Relationship between viscosity and acyl tail dynamics in lipid bilayers

Membrane viscosity is a fundamental property that controls transport phenomena within two-dimensional biological membranes. To better understand the molecular origins of membrane viscosity, we employed quasi-elastic x-ray and neutron scattering techniques to access coherent lipid acyl tail correlation dynamics over time scales from several ps to hundreds of ns. In the fluid phase of dimyristoyl phosphocholine bilayers, two relaxation processes are observed with the relaxation times of ≈ 30 ps and ≈ 500 ps. The fast relaxation was attributed to density fluctuations of the lipid acyl tails while the slow mode was assigned to lipid molecular rearrangements. These two modes became a heterogeneous single mode with relaxation times of 10s to 100s of ns when the temperature was decreased to the lipid gel phase.  Based on the relationship between density fluctuations and viscosity in three-dimensional molecular liquids, we estimated the two-dimensional membrane viscosity both in the lipid fluid phase and in the gel phase. The estimated membrane viscosity values were in the middle of a broad distribution of reported values in the fluid phase and increased by an order of magnitude larger in the vicinity of the melting transition temperature in the gel phase.