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The lipid-water interaction in lyotropic measophases : an NMR study Abdolall, Khaled

Abstract

A nuclear magnetic resonance study of the lipid-water interaction has been carried out in the lamellar mesophase of the sodium laurate-water system. Deuterium quadrupole splittings and spin lattice relaxation time measurements of perdeuterated fatty acid chains and the quadrupole splittings of water (D₂O) and the sodium counter ion are used to study the effects of this interaction. The results indicate that the lipid-water interaction has a strong influence on the conformations and motions of the. lipid chains, particularly those chain segments near the lipid water interface. The details of this interaction are not included in the theories which attempt to explain hydrocarbon chain ordering in bilayer membranes in terms of chain-chain (or lipid-lipid) interactions only. A thermodynamic analysis of the results indicates that a description of the ordering of the hydrocarbon chains entirely in terms of chain-chain interactions is not complete, and that a complete theory should include the lipid-water interaction explicitly. The first experimental evidence for a spin lattice relaxation mechanism between water protons and lipid protons in a lipid water system is also reported. The effect of isotopic modification of the methylene hydrogen nuclei on the proton spin lattice relaxation rate in H₂O and also of the effect of the isotope modification of the water on the relaxation rates of the lipid protons is investigated. Although the measurements show that protons deep in the bilayer make a substantial contribution to the spin-lattice relaxation rate of the water protons, a detailed theoretical analysis demonstrates that the experimental results can be accounted for without invoking deep penetration of the water in the bilayer.

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