A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

High-resolution Fe K-edge XANES spectra of a series of crystalline Fe 2+ - and Fe 3+ -bearing model compounds were measured in an effort to correlate characteristics of the pre-edge feature with oxidation state and local coordination environment of Fe atoms. The model compounds comprise 30 natural minerals and synthetic compounds, with Fe coordination environments ranging from 4 to 12 O atoms for Fe 2+ , including 5-coordinated trigonal bipyramidal Fe 2+ , and from 4 to 6 O atoms for Fe 3+ . Most pre-edge spectra show two components (due to crystal-field splitting) that are located just above the Fermi level. The most useful characteristics of the Fe-K pre-edge for determining Fe oxidation state and coordination number are the position of its centroid and its integrated intensity. The separation between the average pre-edge centroid positions for Fe 2+ and Fe 3+ is 1.4 ± 0.1 eV. Thus, the position of the pre-edge feature can be used as a measure of the average Fe-redox state, with the average pre-edge position for mixed Fe 2+ -Fe 3+ compounds occurring between positions for Fe 2+ and Fe 3+ . The lowest pre-edge normalized heights and integrated intensities are observed for the most centrosymmetric sites of Fe, in agreement with previous studies (see Waychunas et al. 1983). Examination of the pre-edge features of mechanical mixtures of phases containing different proportions of Fe 2+ and Fe 3+ suggests that the pre-edge position and intensity for these mixtures can vary quite non-linearly with the average redox state of Fe. However, distinctly different trends of pre-edge position vs. pre-edge intensity can be observed, depending on the coordination environment of Fe 2+ and Fe 3+ , with an accuracy in redox determination of ±10 mol% provided that the site geometry for each redox state is known. These methods have been used to estimate the Fe 3+ /Fe 2+ ratio in 12 minerals (magnetite, vesuvianite, franklinite, rhodonite, etc.) containing variable/unknown amounts of Fe 2+ /Fe 3+ .

http://www.minsocam.org/msa/ammin/toc/Abstracts/2001_Abstracts/MJ01_Abstracts/Wilke_p714_01.pdf

Oxidation state and coordination of Fe in minerals: An Fe K-XANES spectroscopic study

G-protein-coupled receptors (GPCRs) signal primarily through G proteins or arrestins. Arrestin binding to GPCRs blocks G protein interaction and redirects signalling to numerous G-protein-independent pathways. Here we report the crystal structure of a constitutively active form of human rhodopsin bound to a pre-activated form of the mouse visual arrestin, determined by serial femtosecond X-ray laser crystallography. Together with extensive biochemical and mutagenesis data, the structure reveals an overall architecture of the rhodopsin-arrestin assembly in which rhodopsin uses distinct structural elements, including transmembrane helix 7 and helix 8, to recruit arrestin. Correspondingly, arrestin adopts the pre-activated conformation, with a similar to 20 degrees rotation between the amino and carboxy domains, which opens up a cleft in arrestin to accommodate a short helix formed by the second intracellular loop of rhodopsin. This structure provides a basis for understanding GPCR-mediated arrestin-biased signalling and demonstrates the power of X-ray lasers for advancing the frontiers of structural biology.

/pdf/crystal-structure-of-rhodopsin-bound-to-arrestin-by-15lld79q4m.pdf

Crystal structure of rhodopsin bound to arrestin by femtosecond X-ray laser

X-ray sources such as free-electron lasers offer the potential to study matter at unprecedented spatial and temporal resolution. But that potential is limited by the poor quality of conventional X-ray optical elements. An in situ technique that corrects for wavefront aberrations and allows X-rays to be focused to a spot just 7 nm wide could provide a solution.

/pdf/breaking-the-10-nm-barrier-in-hard-x-ray-focusing-4jjmjnewxj.pdf

Breaking the 10 nm barrier in hard-X-ray focusing

Serial femtosecond crystallography using ultrashort pulses from x-ray free electron lasers (XFELs) enables studies of the light-triggered dynamics of biomolecules. We used microcrystals of photoactive yellow protein (a bacterial blue light photoreceptor) as a model system and obtained high-resolution, time-resolved difference electron density maps of excellent quality with strong features; these allowed the determination of structures of reaction intermediates to a resolution of 1.6 angstroms. Our results open the way to the study of reversible and nonreversible biological reactions on time scales as short as femtoseconds under conditions that maximize the extent of reaction initiation throughout the crystal.

/pdf/time-resolved-serial-crystallography-captures-high-2dq4kt7kb7.pdf

Time-resolved serial crystallography captures high-resolution intermediates of photoactive yellow protein

We present recent results on the inspection of a first diffraction-limited hard X-ray Kirkpatrick-Baez (KB) mirror pair for the Coherent X-ray Imaging (CXI) instrument at the Linac Coherent Light Source (LCLS). The full KB system – mirrors and holders - was under inspection by use of high resolution slope measuring deflectometry. The tests confirmed that KB mirrors of 350mm aperture length characterized by an outstanding residual figure error of <1 nm rms has been realized. This corresponds to the residual figure slope error of about 0.05µrad rms, unprecedented on such long elliptical mirrors. Additional measurements show the clamping of the mirrors to be a critical step for the final – shape preserving installation of such outstanding optics.

Ultra-precise characterization of LCLS hard X-ray focusing mirrors by high resolution slope measuring deflectometry.

The ESRF beamline ID26 is dedicated to XAFS studies on Ultra-diluted Samples (XAUS beamline). The spectral range 2.3-30 keV is covered. The aim of the beamline is to extract structural and electronic information on dilute samples for which the concentration of the absorbing element ranges from a few ppm up to 10 000 ppm. A wide range of applications is covered in biology, catalysis, chemistry, environmental sciences, solid state physics, etc. The beamline accepted the first users in November 1997. We give herein a brief description of the beamline and report on recent results.

The ESRF beamline ID26: X-ray absorption on ultra dilute sample.

The angular acceptance of piezoelectric (Pzt) bimorph mirrors is limited by the maximum length of commercially available Pzt ceramic plates. To overcome this limit and manufacture longer devices, several (2n + 1) 150 mm-long bimorph Pzt stacks were assembled side-to-side. Two prototype mirrors, 450 (n = 1) and 750 (n = 2) mm long, were designed, assembled, polished and optically characterized. They are fully UHV compatible and are now installed in the monochromatic section of the ESRF beamlines ID26 and ID32. Both mirrors cover the full range of required bending radii (1 km concave–3.5 km convex). Junctions between segments do not spoil the optical surface quality. The surface slope error r.m.s. can be kept well below 1 arcsec over the full bending range. Adaptive compensation for low-frequency figure errors was shown to be easy and reliable. After compensation, residual shape errors are of the order of 40 nm r.m.s. over 700 mm.

Multi-segmented piezoelectric mirrors as active/adaptive optics components

This paper reports on the performance of the instrumentation developed for the ESRF beamline ID12A, which is dedicated to spectroscopic applications requiring full control of the polarization at energies ≥2.0 keV. Emphasis is placed on the characterization of various optical components of the beamline and on problems associated with either the control or the conversion of the polarization state. A few examples have been selected to illustrate what sort of new spectroscopic information has already been obtained at the beamline. These include the comparison of X-ray magnetic dichroism spectra recorded with linear or circular polarization and the very first detection of X-ray natural circular dichroism in single crystals known to exhibit a very large non-linear susceptibility at optical wavelengths.

Instrumentation Developments for X-ray Linear and Circular Dichroism at the ESRF Beamline ID12A.

We present the design, fabrication and characterization of a novel adaptive X-ray optic by bringing together bimorph
adaptive technology and the novel Elastic Emission Machining "super-polishing" technique. This super-polished
adaptive mirror provides variable focal distance and local figure control in the sub-nm range. The optic has the potential
to generate distortion-free beams, and enable wavefront control.

Riccardo Signorato

Papers

Ultra-precise characterization of LCLS hard X-ray focusing mirrors by high resolution slope measuring deflectometry.

The ESRF beamline ID26: X-ray absorption on ultra dilute sample.

Multi-segmented piezoelectric mirrors as active/adaptive optics components

Instrumentation Developments for X-ray Linear and Circular Dichroism at the ESRF Beamline ID12A.

A Novel Adaptive Bimorph Focusing Mirror and Wavefront Corrector with Sub-Nanometre Dynamical Figure Control