Glaeser R M, Ceska T A
Biophysics Department, Lawrence Berkeley Laboratory, University of California, Berkeley 94720.
Acta Crystallogr A. 1989 Sep 1;45 ( Pt 9):620-8. doi: 10.1107/s0108767389004599.
Electron diffraction patterns of 45 A thick two-dimensional crystalline arrays of a cell membrane protein, bacteriorhodopsin, have been recorded at two electron voltages, namely 20 and 120 kV. Significant intensity differences are observed for Friedel mates at 20 kV, but deviations from Friedel symmetry are quite small at 120 kV. It does not seem likely that the measured Friedel differences can be accounted for by complex atomic structure factors, by curvature of the Ewald sphere, or by effects that might occur as a result of inelastic scattering (absorption). It is therefore concluded that dynamical diffraction within the single molecular layer of these crystals is responsible for the observed Friedel differences. The results are useful in estimating the maximum specimen thickness for which the kinematic approximation may be safely used in electron crystallography of biological macromolecules at the usual electron voltage of 100 kV, or even at higher voltages. The results show that the Friedel differences are independent of resolution and this opens up the possibility that dynamical effects occurring at lower voltages might be used to phase higher-voltage kinematic diffraction intensities.
已在20 kV和120 kV这两种电子电压下记录了细胞膜蛋白细菌视紫红质45 Å厚的二维晶体阵列的电子衍射图样。在20 kV时，Friedel对映体存在明显的强度差异，但在120 kV时，与Friedel对称性的偏差相当小。测量到的Friedel差异似乎不太可能由复杂的原子结构因子、埃瓦尔德球的曲率或非弹性散射（吸收）可能产生的效应来解释。因此得出结论，这些晶体单分子层内的动态衍射是观察到的Friedel差异的原因。这些结果有助于估计在100 kV甚至更高电压的常用电子电压下，生物大分子电子晶体学中可以安全使用运动学近似的最大样品厚度。结果表明，Friedel差异与分辨率无关，这开辟了一种可能性，即较低电压下发生的动态效应可用于对较高电压下的运动学衍射强度进行相位测定。
