Orientation of rhodopsin alpha-helices in in retinal rod outer segment membranes studied by infrared linear dichroism

Frog retinal rod outer segments, oriented by a magentic field, were shown to contain rhodopsin alpha-helical segments preferentially aligned perpendicular to the plane of the disc membrane, by the technique of infrared linear dichroism. Infrared absorption parallel and perpendicular to the rod axes by peptide C parallel to O groups, whose absorption band contains alpha-helical and random coil components at slightly different frequencies, showed positive dichroism centered on the alpha-helix frequence. We conclude that the alpha-helical portion of the protein has an average orientation in the transmembrane direction. Furthermore, infrared spectra of rods in 2H2O Ringer's solution exhibit two distinct peptide amino group absorption bands: the unexchanged N-2H band, which is nondichroic. This implies that the oriented part of the protein is in the lipid bilayer, supporting a model for rhodopsin with a hydrophobic core containing partially oriented alpha-helices and hydrophilic ends consisting of unoriented polypeptide.