Small-angle scattering is a very powerful method to investigate the structure of disorderd systems from mesoscopic viewpoint. In the present chapter, it is introduced how the inhomogeneity of molecular distribution in liquids or supercritical fluids (density fluctuation) and mixing state in solutions (mixing state) are obtained from the small-angle scattering technique.
Recent structure analyses of collagen-model peptides revealed several important structural features of collagen. (1) Contrary to the Rich and Crick 10/3-helix, the 7/2-helical conformation is preferable as a structure of collagen. (2) The triple-helical structure is stabilized by a repetitive X (C=O) --- HN (Gly) hydrogen bonds. If X is not Pro or Hyp, the water-bridge, Gly (C=O) --- H2O --- HN (X), also stabilize the triple-helical structure. (3) The stabilization of collagen by Hyp can be attributed to the inductive effect of the hydroxyl moiety attached to the Cγ of Pro.
Diol dehydratase is an adenosylcobalamin-dependant enzyme that catalyzes conversion from 1, 2-diol compounds to corresponding aldehydes. The crystal structure of the enzyme shows that B12 compounds are incorporated with the DBI-on mode. The enzyme has a potassium ion at the active site, where the substrate coordinates to it. The structure of the enzyme complexed with adeninylpentylcobalamin reveals the activation mechanism of adenosylcobalamin.
The three-dimensional structure of catechol 2, 3-dioxygenases (metapyrocatechase, MPC), which catalyzes the proximal extradiol cleavage reaction of catechol and 3- or 4-substituted catechol, has been determined at 2.8Å resolution. The enzyme is a homotetramer with non-crystallographic 222 symmetry and each subunit is folded into two similar domains. The active site structure reveals a distorted tetrahedral Fe (II) site coordinated by three endogenous ligands (His 153, His214, and G1u265) and an acetone molecule.
A new single crystal focusing monochromator for protein crystallography has been developed. In order to achieve simultaneous tuning of the beam demagnification rate and the radius of curvature for focusing over a wide wavelength range, an inclined monochromator was cylindrically bent. The monochromator has an ability to tune such two parameters simultaneously with an azimuthal rotation of the crystal. A new monochromator incorporating this idea has been developed for beamline BL6B at the Photon Factory. The monochromator was designed for use in a wavelength range 0.87 Å to 1.90 Å. The evaluation of the beam focusing was performed at wavelengths of 1.04 Å, 1.38 Å and 1.74 Å.
For local structure analyses of low dimensional structures in semiconductors, such as defects, interfaces and surfaces, a new site-selective x-ray absorption fine structure (XAFS) measurement, ′capacitance XAFS′method, is proposed. In this method, the x-ray photon energy dependence of a capacitance involved in various diode structures is measured. The capacitance is changed by an x-ray induced photoemission of a localized electron trapped in the low dimensional structures. Since the photoemission originates from the x-ray absorption of only the low dimensional structures, the site-selective analysis may be realized. Experiments using Schottky barrier diodes of A1GaAs: Se with a defect, ′DX center′, successfully indicate the site-selective XAFS spectra of the defect atom.