Interdependence between Collective Thermal Fluctuations and Elastic and Viscous Properties in Model Lipid Bilayers

Lipid membranes undergo an array of conformational and dynamic transitions, ranging from individual lipid motions to undulations of micron-sized patches of the membrane. However, the dynamics at intermediate length scales are largely unexplored due to experimental challenges in accessing the appropriate length and time scales. Over the past several years our group has used neutron spin echo spectroscopy (NSE) to provide unique insights into these elusive dynamics in model lipid bilayers, measuring collective bending and thickness fluctuations. These thermally induced collective membrane fluctuations are controlled by elastic and viscous properties of the membranes. It has long been known that the bending fluctuations are characterized by the bending modulus, κ, of the membranes and the motion is damped by the viscosity of solvent, η. By contrast, according to a recent theory proposed by Bingham, Smye and Olmsted, the collective thickness fluctuations are characterized by the bilayer area compressibility modulus, KA, which is damped by the membrane and solvent viscosities, μ and η, respectively. Therefore, by measuring these two collective membrane fluctuations the membrane's elastic and viscous parameters can be evaluated. Here we use this novel method to determine these characteristic parameters of lipid bilayers from neutron scattering data for a couple of simple saturated phosphatidylcholine bilayers. The estimated values are κ ∼ 10−19 J, KA ∼ 0.3 to 0.4 N/m, and μ ∼ 10 nPa s m, which are all consistent with literature values.