  The mechanical unfolding of an engineered protein composed of eight domains
of Ig27 is investigated by using atomic force microscopy. Exploiting a
fluctuation relation, the equilibrium free energy as a function of the molecule
elongation is estimated from pulling experiments. Such a free energy exhibits a
regular shape that sets a typical unfolding length at zero force of the order
of 20 nm. This length scale turns out to be much larger than the kinetic
unfolding length that is also estimated by analyzing the typical rupture force
of the molecule under dynamic loading.
