Transient Effects of Excluded Volume Interactions on the Translational Diffusion of Hydrodynamically Anisotropic Molecules.

We have investigated effects of excluded volume interactions on the translational diffusion of hydrodynamically anisotropic molecules. For that, we have performed rigid-body Brownian dynamics simulations of aqueous solutions of hen egg-white lysozyme (HEWL), at concentrations ranging from 1.25 mg/mL to 250 mg/mL and evaluated the lysozyme's self-diffusion. In the long time limit (above 1 μs), the protein's translational diffusion is isotropic, regardless the solution concentration. However, on the time scale of the order of up to hundreds of nanoseconds, the anisotropic translational diffusion is observed, with the transition time from the anisotropic to isotropic translational diffusion depending on the lysozyme concentration. The magnitude of the translational diffusion anisotropy in this transient regime is also concentration-dependent and steric interactions enhance the anisotropy. Moreover, steric interactions cause the anisotropy to be a nonmonotonic function of time. When the hydrodynamic anisotropy of the protein is neglected in Brownian dynamics simulations and its diffusion tensor is replaced with average translational and rotational diffusion coefficients, the lysozyme's translational dynamics in the long-time limit is similar to that in the case of the corresponding hydrodynamically anisotropic object. However, such a similarity is not observed below 1 μs and in this time regime the translational dynamics of lysozyme molecules modeled with isotropic diffusion coefficients substantially deviates from that derived from Brownian dynamics simulations of their hydrodynamically anisotropic counterparts.