Abstract
In an effort to elucidate the state of molecular motion in aqueous lecithin systems, the nuclear magnetic relaxation properties of unsonicated lecithin crystal and liquid crystal phases have been investigated. The presence of a narrow component in the nmr spectrum of aqueous lamellar lecithin crystals is taken to indicate a significant degree of motion of the lecithin methyl groups in this solid phase. Also in the crystal state of lecithin, nmr linewidth measurements, together with thermal analysis experiments appear to indicate that a structural rearrangement of either the lecithin or the bound water takes place, which is not accompanied by an increase in molecular mobility. Delayed Fourier Transform and wide-line nmr experiments show that at the crystal -> liquid crystal (bilayer) phase transition temperature, the lecithin choline methyl, terminal methyl, and hydrocarbon chain methylene protons are all simultaneously mobilized. A previously noted field dependence of the proton magnetic resonance linewidth of aqueous lecithin bilayers is shown to be accounted for by the chemical shift differences among the various kinds of protons. Spin lattice relaxation rates have been measured for these protons as a function of temperature and frequency, and these data have been interpreted in terms of models for the segmental motion of the choline head groups and the hydrocarbon chains. The influence of spin diffusion on the relaxation behavior of the various protons is also discussed.
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