With the development of the XFEL （X-ray free electron laser）, high quality diffraction patterns from nanocrystals have been achieved. The nanocrystals with different sizes and random orientations are injected to the XFEL beams and the diffraction patterns can be obtained by the so-called ＂diffraction-and-destruction＂ mode. The recovery of orientations is one of the most critical steps in reconstructing the 3D structure of nanocrystals. There is already an approach to solve the orientation problem by using the automated indexing software in crystallography. However, this method cannot distinguish the twin orientations in the cases of the symmetries of Bravais lattices higher than the point groups. Here we propose a new method to solve this problem. The shape transforms of nanocrystals can be determined from all of the intensities around the diffraction spots, and then Fourier transformation of a single crystal cell is obtained. The actual orientations of the patterns can be solved by comparing the values of the Fourier transformations of the crystal cell on the intersections of all patterns. This so-called ＂multiple-common-line＂ method can distinguish the twin orientations in the XFEL diffraction patterns successfully.
The first step of phasing in any de novo protein structure determination using isomorphous replacement （IR） or anomalous scattering （AD） experiments is to find heavy atom positions. Traditionally, heavy atom positions can be solved by inspecting the difference Patterson maps. Due to the weak signals in isomorphous or anomalous differences and the noisy background in the Patterson map, the search for heavy atoms may become difficult. Here, the direct demodulation （DD） method is applied to the difference Patterson maps to reduce the noisy backgrounds and sharpen the signal peaks. The real space Patterson search by using these optimized maps can locate the heavy atom positions more accurately. It is anticipated that the direct demodulation method can assist in heavy atom position determination and facilitate the de novo structure determination of proteins.
In this paper the solution conformation of the response regulator proteins from Deinococcus radiodurans was studied by small-angle X-ray scattering （SAXS）. The SAXS curves of Dr-rrA in solutions were obtained at Beamline 1W2A of Beijing Synchrotron Radiation Facility （BSRF）. Two possible conformations of the response regulator proteins, compact and incompact conformations, have been represented by the known crystallographic structures. And theoretical solution scattering curves of the two possible conformations were calculated and fitted to the experimental scattering curve of Dr-rrA, respectively. The result indicates that the solution conformation of the response regulator proteins is inclined to the compact one, which is in agreement with the result of biochemical experiments.
A computer program for small angle X-ray scattering （SAXS） data processing and analysis named S.exe written in Intel Visual Fortran has been developed. This paper briefly introduces its main theory and function.
With high concentrations of hemoglobin （Hb） in red blood cells, self-interactions among these molecules could increase the propensities of their polymerization and aggregation. In the present work, high concentration Hb in solution and red blood cells were analyzed by small-angle X-ray scattering. Calculation of the effective structure factor indicates that the interaction of Hb molecules is the same when they are crowded together in both the cell and physiological saline. The Hb molecules stay individual without the formation of aggregates and clusters in cells.
