Magnetic resonance studies on membrane and model membrane system: proton magnetic relaxation rates in sonicated lecithin dispersions.

We have used Fourier transform nuclear magnetic resonance methods to measure the spin-lattice and transverse relaxation times at 220 MHz of the choline N-methyl and the fatty-acid alpha-carbonyl, allyl, vinyl, methylene, and methyl protons of sonicated egg-yolk lecithin. Over the temperature range investigated the T(1) values were, in general, similar to, but different from, one another, suggesting that the relaxation rates of all of the fatty-acid protons are not determined solely by spin-diffusion to a heat sink. Arrhenius plots of the T(1) data gave activation energies similar to those for the barriers to internal rotation in alkanes. The values of the transverse relaxation rate, T(2), showed a relatively large variation among the proton resonances; about 20% of the methylene protons had a T(2) of 56 msec, while the remaining protons relaxed according to a distribution of values all shorter than 20 msec. Such a distribution of relaxation times is envisioned to arise from a distribution of correlation times stemming from complex motions in which extended angular excursions of the fatty acid chain are coupled to trans-->gauche conformational transitions.