Concerted dihedral rotations give rise to internal friction in unfolded proteins.

Protein chains undergo conformational diffusion during folding and dynamics, experiencing both thermal kicks and viscous drag. Recent experiments have shown that the corresponding friction can be separated into wet friction, which is determined by the solvent viscosity, and dry friction, where frictional effects arise due to the interactions within the protein chain. Despite important advances, the molecular origins underlying dry friction in proteins have remained unclear. To address this problem, we studied the dynamics of the unfolded cold-shock protein at different solvent viscosities and denaturant concentrations. Using extensive all-atom molecular dynamics simulations we estimated the internal friction time scales and found them to agree well with the corresponding experimental measurements (Soranno et al. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 17800-17806). Analysis of the reconfiguration dynamics of the unfolded chain further revealed that hops in the dihedral space provide the dominant mechanism of internal friction. Furthermore, the increased number of concerted dihedral moves at physiological conditions suggest that, in such conditions, the concerted motions result in higher frictional forces. These findings have important implications for understanding the folding kinetics of proteins as well as the dynamics of intrinsically disordered proteins.