PMID: 619997;Abstract:
The structural changes in the polar head group region of unsonicated bilayer membranes of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine produced by addition of cholesterol have been determined using deuterium and phosphorus-31 NMR. Incorporation of up to 50 mol % cholesterol produces little change in the phosphorus-31 chemical shielding anisotropies, compared with the values in pure bilayers above the phase transition temperatures, while some of the deuterium quadrupole splittings are reduced by almost a factor of two. Adjustment of the head group torsion angles by only a few degrees accounts for the observed spectral changes. Addition of cholesterol therefore has opposite effects on the hydrocarbon and polar regions of membranes: although cholesterol makes the hydrocarbon region gel-like, with an increased probability of trans conformations, the conformation of the polar head groups is very similar to that found in the liquid crystalline phase of pure phospholipid bilayers. © 1978 American Chemical Society.
PMID: 889780;Abstract:
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.
PMID: 1932556;PMCID: PMC1260117;Abstract:
A partial phase diagram of the ternary system dioleoylphosphatidylethanolamine (DOPE)/sodium cholate/water has been determined using 31P Nuclear Magnetic Resonance (NMR) spectroscopy. In the absence of cholate, it is well known that the DOPE/water system forms a reversed hexagonal (H(II)) phase. We have found that addition of even small amounts of cholate to the DOPE/water system leads to a transition to a lamellar (L(α)) phase. At higher cholate concentrations, a cubic (I) phase (low water content) or a micellar solution (L1) phase (high water content) is present. Thus, cholate molecules have a strong tendency to alter the lipid monolayer curvature. Increasing the concentration of cholate changes the curvature of DOPE from negative (H(II) phase), through zero (L(α) phase), and finally to a phase of positive curvature (micellar solution). This observation can be rationalized in terms of the molecular structure of cholate, which is amphipathic and has one hydrophobic and one hydrophilic side of the steroid ring system. The cholate molecules have a tendency to lie flat on the lipid aggregate surface, thereby increasing the effective interfacial area of the polar head groups, and altering the curvature free energy of the system.
