SMALL ANGLE X-RAY SCATTERING

Small angle scattering of x-ray is observed from almost all kinds of matters, and it is widely used in structural studies of non-crystalline materials at relatively low resolution. The term "small angle" here refers to the angular range within a few degrees, containing structural information on the order of approximately a nanometer to submicrometers. Many applications of small angle x-ray scattering technique are found in structural biology, polymer science, colloid chemistry and materials science. In structural biology this technique complements crystallographic structural analysis, which requires hard-to-get high quality crystals of macromolecules, and is one of a few structural techniques for studying proteins in solution such as those in unfolded states or simply those whose crystallization conditions have not been determined. Biological fibers such as skeletal muscle are quasi-crystalline but not as well ordered as crystals, thus giving relatively broad diffraction peaks mostly at small angles. Many biological lipids exist as vesicles or liquid crystals, physical states that are rather poorly ordered. Micellar structures and synthetic polymer materials are often studied with this technique. Small angle scattering has been used in studying crystallization processes within polymers or alloys, and is also useful in studying a certain component within an amorphous material using anomalous scattering effects.

There are a few aspects of synchrotron radiation that have made small angle scattering studies much more effective: very small beam divergence, high beam flux, and energy tunability. It is crucial to have small beam divergence in order to isolate weak scattering at very small angles from the direct beam which is orders of magnitude stronger. The high beam flux allows one to use a smaller beam size, resulting in better isolation of scattering from the direct beam. The flux level at a synchrotron source is usually several orders of magnitude higher than those from conventional x-ray sources, thus studies of weak scatterers have become much more practical. The high flux beam also made it possible to conduct time-resolved measurements of small angle scattering. It is possible to conduct anomalous small angle x-ray scattering only when beam energy tunability of synchrotron radiation is used.

At SSRL there are two stations that are routinely used for small angle x-ray scattering studies: BL 1-4 and BL 4-2. In last few years BL 1-4 has been used primarily for the studies of synthetic polymers (e.g. diblock copolymers, crystallization), and the majority of BL 4-2 users study biological problems, including protein folding (static and stopped-flow time-resolved), kinetics of allosteric transition, association/dissociation of proteins in solution, muscle fiber/tissue diffraction, solution scattering from virus particles. Low angle single crystal diffraction studies of viruses and large proteins have recently begun. In materials science, amorphous alloy materials and metal-containing clay compounds have been studied with anomalous small angle scattering technique.

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