Viscosity of mixed lipid bilayers: Comparison between atomistic and coarse grained models

Membrane fluidity, often characterized by surface viscosity, plays a critical role in regulating a wide range of biological processes, including protein diffusion and function, receptor binding, signal transduction, enzyme and drug interactions, and cellular events such as membrane fusion. While the viscosity of single-component lipid membranes has been extensively studied, significantly less is known about how compositional heterogeneity influences surface viscosity in mixed lipid bilayers. In this work, we employ both atomistic and coarse-grained molecular dynamics simulations to examine surface viscosity trends in binary lipid mixtures composed of lipids with distinct structural characteristics: (i) mismatch in acyl chain lengths, (ii) opposite spontaneous curvatures, and (iii) variations in head group structure. The simulation data are analyzed using the Redlich-Kister model to quantify deviations from ideal mixing behavior. Our findings reveal that the greatest non-ideality in surface viscosity arises when the constituent lipids exhibit opposite spontaneous curvatures. Notably, coarse-grained models fail to capture the correct viscosity trends in systems with more complex intermolecular interactions, indicating that interfacial friction is not accurately represented at reduced resolution.