COLLECTIVE LIPID MOTIONS IN BILAYER-MEMBRANES STUDIED BY TRANSVERSE DEUTERON SPIN RELAXATION

The anisotropy and pulse frequency dispersion of the spin-spin relaxation time T2E(CP) from Carr-Purcell-Meiboom-Gill pulse sequences is employed to evaluate the major contribution to transverse H-2 spin relaxation in bilayer membranes. Analysis of the experiments is achieved in terms of a density operator formalism, employing the stochastic Liouville approach. From a comparison of the observed angular and frequency dependences of T2E(CP) with those predicted for order director fluctuations, we conclude that collective lipid motions constitute the dominant transverse relaxation process. Computer simulations provide the viscoelastic parameters of the lipid membranes. For 1,2-dimyristoyl-sn-glycero-3-phosphocholine bilayers at T = 318 K an average elastic constant of K = 2 x 10(-11) N and an effective viscosity of eta = 0.1 P have been determined. Using the experimentally accessible value for the long wavelength cutoff of the elastic modes, one obtains the mean square amplitude of the director fluctuations <theta-0(2)> = 0.04. This corresponds to an order parameter of S(OF) = 0.94. Apparently, the contributions of the collective motions to the measured order parameters are marginal.