Rotational friction on globular proteins combining dielectric and hydrodynamic effects

Rotational friction on proteins and macromolecules is known to derive contributions from at least two distinct sources - hydrodynamic (due to viscosity) and dielectric friction (due to polar interactions). In the existing theoretical approaches, the effect of the latter is taken into account by increasing the size of the protein with the addition of a hydration layer. Here, we calculate the rotational dielectric friction on a protein (ζDF) by using a generalized arbitrary charge distribution model (where the charges are obtained from quantum chemical calculation) and the hydrodynamic friction with stick boundary condition, by using the sophisticated theoretical technique known as tri-axial ellipsoidal method (ζTR). The calculation of hydrodynamic friction is done with only the dry volume of the protein (no hydration layer). We find that the total friction thus obtained by summing up ζDF and ζTR, gives reasonable agreement with the experimental results, i.e., ζexp ≈ ζDF + ζTR.