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Two-State Folding over a Weak Free-Energy Barrier

Complex Systems Division, Department of Theoretical Physics, Lund University, Sölvegatan 14A, SE-223 62 Lund, Sweden

Correspondence: Address reprint requests to A. Irbäck, Tel.: +46-46-2223493; Fax: +46-46-2229686; E-mail: anders{at}thep.lu.se.

We present a Monte Carlo study of a model protein with 54 amino acids that folds directly to its native three-helix-bundle state without forming any well-defined intermediate state. The free-energy barrier separating the native and unfolded states of this protein is found to be weak, even at the folding temperature. Nevertheless, we find that melting curves to a good approximation can be described in terms of a simple two-state system, and that the relaxation behavior is close to single exponential. The motion along individual reaction coordinates is roughly diffusive on timescales beyond the reconfiguration time for a single helix. A simple estimate based on diffusion in a square-well potential predicts the relaxation time within a factor of two.

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