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

生物大分子组装体的单位晶胞体积是蛋白质晶体的十几倍。在生物大分子组装体的X射线衍射实验中，仍有几个问题需要解决。在大晶胞晶体的情况下，衍射点之间的距离很短，很难单独测量每个衍射光的强度。由于每个衍射点的强度与晶体的单位晶胞体积的倒数成正比，大晶胞晶体的衍射光太弱，不能准确测量其强度。因此，我们发起了这项研究，以开发收集生物大分子组合的X射线衍射数据的新方法。本研究的结果总结如下：任意方向的小角振荡法将每个衍射点分裂，并增加强度数据的S/N。因此，晶胞尺寸为500的晶体的强度数据可以用0.5度振荡的方法进行测量。小角振荡法不仅可以减少衍射点之间的重叠，而且可以提高每个衍射点强度的精度。任意方向的振荡方法减小了观测烈度数据的系统误差。

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.