The Saffman-Delbrück hydrodynamic model for lipid-bilayer membranes is modified to account for the periodic boundary conditions commonly imposed in molecular simulations. Predicted lateral diffusion coefficients for membrane-embedded solid bodies are sensitive to box shape and converge slowly to the limit of infinite box size, raising serious doubts for the prospects of using detailed simulations to accurately predict membrane-protein diffusivities and related transport properties. Estimates for the relative error associated with periodic boundary artifacts are 50% and higher for fully atomistic models in currently feasible simulation boxes. MARTINI simulations of LacY membrane protein diffusion and LacY dimer diffusion in DPPC membranes and lipid diffusion in pure DPPC bilayers support the underlying hydrodynamic model.
REFERENCES
This brief derivation provides the correct result, but has ignored the forces of constraint associated with the dimer. A rigorous derivation may be found in Section V of Ref. 35.
In computing these diffusion coefficients, bilayer center of mass motion was removed. Trajectories were converted from GROMACS to CHARMM format, and CHARMM’s correl command was used to calculate MSD curves. Diffusion coefficients were obtained by fitting to the linear portion of the MSD curve.