Improved EXAFS Analysis Program for Alternative Pair Distribution Functions

A curve fitting approach has been developed which permits correct treatment of both anharmonicity and curved-wave and multiple scattering effects in XAFS. This approach has been tested on a dinuclear Rh system and was found to yield better results than ad hoc approaches to incorporating asymmetry into atomic distribution function

QUANTITATIVE Cu X-RAY ABSORPTION EDGE STUDIES : OXIDATION STATE AND SITE STRUCTURE DETERMINATION

X-ray absorption edge studies of Cu(I) complexes with different coordination number and covalency reveal that the 8983-8984 eV feature (assigned as the 1s→4p transition) can be correlated with ligation and site geometry. These Cu(I) features have been qualitatively interpreted using a Ligand Field model, and this has been applied to analyze the polarized single crystal, pH dependent edge spectra of reduced plastocyanin. In addition, normalized difference X-ray absorption edge analysis has been used to quantitatively determine the percent of Cu(I) in several derivatives of the multicopper oxidase, laccase

POLARIZED X-RAY ABSORPTION NEAR EDGE STRUCTURE

Polarized measurements of oriented single crystals can be used to simplify the interpretation of X-ray absorption near edge structure (XANES) spectra by permitting a direct determination of the symmetry properties of a particular transition. We have utilized this technique to study the XANES spectra for several first-row transition metal complexes. Applications to the weak, 1s→3d transition, to intense near edge features, and to continuum transitions are discussed

NSLS (National Synchrotron Light Source) X-19A beamline performance for x-ray absorption measurements

Characterization of the X-19A beamline at the National Synchrotron Light Source (NSLS) is described. The beamline is designed for high resolution x-ray absorption spectroscopy over a wide energy range. All of the beamline optical components are compatible with ultrahigh vacuum (UHV) operation. This permits measurements to be made in a window-less mode, thereby facilitating lower energy (<4 KeV) studies. To upgrade the beamline performance, several possible improvements in instrumentation and practice are discussed to increase photon statistics with an optimum energy resolution, while decreasing the harmonic contamination and noise level. A special effort has been made to improve the stability and UHV compatibility of the monochromator system. Initial x-ray absorption results demonstrate the capabilities of this beamline for x-ray absorption studies of low Z elements (e.g. S) in highly dilute systems. The future use of this beamline for carrying out various x-ray absorption experiments is presented. 10 refs., 4 figs

The Photoactive Excited State of the B12-Based Photoreceptor CarH

7 pags., 5 figs.We have used transient absorption spectroscopy in the UV-visible and X-ray regions to characterize the excited state of CarH, a protein photoreceptor that uses a form of B12, adenosylcobalamin (AdoCbl), to sense light. With visible excitation, a nanosecond-lifetime photoactive excited state is formed with unit quantum yield. The time-resolved X-ray absorption near edge structure difference spectrum of this state demonstrates that the excited state of AdoCbl in CarH undergoes only modest structural expansion around the central cobalt, a behavior similar to that observed for methylcobalamin rather than for AdoCbl free in solution. We propose a new mechanism for CarH photoreactivity involving formation of a triplet excited state. This allows the sensor to operate with high quantum efficiency and without formation of potentially dangerous side products. By stabilizing the excited electronic state, CarH controls reactivity of AdoCbl and enables slow reactions that yield nonreactive products and bypass bond homolysis and reactive radical species formation.This work was supported by grants from the National Science Foundation NSF-CHE 1464584 and NSFCHE 1836435 to R.J.S., NSF-CHE 1565795 to K.J.K., NSFCHE 1608553 and NSF-CHE-1904759 to E.N.G.M., and NSF-CHE 1945174 to M.K.; from the Agencia Estatal de Investigación (AEI)-Spain and the European Regional Development Fund (FEDER) grants PGC2018-094635-BC21 (to M.E.-A.) and PGC2018-094635-B-C22 (to S.P.); and from the Fundacion Seneca (Murcia)-Spain grant 20992/PI/ ́ 18 (to M.E.-A.). Portions of this work were carried out in the Laboratory for Ultrafast Multidimensional Optical Spectroscopy (LUMOS) supported by NSF-CHE 1428479. Use of the Linac Coherent Light Source (LCLS), SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under contract no. DE-AC02-76SF00515