PROBING STATIC DISORDER IN PROTEIN UNFOLDING AND CHEMICAL REACTIONS BY SINGLE-MOLECULE FORCE SPECTROSCOPY

The work presented in this dissertation focuses on the kinetics of biomolecular reactions under mechanical force, including protein unfolding and disulfide-bond reduction, probed at the single-molecule level. The advent of single-molecule force spectroscopy has allowed the direct measure of force-dependent reaction rates, providing a powerful approach to extract the kinetic information and to characterize the underlying energy landscape that governs the reaction. The widely accepted two-state kinetic model for protein unfolding describes that the protein unfolds by crossing over a single energy barrier, with the implicit assumption of a single transition state and a well-defined activation energy barrier. Based on this assump-tion, the ensemble-averaged survival probability is expected to follow single exponential time dependence. However, it has become increasingly clear that the saddle point of the free-energy surface in most reactions is populated by ensembles of conformations, leading to nonexponential kinetics. Here we present a theory that generalizes the two-state model to include static disorder of conformational degrees of freedom to fully account for a diverse set of unfolding pathways. Using single-molecule force-clamp spectroscopy, we study the non