Two crystal forms of the purple membrane from Halobacterium halobium are being investigated. One is the native p3 form which occurs naturally, and the other is an orthorhombic p22121 form which is made artificially with detergent.Both forms diffract to better than 3 Å resolution. They have been studied by electron diffraction at high (3 Å) resolution, and at lower resolution (6.5-7 Å) by electron microscopy. Both crystal forms occur as membrane sheets, containing a single molecular layer of protein molecules with a lipid bilayer filling the space between the protein molecules, the whole having a thickness of about 45-50 Å. The electron microscopic and diffraction analyses have been carried out in three dimensions using specimens tilted at angles of up to 60° to the incident electron beam.The resulting Fourier maps of the two structures enable a common shape for a single molecule of bacteriorhodopsin to be determined, which is free of the ambiguity of molecular boundary normally found in crystal structure analyses at lower resolution than that required to trace the path of the polypeptide chain.

Henderson, R., Baldwin, J. M., Agard, D. A., & Leifer, D. (1983). High-resolution structural analysis of purple membrane. Proceedings, Annual Meeting, Electron Microscopy Society of America, 41, 418–421.