Bacteriorhodopsin is an inside-out protein.

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Abstract
Neutron scattering is particularly useful when parts of a structure can be deuterated. By biosynthetic incorporation, from Halobacterium halobium, purple membranes were obtained in which all of the valines or all of the phenylalanines are present in deuterated form. Difference Fourier techniques permit a general assessment of the distribution of valine and phenylalanine in projections of the purpose membrane structure. These show that valine is distributed toward the periphery of a single bacteriorhodopsin molecule, whereas phenylalanine is distributed toward its center. The facts that the amino acid sequence is known and that much of it can be assigned to the .alpha.-helices of the bacteriorhodopsin structure are used to interpret the results. Comparison of maps with the distribution of valine and phenylalanine around .alpha.-helical perimeters establishes the distribution of other amino acids and leads to the conclusion that the charged and polar groups of the bacteriorhodopsin molecule tend to lie at the molecular interior, away from contact with lipid, while the nonpolar surfaces are directed outward, making contact with the lipid regions. Thus, the protein is inside-out compared with the organization of soluble proteins.