Motions of molecules and their assemblies are fundamental to biological functions. The hierarchical lipid membrane motions span individual molecular movements to undulations of micron-sized patches and vary with the degree of complexity of the membranes. Over this wide space and time range dynamics on the nanometer scale remain 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 (NSE) spectroscopy to provide unique insights into these elusive dynamics in model lipid bilayers. NSE measures collective motions of membranes on the nanometer and nanosecond scales, such as undulation and thickness fluctuations. These thermal fluctuations of lipid molecules are controlled by the elastic and viscous properties of the membranes and provide a basis for understanding the biomembrane’s mechanical properties. Undulation fluctuations have long been studied to understand membrane elasticity as the fluctuations are characterized by the membrane bending modulus, κ, and are damped by the solvent viscosity, η. Thickness fluctuations, on the other hand, have been more challenging to observe experimentally. Our group showed that NSE experiments can capture the thickness fluctuation dynamics and provide a measure of the fluctuation amplitude, Δh, and time scale, τ. Statistical mechanics predicts a relation between the area compressibility modulus, KA, and the fractional change in thickness, Δh/h, under the volume conservation condition. This relationship allows us to estimate KA values from the experimental Δh measurements. Moreover, recent theoretical developments by Bingham and colleagues suggest that τ is characterized by KA and the membrane viscosity, μ. We have applied the above discussion to the analysis of the thickness fluctuations dynamics, and it is now possible to determine μ as the only fit parameter to the experimental data. We will explain the method and review some of the results in phospholipid membranes with and without small molecule and peptide additives.

Elastic and viscous properties of lipid bilayers: Application of neutron spin echo spectroscopy

253rd National Meeting (2017)

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