Unfolding proteins under external forces: a solvable model under the self-consistent pair contact probability approximation.

We extend a model of Micheletti et al. [Phys. Rev. Lett. 87, 088102 (2001)]] used to study protein con-formations to the case in which there is an external force field. Under the self-consistent pair contact probability approximation, this residue-level resolution model can still be solved under pulling forces. We implement the algorithm using heterogeneous parameters and study the force-induced unfolding of a helical segment from the protein transformylase and of the beta-stranded domains from the protein titin. The results are qualitatively consistent with the results from more expensive, atomistic dynamics simulation. Despite the mean-field-like approach, we observed a sharp and cooperative unfolding transition.