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 3.9Å 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°, 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