Tail length dependence of acyl tail correlation dynamics and membrane viscosity in lipid bilayers*

Biological membranes are unique borders that define cell boundaries and separate the cell interior from its surrounding environment. These membranes are rigid enough to maintain the cell shape and protect the cell from collapsing and leaking its components, and at the same time, are soft enough to deform and exchange molecules into and out of the cell. The bending and compression elasticity describe the membrane rigidity and the extent of the deformation, but these properties do not explain how fast embedded components can move inside the membrane. Instead, the viscous properties of the membranes determine these response times. Among various techniques to measure membrane viscosity, acyl tail correlation dynamics measured by neutron scattering can help identify the molecular origins of membrane viscosity. In addition to the previous studies on dimyristoyl-phosphocholine (DMPC) bilayers, we performed further neutron spin echo studies on dipalmitoyl- and distearoyl-phosphocholine (DPPC and DSPC) bilayers. We will discuss how the tail length differences, with the number of carbons ranging from 14 to 18, changes the acyl tail correlation dynamics and compare the estimated values of the membrane viscosity with those estimated from the collective membrane thickness fluctuations.