Andrea Hamill,1 Michihiro Nagao,1, 2,* and Paul Butler1


1 NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899-6102, USA 2 Center for Exploration of Energy and Matter, Indiana University, Bloomington, IN 47408, USA



Lipid membranes are self-assembled highly flexible structures that have the ability to undergo an array of conformational and dynamic transitions which are essential for many biological functions. These dynamic motions range from individual lipid oscillation, rotation and diffusion to the undulation of large (micron size) patches of the membrane. Spectroscopic techniques have been used successfully to probe lipid membrane dynamics at the molecular length while microscopic techniques are used at long length scales. The dynamics at intermediate length scales are fundamental to understanding how mesoscopic motions emerge from microscopic atomic interactions yet they are the most experimentally elusive. Such thermal fluctuations are also thought to be the physical basis of many cellular processes. Specifically the ability to measure membrane thickness fluctuations, which have been theoretically suggested, has only recently been demonstrated. Neutron spin echo (NSE) spectroscopy was used to experimentally access such fluctuations in a surfactant membrane system. In this study we report, for the first time, thickness fluctuations in a lipid membrane. Thickness fluctuations were measured in phosphocholine lipid bilayers as a function of both lipid tail length and temperature. Unilamellar vesicles composed of a single lipid DMPC, DPPC, or DSPC were prepared and measured at temperatures both above and below their respective melting transition temperature. Surprisingly, unlike undulation fluctuations the thickness fluctuation amplitude was found to be independent of both temperature and lipid tail length with an amplitude of 4 .