Rotational friction on small globular proteins: Combined dielectric and hydrodynamic effect

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 in an ad hoc manner, 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, (ζ hyd ) by using the sophisticated theoretical technique known as tri-axial ellipsoidal method, formulated by Harding [S. E. Harding, Comp. Biol. Med. 12, 75 (1982)]. The calculation of hydrodynamic friction is done with only the dry volume of the protein (no hydration layer). We find that the total friction obtained by summing up ζDF and ζ stick hyd gives reasonable agreement with the experimental results, i.e., ζexp ≈ ζDF + ζ stick hyd . Email: [email protected]