. 136, 144311 (2012); selected also for Virtual J. Biological Phys. Research 23 (2012)
  L. Weinhardt, A. Benkert, F. Meyer, M. Blum, R.G. Wilks, W. Yang, M. Bär, F. Reinert, and C. Heske
  
• Non-equivalent carbon atoms in the resonant inelastic soft X-ray scattering map of cysteine, 138, 034306 (2013)
  F. Meyer, L. Weinhardt, M. Blum, M. Bär, R. G. Wilks, W. Yang, C. Heske, and F. Reinert
  
• Ion-solvation-induced molecular reorganization in liquid water probed by resonant inelastic soft x-ray scattering, J. Phys. Chem. Lett. 5, 4143−4148 (2014)
  Y. L. Jeyachandran, F. Meyer, S. Nagarajan, A. Benkert, M. Bär, M. Blum, W. Yang, F. Reinert, C. Heske, L. Weinhardt, and M. Zharnikov
  
• Setup for in situ investigation of gases and gas/solid interfaces by soft X- ray emission and absorption spectroscopy. Rev. Sci. Instr. 85, 015119 (2014)
  A. Benkert, M. Blum, F. Meyer, R. G. Wilks, W. Yang, M. Bär, F. Reinert, C. Heske, and L. Weinhardt
  
• ”Building-block picture” of the electronic structure of aqueous cysteine derived from resonant inelastic soft x-ray scattering, J. Phys. Chem. B 118, 13142−13150 (2014)
  F. Meyer, M. Blum, A. Benkert, D. Hauschild, S. Nagarajan, R. G. Wilks, J. Andersson, W. Yang, M. Zharnikov, M. Bär, C. Heske, F. Reinert, and L. Weinhardt