Development of novel X-ray diffraction measurement for biological macromolecular assemblies

生物大分子集合体的晶胞体积是蛋白质晶体的十倍以上。生物大分子组装体的X射线衍射实验还存在一些问题需要解决。在具有大晶胞的晶体的情况下，衍射斑点之间的距离如此之短，以至于难以分别测量每个衍射的强度。由于每个衍射点的强度与晶体的晶胞体积的倒数成比例，因此来自具有大晶胞的晶体的衍射太弱而不能精确地测量它们的强度。本研究的主要结果如下：任意方向的小角振荡法使衍射点分离，提高了强度数据的信噪比。因此，具有500个晶胞尺寸的晶体的强度数据能够通过具有0.5度振荡的方法进行。小角度振荡方法不能减少衍射点之间的重叠，但也增加了每个衍射的强度的精度。任意方向的振荡方法减少了观测强度数据中的系统误差。

Unit cell volumes of biological macromolecular assemblies are as large as more than ten times of those of protein crystals. There remain several problems to be solved in x-ray diffraction experiments for the biological macromolecular assemblies. In the case of crystals with large unit cell, distances between diffraction spots so short that it is difficult to measure intensity of each diffraction separately. Since intensity of each diffraction spot is proportional to reciprocal of unit cell volume of crystal, diffraction from crystals with large unit cells is too weak to measure their intensity accurately. Thus we have initiated this research to develop new methods to collect-x-ray diffraction data of the biological macromolecular assemblies.Results obtained in the present research are summarized as follows : Small angle oscillation method about any direction splits each diffraction spot from the others and increases S/N of intensity data. Consequently, intensity data of crystals with unit cell dimensions of 500 is able to carried out by the method with 0.5 degree oscillation. The small angle oscillation method does not enable to decrease overlapping between diffraction spots, but also increase accuracy of intensity of each diffraction. The oscillation method about any direction reduces systematic error in observed intensity data.