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Henderson and Unwin membrane theory
Henderson and Unwin have studied the purple membrane by electron microscopy using a method for determining the projected structures of unstained crystalline specimens. By applying the method to tilted specimens, and using the principles put forward by De Rosier and Kulg for the combination of such two- dimensional views, they have obtained a three dimensional map of the membrane at 7 A resolution. The map reveals the location of the protein and lipid components, the arrangement of the polypeptide chains within each protein molecule, and the relationship of the protein molecules in the lattice. High-resolution micrographs of crystalline arrays of membrane proteins, taken at a low dose of electrons to minimize radiation damage , have been exploited to determine the three-dimensional structure by Fourier transform. Recent studies on negatively stained rat hepatocyte gap junctions subjected to three-dimensional Fourier reconstruction ( of low-dose electron micrographs ) indicate that the six protein subunits are arranged in a cylinder slightly tilted Tangentially, enclosing a channel 2nm wide at the extracellular region. The dimensions of the channel within the membrane were narrower but could not be resolved ( Unwin and Zampighi ,1980). A small radical movement of the subunits at the cytoplasmic ends could reduce the subunit inclination tangential to six-fold axis and close the channel.

Further details of the molecular organization should emerge as preparative methods become available so that high-resolution three dimensional images comparable to the purple membranes. By using ingenious procedure for the analysis of periodic arrays of bio macromolecules in which data from low-dose electron images and diffraction patterns were combined, Henderson and Unwin (1975) reconstructed a three-dimensional image of purple membranes at 0.7 nm resolution. Glucose embedding was employed to alleviate the dehydration damage and low doses (<0.5 e\A) to reduce the irradiation damage. The electron micrographs of unstained membranes were recorded such that the only source of contrast was a weak phase contrast induced by defocusing.

In their experiment, Unwin and Henderson found that Protein extends to both sides of the lipid bilayer  and is comprised of seven  α-helices packed about  1-1.2nm apart and 3.5-4.0 nm in length ,running perpendicular to the plan of membrane (fig.). The molecules are organized around a threefold axis with a 2 nm-wide space at the center that is filled with lipids. This elegant work represents the most significant step forward thus far, as it has for the first time provided us with the structure of an integral membrane protein in situ. The availability of the amino acid sequence, together with the information about the electron scattering density from the work of Henderson and Unwin (1975 ), has stimulated the model-building efforts (Engleman et al., 1980 ) to fit the bacteriorhodopsin sequence information into a series of α- helical segment. Some of the features of the model proposed by Engleman et al.