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Nonresonant N K, O K, C K, and S L2,3 X-ray emission spectra of the 20 most common proteinogenic amino acids in their solid zwitterionic form are reported. They represent a comprehensive database that can serve as a reliable basis for the X-ray absorption spectroscopy (XES) studies of peptides and proteins. At the most important N and O K edges, clear similarities and differences between the spectra of certain amino acids are observed and associated with the specific chemical structure of these molecules and their functional groups. Analysis of these spectra allows the generation of spectral fingerprints of the protonated amino group, the deprotonated carboxylic group, and, using a building block approach, the specific nitrogen- and oxygen-containing functional groups in the side chains of the amino acids. Some of these fingerprints are compared to the spectra of reference compounds with the respective functional groups; they exhibit reasonable similarity, underlining the validity of the spectral fingerprint approach. The C K and S L2,3 XES spectra are found to be specific for each amino acid, in accordance with the different local environments of the involved C and S atoms, respectively. [ABSTRACT FROM AUTHOR]

Understanding the molecular structure of the hydration shells and their impact on the hydrogen bond (HB) network of water in aqueous salt solutions is a fundamentally important and technically relevant question. In the present work, such hydration effects were studied for a series of representative salt solutions (NaCl, KCl, CaCl2, MgCl2, and KBr) by soft X-ray emission spectroscopy (XES) and resonant inelastic soft X-ray scattering (RIXS). The oxygen K-edge XES spectra could be described with three components, attributed to initial state HB configurations in pure water, water molecules that have undergone an ultrafast dissociation initiated by the X-ray excitation, and water molecules in contact with salt ions. The behavior of the individual components, as well as the spectral shape of the latter component, has been analyzed in detail. In view of the role of ions in such effects as protein denaturation (i.e., the Hofmeister series), we discuss the ion-specific nature of the hydration shells and find that the results point to a predominant role of anions as compared to cations. Furthermore, we observe a concentration-dependent suppression of ultrafast dissociation in all salt solutions, associated with a significant distortion of intact HB configurations of water molecules facilitating such a dissociation. [ABSTRACT FROM AUTHOR]

The electronic structure of gas-phase methanol molecules (H3COH, H3COD, and D3COD) at atmospheric pressure was investigated using resonant inelastic soft X-ray scattering (RIXS) at the O K and C K edges. We observe strong changes of the relative emission intensities of all valence orbitals as a function of excitation energy, which can be related to the symmetries of the involved orbitals causing an angularly anisotropic RIXS intensity. Furthermore, all observed emission lines are subject to strong spectator shifts of up to -0.9 eV at the O K edge and up to -0.3 eV at the C K edge. At the lowest O K resonance, we find clear evidence for dissociation of the methanol molecule on the time scale of the RIXS process, which is illustrated by comparing X-ray emission spectra of regular and deuterated methanol. [ABSTRACT FROM AUTHOR]

Theelectronic structure of the amino acid l-cysteinein an aqueous environment was studied using resonant inelastic softX-ray scattering (RIXS) in a 2D map representation and analyzed inthe framework of a “building block” approach. The elementselectivity of RIXS allows a local investigation of the electronicstructure of the three functional groups of cysteine, namely, thecarboxyl, amino, and thiol groups, by measuring at the O K, N K, andS L2,3edges, respectively. Variation of the pH value allowsan investigation of molecules with protonated and deprotonated functionalgroups, which can then be compared with simple reference moleculesthat represent the isolated functional groups. We find that such buildingblocks can provide an excellent description of X-ray emission spectroscopy(XES) and RIXS spectra, but only if all nearest-neighbor atoms areincluded. This finding is analogous to the building block principlecommonly used in X-ray absorption spectroscopy. The building blocksshow a distinct spectral character (fingerprint) and allow a comprehensiveinterpretation of the cysteine spectra. This simple approach opensthe path to investigate the electronic structure of more complex biologicalmolecules in aqueous solutions using XES and RIXS. [ABSTRACT FROM AUTHOR]

Resonant inelastic soft X-ray scattering (RIXS) has beenused tostudy the electronic structure of glycine and lysine in aqueous solution.Upon variation of the pH value of the solution from acidic to basic,major changes of the nitrogen K edge RIXS data are observed for bothamino acids, which are associated with the protonation and deprotonationof the amino groups. The experimental results are compared with simulationsbased on density functional theory, yielding a detailed understandingof the spectral changes, as well as insights into the ultrafast protondynamics in the intermediate core-excited/ionized state of the RIXSprocess. [ABSTRACT FROM AUTHOR]