First Observation of Dynamics in Lipid Multilayers using X-ray Photon Correlation Spectroscopy (XPCS)

Collective modes of layer undulations in lipid multilayers are of considerable interest because they can be used to measure the elastic moduli and viscosity of the lipid bilayers as a function of interlamellar spacing, properties of the interlayer aqueous channels, and temperature. These are fundamental quantities required to calculate the configurations and fluctuations of lipid membranes, relevant in modeling many biomembrane functionalities (e.g., intermembrane interactions and polyvalent ligand recognition) that depend on elasticity and dynamics of membrane phases. However, only relatively few investigations have been made of lipid systems, with e.g. dynamical light scattering, neutron spin echo and inelastic neutron scattering. There remains a gap in time scales and length scales which the technique of x-ray photon correlation spectroscopy (XPCS) can fill.We present here the first XPCS measurements of the dynamics of 1,2-dioleoyl-sn-glycero-3-phtosphocholine (DOPC) multilayers at relative humidity of 99% and temperature of 28°C. The measurements were done at and near the 1st Bragg peak of the multilayer, in which range the intensity-intensity autocorrelation function includes heterodyne oscillations [1] due to a large static component as well as homodyne oscillations predominated at q-values off the Bragg peak. According to de Jeu's theory [2], there are two different modes of relaxation time in the system: a slow and a fast one. Our experiment reveals the existence of the slow mode, which exhibits a plateau in relaxation time over a range of q (10e-5 A-1∼10e-4 A-1). The results of the analysis of these correlation functions according to the model of de Jeu et al. will be presented.1. C. Gutt et al, Phys. Rev. Lett. 91, 076104 (2003)2. W. H. de Jue et al, Rev. Mod. Phys., 75, 181-235 (2003)