  We consider the relation between the microscopic and effective descriptions
of the unfolding experiment on a model polypeptide. We evaluate the probability
distribution function of the performed work by Monte Carlo simulations and
compare it with that obtained by evaluating the work distribution generating
function on an effective Brownian motion model tailored to reproduce exactly
the equilibrium properties. The agreement is satisfactory for fast protocols,
but deteriorates for slower ones, hinting at the existence of processes on
several time scales even in such a simple system.
