Multiple stepwise refolding of immunoglobulin domain I27 upon force quench depends on initial conditions

Mechanical folding trajectories for polyproteins starting from initially stretched conformations generated by single-molecule atomic force microscopy experiments [Fernandez, J. M. & Li, H. (2004) Science 303, 1674-1678] show that refolding, monitored by the end-to-end distance, occurs in distinct multiple stages. To clarify the molecular nature of folding starting from stretched conformations, we have probed the folding dynamics, upon force quench, for the single I27 domain from the muscle protein titin by using a C(alpha)-Go model. Upon temperature quench, collapse and folding of I27 are synchronous. In contrast, refolding from stretched initial structures not only increases the folding and collapse time scales but also decouples the two kinetic processes. The increase in the folding times is associated primarily with the stretched state to compact random coil transition. Surprisingly, force quench does not alter the nature of the refolding kinetics, but merely increases the height of the free-energy folding barrier. Force quench refolding times scale as tau(F) approximately tau(F)(0)exp(f(q)Deltax(f)/k(B)T), where Deltax(f) approximately 0.6 nm is the location of the average transition state along the reaction coordinate given by end-to-end distance. We predict that tau(F) and the folding mechanism can be dramatically altered by the initial and/or final values of force. The implications of our results for design and analysis of experiments are discussed.