  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 an individual helix. A simple estimate based on diffusion in a square-well
potential predicts the relaxation time within a factor of two.
