ORIENTATIONAL CORRELATIONS AND SPIN RELAXATION IN LAMELLAR FLUID MEMBRANE PHASES

Thermal fluctuations of fluid membranes in multilamellar systems have been extensively studied during the past decade by means of nuclear sp in relaxation. Such data have generally been analyzed in terms of an e ffectively two-dimensional membrane model, which does not properly inc orporate the mutual coupling of the individual membranes. Here we pres ent a comprehensive theory of spin relaxation induced by small-amplitu de, long-wavelength elastic distortions in a multilamellar stack of fl uid membranes. In contrast to previous theoretical treatments, we find that membrane coupling can profoundly affect the spin relaxation beha vior via its effect on the amplitudes and rates of membrane distortion modes. A physical basis for the resulting, rather intricate, spin rel axation behavior is provided by analyzing the spatial correlation func tion for the local membrane orientation. We find that the decay of thi s function involves two correlation lengths: one is related to interac tions with the two adjacent membranes, and the other reflects the cohe rent fluctuation modes in the entire membrane stack. This analysis exp lains why the time correlation function has the asymptotic form 1/tau( 2) rather than 1/tau, as expected for a two-dimensional system. A rein terpretation of existing low-frequency spin relaxation data from multi lamellar phospholipid-water dispersions in terms of our theory should provide valuable insights into the nature of intermembrane forces.