Deuterium nuclear magnetic resonance spectra of dimyristoylphosphatidylcholines specifically labeled in positions 2ʹ, 3ʹ, 4ʹ, 6ʹ, 8ʹ, 10ʹ, 12ʹ, and 14ʹ, of the 2 chain, of an N-deuteriomethylphosphatidylcholine, and of cholesterol-3β-d1, have been obtained by the Fourier transform method at 5.46 and 3.52 T on two “home-built” widebore superconducting magnet spectrometers, as a function of temperature and composition. Data on the specifically deuterium-labeled cholesterol molecule (in nonsonicated membrane systems) permits an estimate of the most probable angle of tilt of the sterol in the membrane, and evaluation of the order parameter (Sα) describing rigid body motions in the bilayer. Segmental order parameters derived from the data presented allow calculation of individual chain segment projections onto the director axis and, consequently, estimation of effective chain length. It is shown that mathematical models which include chain tilt as well as those which neglect this type of rigid body motion give essentially identical results when applied to the dimyristoylphosphatidylcholine and dimyristoylphosphatidylcholine-cholesterol bilayer systems (in excess water, between 23 and 60 °C). Results of calculations of chain length and membrane thickness of a dimyristoylphosphatidylcholine-30 mol % cholesterol membrane system at 23 °C give excellent agreement when compared with recent highreso lution neutron diffraction data obtained on specifically deuterium labeled lecithin-cholesterol systems. No evidence for formation of lecithin-cholesterol complexes having lifetimes of ~30 ms has been found. Below the pure-lipid gel-liquid crystal phase transition temperature Tc but in the presence of cholesterol, we have obtained further evidence for 1-chain/ 2-chain nonequivalence. At 10 °C, the 2' segment of the 2 chain, but not the 2ʹ segment of the 1 chain or the 3ʹ, 6ʹ, or 12ʹ segments of the 2 chain, is broadened almost beyond detection. These results are in agreement with similar effects reported recently for the dipalmitoylphosphatidylcholine-cholesterol system and may indicate a bent configuration for the 2 chain, in the lecithin-cholesterol system. Further cooling below Tc results in loss of the 1-chain 2ʹ-position signal intensity plus 2-chain 3ʹ, 6ʹ, and 12ʹ signals simultaneously. The increase in length of the 2 chain of dimyristoylphosphatidylcholine upon addition of 30 mol % cholesterol of 23 °C is about 2.3 Å. Addition of cholesterol to a choline-labeled lecithin results in complex behavior of the head group deuterium quadrupole splitting as a function of temperature, and cholesterol mole fraction. Above ~20 mol % cholesterol, the main effect is a decrease in quadrupole splitting as cholesterol content increases, the opposite effect to that observed with hydrocarbon chains.
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