Internal friction in the ultrafast folding of the tryptophan cage

Protein folding is a diffusional process, and the speed of folding is controlled by the frictional forces that act on the polypeptide chain. Several previous studies have suggested that the bulk viscosity of the solvent is the only important source of friction in folding reactions. By contrast, our studies of the folding dynamics of the Tryptophan Cage, a small, ultrafast-folding protein, show clear evidence for additional frictional forces that are independent of the solvent viscosity. The nature of this internal friction appears qualitatively different for the folding vs. unfolding reactions. We expect that this phenomenon could significantly influence the rate of many folding processes that proceed on sufficiently fast (∼μs) time scales. In particular, it may limit folding speeds to values substantially slower than the diffusion limits estimated from simple contact-formation studies in polypeptides.