Phase accuracy in high-resolution electron microscopy of trigonal and orthorhombic purple membrane.

High-resolution images of orthorhombic purple membrane have been obtained by electron cryomicroscopy with spot-scan illumination, and the projection structure at 3.9 A resolution calculated after image processing and averaging of the data. Since the phases of the structure factors in the projection down the orthorhombic twofold axis should be either 0 or 180 degrees , this offers the first opportunity to make an independent test of the estimated accuracy of high-resolution phases obtained by electron microscopy. The results show the final phases are less accurate than previously estimated by a small factor (1.3). Careful comparison of the new orthorhombic structure to the known trigonal structure shows only small differences after account is taken of a slight difference in the tilt angle of the molecules in the two crystals. This is consistent with the available kinetic and spectroscopic data which show very small differences in behavior.