Interpretation of 100- and 360-MHz Proton Magnetic Resonance Spectra of Retinal Rod Outer Segment Disk Membranes

Well resolved proton nuclear magnetic resonance (¹H NMR) spectra of bovine retinal rod outer segment (ROS) disk membranes have been obtained at 100 and 360 MHz. The resolved ¹H resonances of the ROS membranes are due to phospholipids, with little contribution from rhodopsin. The spectra of both the ROS membranes and bilayer vesicles prepared from purified ROS phospholipids (liposomes) appear to represent a superposition of relatively sharp resonance components and a broad, underlying background. The distribution between sharp and broad spectral components is sensitive to sonication and temperature. The percentage of choline methyl protons which are resolved in the ROS membrane spectra as sharp resonance components increases from approximately 35 to 100% and the average of the lipid hydrocarbon chain protons from approximately 20 to 40% over the temperature range 5-50 °C. The motional state of terminal CH₃ groups on the polyunsaturated docosahexenoic acid (C22:6ω3) side chains cannot be very different from those on the less unsaturated side chains, since the observed terminal CH₃ resonance consists of components from the C22:6ω3 and other side chains in proportions which reflect their composition ratios. The observation of a comparable fraction of phospholipids yielding high-resolution spectral components and similar resonance line widths for the ROS membranes and ROS liposomes suggests that rhodopsin does not greatly alter the lower frequency segmental motions of phospholipids in the ROS membrane. The NMR data are discussed in terms of models for the organization of phospholipids in the disk membrane and their interaction with rhodopsin.