Possibilities of X-Ray Absorption Spectroscopy in the Total External Reflection Geometry for Studying Protein Films on L
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ACE AND THIN FILMS
Possibilities of X-Ray Absorption Spectroscopy in the Total External Reflection Geometry for Studying Protein Films on Liquids N. N. Novikovaa,*, S. N. Yakunina, M. V. Koval’chuka, E. A. Yur’evab, N. D. Stepinac, A. V. Rogacheva, M. A. Kremennayad, G. E. Yalovegad, O. V. Kosmachevskayae, and A. F. Topunove a National
Research Centre “Kurchatov Institute,” Moscow, 123182 Russia Veltischev Research and Clinical Institute for Pediatrics, Moscow, 117997 Russia c Shubnikov Institute of Crystallography, Federal Scientific Research Centre “Crystallography and Photonics,” Russian Academy of Sciences, Moscow, 119333 Russia d Southern Federal University, Rostov-on-Don, 344006 Russia e Bach Institute of Biochemistry, Federal Research Centre “Fundamentals of Biotechnology,” Russian Academy of Sciences, Moscow, 119071 Russia *e-mail: [email protected] b
Received July 15, 2019; revised July 15, 2019; accepted July 25, 2019
Abstract—XANES spectra of protein films (hemoglobin and alkaline phosphatase) formed on the surface of a liquid subphase in a Langmuir trough have been obtained experimentally. The potential of X-ray absorption spectroscopy in the total external reflection geometry is demonstrated by the example of a protein film based on hemoglobin subjected to the action of urea. It is established that the presence of 0.09 M urea solution in the subphase enhances significantly the ability of hemoglobin to bind zinc and iron ions. Information about the local atomic environment of zinc ions bound with a hemoglobin molecule is obtained. It is shown that each zinc ion is coordinated by four ligands, two of which are amino acid residues of cysteine and histidine. The general concepts of the molecular mechanisms of accumulation of metal ions under the action of disturbing factors are formulated. DOI: 10.1134/S1063774519060130
INTRODUCTION X-ray absorption spectroscopy is one of the most informative methods for studying the local structure of materials. X-ray absorption spectra are generally divided into two parts: X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). An analysis of the specific features of XANES spectra allows one to determine the coordinates of neighboring atoms, interatomic distances (chemical bond lengths), and the symmetry of local environment of absorbing atom (chemical bond angles). Using XANES measurements, one can also obtaine information about the energy of unoccupied molecular orbitals in molecules or electron bands above the Fermi level in condensed matter, electrondensity distribution in atoms, and oxidation state of the absorbing atom [1, 2]. XANES spectroscopy was repeatedly applied to study the local environment of metals at the active centers of metalloproteins [3, 4]. Measurements of XANES spectra in the fluorescent mode under the conditions of total external reflection (TER) provide new possibilities for analyzing objects with an extremely low concentration of absorbing atoms, such
as metalloproteins. In this case, the graz
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