Extended X-ray absorption fine structure

About: Extended X-ray absorption fine structure is a research topic. Over the lifetime, 10452 publications have been published within this topic receiving 276744 citations.

Structure and kinetics of the photoproduct of carboxymyoglobin at low temperatures: an X-ray absorption study.

Abstract: Photolysis and recombination of carboxymyoglobin at low temperatures have been studied by a variety of methods. This paper combines optical and structural studies of carboxymyoglobin photolysis and recombination in the temperature range 4-120 K. The absorbance changes indicate ablation of the characteristic optical transitions of carboxymyoglobin and formation of a photoproduct (Mb*CO) differing from deoxymyoglobin. When the X-ray absorption changes in the 7150-7200-eV region of the X-ray absorption spectrum are used as an indicator of structural change, the photoproduct at 4 K as measured with respect to the unphotolyzed sample is 60% of that observed for the chemically produced deoxy form. Saturation of the change is obtained with repetitive flashes totaling several thousand joules of energy from a xenon flash lamp by using a thin sample (1 mm) at 4 mM concentration as measured by both optical transmission and X-ray absorption criteria. The kinetics of the reaction show the change to occur at 10 K within the resolving time currently available (2 s) in the X-ray absorption measurements. The amplitude of the light-induced change decreases to half its maximal value at 40 K and to zero at 90 K. Steady illumination suggests at least two recombination processes. Analysis of the extended X-ray absorption fine structure (EXAFS) data on Mb*CO indicates small distance changes in the first shell of Fe-N and Fe-C that can be attributed to lengthening of the pyrrole nitrogen bonds and proximal histidine motion, together with a small displacement of the CO molecule on photolysis--a form here designated Mb*CO. This structure of the germinate state, Mb*CO, may elucidate the nature of elementary steps in chemical reactions and in tunneling processes.

Structural investigations of TiO2SiO2 glassy and glass-ceramic materials prepared by the sol-gel method

Abstract: In the present paper we will show by both extended X-ray absorption fine structure (EXAFS) and near-edge (XANES) spectroscopies that the coordination of Ti 4+ ions is only tetrahedral in TiO 2 SiO 2 very homogeneous glasses obtained by the sol-gel method. The coordination of Ti 4+ is also tetrahedral in the essicated gels. An “intermediate” ordered phase, not yet well defined, has been observed at the beginning of the crystallization process. Moreover, the thermal treatment causes a progressive ordering of the T d tetrahedral units going from the essicated gels to the more and more dense glasses. The presence of six-fold Ti-sites has been clearly observed by both EXAFS and near-edge spectroscopies only in a partially crystallized glass containing the highest TiO 2 content (19 wt%), at 1200°C. On the contrary, in all the glasses having the lowest TiO 2 content (4.5 wt%) a four-fold Ti coordination has been observed until 1200°C. The crystalline phase, which appears at high temperatures, is anatase having a crystalline size in the range 50–100 A for thermal treatments in the range 1000–1200°C for the sample richer in TiO 2 . An appearance of crystallinity at higher temperatures for lower TiO 2 concentrations has been observed by X-ray diffraction (XRD) analysis, in agreement with other authors.

Atomic environment of high-field strength Nd and Al cations as dopants and major components in silicate glasses: a Nd LIII-edge and Al K-edge X-ray absorption spectroscopic study

Abstract: The local atomic structure around Nd and Al ions present as dopants in SiO 2 glasses and as major constituents in other silicate glasses have been studied using Nd L III -edge EXAFS and Al K-edge XANES and EXAFS spectroscopy. The Nd EXAFS results provide the first direct evidence of clustering of Nd ions through the formation of Nd–O–Nd bonds in a SiO 2 glass doped with 2400 ppm Nd 2 O 3 . The Nd–O and Nd–Nd distances in this glass are found to be 2.35 and 3.80 A, respectively. Codoping of this glass with 7300 ppm Al 2 O 3 is shown to result in a homogeneous dispersion of the Nd atoms. The Nd–O coordination polyhedra in this glass relax to a larger volume with respect to that in its Al-free counterpart with a significant increase in the Nd–O distance to 2.62 A. Moreover, Nd–O–Si/Al bonds are formed at the expense of Nd–O–Nd bonds on addition of Al, indicating incorporation of Nd–O polyhedra into the silicate network; the corresponding Nd–Si distance is found to be 3.51 A. A similar atomic environment for Nd is also observed in Nd 2 Al 2 Si 3 O 12 glass where Nd is present as a major component rather than as a dopant. However, no such Nd–O–Si bonds are observed in a K–Nd-silicate glass indicating the fact that incorporation of Nd–O polyhedra into the silicate network without Al creates significant radial disorder in the next-nearest neighbor Si shell around Nd. The Al XANES and EXAFS spectra indicate that Al ions in all these glasses exist in four-fold coordination with Al–O distances ranging between 1.76 and 1.77 A. A second shell of Si is observed in all compositions at distances ranging between 2.92 and 3.00 A. Al–O–Nd bonds are also observed in the Nd 2 Al 2 Si 3 O 12 glass. It is suggested that Nd, when incorporated into the Si, Al–O network, plays the role of charge-balancing cation for [AlO 4 ] −1 units in aluminosilicate glasses.

Determination of the iron-sulfur distances in rubredoxin by x-ray absorption spectroscopy.

Abstract: The high intensity x-ray flux from the synchrotron radiation at the Stanford Synchroton Radiation Project has been used to study the extended x-ray absorption fine structure (EXAFS) of the iron-sulfur protein Peptococcus aerogenes rubredoxin. Absorption measurements were made from 7080 eV, which is below the K-edge of iron, to about 650 eV above the edge and structure was obtained over the entire region. By means of a model iron-sulfur compound for evaluating the phase shifts, the variation of the absorption above the edge of lyophilized, oxidized rubredoxin was converted to iron-sulfur distances. The data were fitted with a least squares program to a model in which three distances R3 were kept equal and the fourth R1 was allowed to differ. The mean square error was constant over a region of this parameter space, becoming twice as large at R3 = 2.217, R1 = 2.389 and R3 = 2.268, R1 = 2.108 A. These values, which are the extreme differences allowed by the present data, are definitely closer to being equal than those found by the determination of the x-ray diffraction crystal structure of the similar protein from Clostridium pasteurianum. However, the average distance from our experiment is in excellent agreement with the average distance from the crystal structure determination. Preliminary EXAFS measurements were also made on the oxidized rubredoxin in solution at pH 7.0. The spectra were unchanged, indicating that the average iron-sulfur distance change is less than 0.02 A. Upon reduction the average iron-sulfur bond length increased by about 0.05 A. Since the EXAFS measurements can give accurate determinations of distances in proteins both in crystals and solution, the technique should be widely applicable.