Folding kinetics of proteins and cold denaturation.

Folding kinetics of a lattice model of protein is studied. It uses the random energy model for the intrachain couplings and a temperature dependent free energy of solvation derived from a realistic hydration model of apolar solutes. The folding times are computed using Monte Carlo simulations in the region of the phase diagram where the chain occurs in the native structure. These folding times are roughly equals for the temperatures of cold and warm denaturations for a large range of solvent quality. Between these temperatures, the folding times reach maxima and thus, at low temperatures, the kinetics of the chain always speeds up as the temperature is decreased. The study of the conformational space as function of the temperature permits to elucidate this phenomenon. At low temperature, it shows that the activation barriers of the system decrease faster than the temperature as the temperature is decreased. At high temperature, the rate of the barriers over the temperature decreases as the temperature is increased because the height of the barrier is almost constant.