Restoration of the missing cone in protein electron crystallography by direct methods: bacteriorhodopsin

Abstract

Abstract The x-ray crystal structure of bacteriorhodopsin was used to generate model electron diffraction amplitudes and phases, referring to an electron crystallographic determination to isolate the details of a 52 Å-thick single layer in plane group p3 will cell constants: a = b = 62 Å. For these ideal data, phase extension by direct methods (Sayre equation) was shown to restore phase values within missing cones (defined by limited goniometric tilts of ±45, ±60, and ±70°) to reasonable accuracy. Restoration of amplitude information was much less satisfactory than the result found for phase terms. Nevertheless, with a crude amplitude estimate, it was found that, even at 6 Å diffraction resolution, details of the peptide folds linking the transmembrane helices could be observed in the three dimensional potential maps.