The 18ID undulator beamline of the Biophysics Collaborative Access Team at the Advanced Photon Source, Argonne, IL, USA, is a highperformance instrument designed for, and dedicated to, the study of partially ordered and disordered biological materials using the techniques of small-angle X-ray scattering, ®ber diffraction, and X-ray absorption spectroscopy. The beamline and associated instrumentation are described in detail and examples of the representative experimental results are presented.

Crystallization of human membrane proteins in lipidic cubic phase often results in very small but highly ordered crystals. Advent of the sub-10 mm minibeam at the APS GM/CA CAT has enabled the collection of high quality diffraction data from such microcrystals. Herein we describe the challenges and solutions related to growing, manipulating and collecting data from optically invisible microcrystals embedded in an opaque frozen in meso material. Of critical importance is the use of the intense and small synchrotron beam to raster through and locate the crystal sample in an efficient and reliable manner. The resulting diffraction patterns have a significant reduction in background, with strong intensity and improvement in diffraction resolution compared with larger beam sizes. Three high-resolution structures of human G protein-coupled receptors serve as evidence of the utility of these techniques that will likely be useful for future structural determination efforts. We anticipate that further innovations of the technologies applied to microcrystallography will enable the solving of structures of ever more challenging targets.

A generalized dynamical theory has been developed that extends previous models of x-ray diffraction from crystals and multilayers with vertical strains to the cases of grazing incidence and/or exit below the critical angle for total specular reflection. This provides a common description for extremely asymmetric diffraction, surface ͑''grazing-incidence''͒, and grazing Bragg-Laue diffraction, thus providing opportunities for the applications of grazing geometries to the studies of thin multilayers. The solution, obtained in the form of recursion formulas for ͑2ϫ2͒ scattering matrices for each individual layer, eliminates possible divergences of the ͑4ϫ4͒ transfer-matrix algorithm developed previously. For nongrazing x-ray diffraction in the Bragg geometry and for grazing-incidence x-ray specular reflection out of the Bragg diffraction conditions, the matrices are reduced to scalars and the recursion formulas become equivalent to the earlier recursion formulas by Bartels et al. ͓Acta Cryst. A 42, 539 ͑1986͔͒ and Parratt ͓Phys. Rev. 95, 359 ͑1954͔͒, respectively. The theory has been confirmed by an extremely asymmetric x-ray-diffraction experiment with a strained AlAs/GaAs superlattice carried out at HASYLAB. A solution to the difficulties due to dispersion encountered in extremely asymmetric diffraction measurements has been demonstrated. Finally, the validity of Ewald's expansion for thin layers and the relation of the matrix method to the Darwin theory, as well as the structure of x-ray standing waves in multilayers are discussed. ͓S0163-1829͑98͒05408-3͔

The high-brilliance X-ray beams from undulator sources at third-generation synchrotron facilities are excellent tools for solving crystal structures of important and challenging biological macromolecules and complexes. However, many of the most important structural targets yield crystals that are too small or too inhomogeneous for a 'standard' beam from an undulator source, $ 25-50 mm (FWHM) in the vertical and 50-100 mm in the horizontal direction. Although many synchrotron facilities have microfocus beamlines for other applications, this capability for macromolecular crystallography was pioneered at ID-13 of the ESRF. The National Institute of General Medical Sciences and National Cancer Institute Collaborative Access Team (GM/CA-CAT) dual canted undulator beamlines at the APS deliver high-intensity focused beams with a minimum focal size of 20 mm  65 mm at the sample position. To meet growing user demand for beams to study samples of 10 mm or less, a 'mini-beam' apparatus was developed that conditions the focused beam to either 5 mm or 10 mm (FWHM) diameter with high intensity. The mini-beam has a symmetric Gaussian shape in both the horizontal and vertical directions, and reduces the vertical divergence of the focused beam by 25%. Significant reduction in background was achieved by implementation of both forward-and back-scatter guards. A unique triple-collimator apparatus, which has been in routine use on both undulator beamlines since February 2008, allows users to rapidly interchange the focused beam and conditioned mini-beams of two sizes with a single mouse click. The device and the beam are stable over many hours of routine operation. The rapid-exchange capability has greatly facilitated sample screening and resulted in several structures that could not have been obtained with the larger focused beam.

Radiation damage is a major limitation in crystallography of biological macromolecules, even for cryocooled samples, and is particularly acute in microdiffraction. For the X-ray energies most commonly used for protein crystallography at synchrotron sources, photoelectrons are the predominant source of radiation damage. If the beam size is small relative to the photoelectron path length, then the photoelectron may escape the beam footprint, resulting in less damage in the illuminated volume. Thus, it may be possible to exploit this phenomenon to reduce radiation-induced damage during data measurement for techniques such as diffraction, spectroscopy, and imaging that use X-rays to probe both crystalline and noncrystalline biological samples. In a systematic and direct experimental demonstration of reduced radiation damage in protein crystals with small beams, damage was measured as a function of micron-sized X-ray beams of decreasing dimensions. The damage rate normalized for dose was reduced by a factor of three from the largest (15.6 μm) to the smallest (0.84 μm) X-ray beam used. Radiation-induced damage to protein crystals was also mapped parallel and perpendicular to the polarization direction of an incident 1-μm X-ray beam. Damage was greatest at the beam center and decreased monotonically to zero at a distance of about 4 μm, establishing the range of photoelectrons. The observed damage is less anisotropic than photoelectron emission probability, consistent with photoelectron trajectory simulations. These experimental results provide the basis for data collection protocols to mitigate with micron-sized X-ray beams the effects of radiation damage.microcrystallography | synchrotron radiation T he brilliance of synchrotron radiation from undulator devices on third-generation sources has been an enormous boon to crystallography of biological macromolecules. The high flux density and low divergence of undulator beams led to a rapid decrease in the minimum crystal size and minimum beam size that can yield usable diffraction data (1-4). However, the resulting decrease in diffracting volume necessitates an increase in X-ray exposure per unit sample volume, increasing radiation damage and severely compromising the substantial benefits of brilliant undulator sources. Thus, there is considerable interest in understanding the mechanism and spatial extent of X-rayinduced damage to crystals of biological macromolecules.Diffraction experiments are typically performed at cryotemperatures (approximately 100 K) to prevent the diffusion of free radicals, which are a major source of damage in crystals exposed to X-rays at higher temperatures (5), but cryocooling does not eliminate X-ray damage. Many experimental approaches to circumventing the effects of radiation damage have been investigated (6-10) but have not yet yielded a breakthrough result. Zero-dose diffraction intensities have been extrapolated from measured values by mathematical modeling (7-9). The effects of radiation damage have also been exploited for crystallograp...