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1H, 2H, and 31P NMR methods have been employed in the study of dimyristoyl lecithin bilayers hydrated with D2O in the gel (L beta'), intermediate (P beta') and liquid-crystalline (L alpha) phases. For D2O/lipid molar ratios, n, in the range 7 less than or equal to n less than or equal to 11 discontinuities are observed in the deuterium NMR splittings at both main and pretransitions. A partial phase diagram based on NMR and differential scanning calorimetry data is presented. 1H NMR dipolar splittings are observed for macroscopically oriented samples in all three phases. Changes in the 1H splittings are correlated with 2H and 31P data and interpreted to show that the chain tilt in the gel phase undergoes a discontinuous change on transition to the intermediate phase, which brings the chain axes closer to the bilayer normal. An estimate of chain tilt in the gel phase is made on the basis of NMR data and found to be approximately 23 degrees for a sample with n = 11 at 18 degrees C.

The angular dependence of 'H and *H NMR relaxation rates has been investigated for macroscopically oriented multi-bilayers of egg yolk lecithin and dimytistoyl lecithin in the La phase. Proton TI, values show a strong dependence on the orientation of the bilayer normals to the magnetic field, while deuterium TI, and TI values and proton TI'S show little or no orientation dependence. The results are interpreted on the basis of a simple modification of the theory of Ukleja, Pirs and Doane for relaxation in oriented thermotropic phases. The results confirm the presence of collective fluctuations in the density of lipid packing in the plane of the bilayer which dominate proton relaxation at low frequencies. There is no evidence of a significant contribution from collective order fluctuations.

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