Folding-unfolding of immunoglobulin domains in titin: a simple two-state model.

The folding-unfolding reaction rate process in the giant protein titin is studied within a simple two-state model. The molecule is assumed to be stretched by an external force which modulates the potential barrier associated with the folded state. A two-state model for this process is assumed (i.e., the immunoglobulin domains are considered to be either folded or unfolded, with no intermediate states at all). Simple calculations yield a relation between the force and the pulling speed that agrees fairly well with data from experiments and Monte Carlo simulations performed recently. Moreover, in a regime involving ultrafast pulling, the results show that the detailed form of the potential barrier is irrelevant, a conclusion that agrees with the current theoretical work on molecular dynamics.