Upper limit of the time scale for diffusion and chain collapse in chymotrypsin inhibitor 2.

The rates of folding of wild-type chymotrypsin inhibitor 2 (CI2) (t1/2 = 12 ms) and of faster (t1/2 = 2 ms) and slower (t1/2 = 350 ms) folding mutants are accelerated in parallel by increasing concentrations of sucrose, despite the increases in viscosity. At a viscosity 26 times that of water, the folding rate constant of wild-type CI2 is accelerated four-fold (t1/2 = 2.7 ms). From this, we can estimate that the diffusional chain collapse in CI2 occurs in less than 100 micros in water, and is not rate-determining in folding.