Shear viscosity of lipid membranes dictates how fast
lipids, proteins,
and other membrane constituents travel along the membrane and rotate
around their principal axis, thus governing the rates of diffusion-limited
reactions taking place at membranes. In this framework, the heterogeneity
of biomembranes indicates that cells could regulate these rates via
varying local viscosities. Unfortunately, experiments to probe membrane
viscosity under various conditions are tedious and error prone. Molecular
dynamics simulations provide an attractive alternative, especially
given that recent theoretical developments enable the elimination
of finite-size effects in simulations. Here, we use a variety of different
equilibrium methods to extract the shear viscosities of lipid membranes
from both coarse-grained and all-atom molecular dynamics simulations.
We systematically probe the variables relevant for cellular membranes,
namely, membrane protein crowding, cholesterol concentration, and
the length and saturation level of lipid acyl chains, as well as temperature.
Our results highlight that in their physiologically relevant ranges,
protein concentration, cholesterol concentration, and temperature
have significantly larger effects on membrane viscosity than lipid
acyl chain length and unsaturation level. In particular, the crowding
with proteins has a significant effect on the shear viscosity of lipid
membranes and thus on the diffusion occurring in the membranes. Our
work also provides the largest collection of membrane viscosity values
from simulation to date, which can be used by the community to predict
the diffusion coefficients or their trends via the Saffman–Delbrück
description. Additionally, it is worth emphasizing that diffusion
coefficients extracted from simulations exploiting periodic boundary
conditions must be corrected for the finite-size effects prior to
comparison with experiment, for which the present collection of viscosity
values can readily be used. Finally, our thorough comparison to experiments
suggests that there is room for improvement in the description of
bilayer dynamics provided by the present force fields.