Lipid membranes are highly complex systems with a hierarchy of structure and dynamics that span several decades in length and time scales and that in turn significantly influence membrane functions. While a large body of research has been dedicated to multiscale structural investigations in these systems, nanoscale dynamical characterization has lagged far behind. The experimental challenge here is the need to simultaneously access the desired length and time scales for the motions of interest. Over the past decade, neutron spin echo spectroscopy has proven to be a unique tool for capturing nanoscale collective lipid membrane dynamics. For instance, lipid membrane bending moduli can be estimated from the relaxation time of single membrane fluctuations in unilamellar vesicles. More recently, lipid membrane thickness fluctuations have also been measured with relaxation times on the order of 100 ns and with amplitudes of a few tenths of nanometers in fluid lipid bilayers. We show that hydrophobic mismatch between lipids with different acyl chain lengths tunes the thickness fluctuation amplitude and relaxation times in a way not achievable in single component systems. Using deformation free energy calculations, we contextualize these results in terms of bilayer elastic parameters, the total compressibility modulus and the fluctuation wavelength. In the mixed lipid bilayers, the increase in total compressibility compared to the pure component membranes enhances the fluctuation amplitude, while changes in the lateral arrangement of the membrane structure are expected to modify the fluctuation time scale. The fluctuation wavelength is also influenced by lipid composition and is on the order of 20 nm in single component bilayers and increases with temperature in mixed lipid bilayers. It is noted that these fluctuation wavelengths are on the same size scale as laterally inhomogeneous bilayer structures such as the ripple phase and raft domains, which may suggest possible interplay between collective membrane dynamics and intra-bilayer structural arrangements.

Hydrophobic mismatch tunes lipid bilayer dynamics

251st National Meeting (2016)

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