| The current thesis investigates the ultra-fast folding behavior of two coiled coil variants. Chapter two addresses whether helix formation or chain collision is the first step of folding. The relative positions of these events leading up to the free energy barrier was determined using a dimeric version of a coiled coil with such low intrinsic helicity that far fewer than 1% of collisions occur between helical, rather than unstructured chains. Folding of this protein approaches the diffusion-limited rate and is much faster than possible if folding is contingent upon pre-collision helix formation. Thus, the collision of two unstructured chains is the initial step of the dominant kinetic pathway, while helicity exerts its influence only at a later step. Folding from an unstructured encounter complex may be efficient and robust, which has implications for any biological process that couples folding to binding.; In chapter three, a second coiled coil is investigated with results pertinent to the identification of the elusive downhill folding behavior predicted by theory. Because this protein is among the fastest and most stable yet described, it is a prime candidate for downhill folding. A combination of traditional stop-flow experiments and native state hydrogen exchange methods demonstrate that the kinetic and thermodynamic characteristics of folding in the absence of denaturant are consistent with those at the folding transition, where behavior is known to be barrier limited. Therefore, folding of this molecule remains cooperative and barrier limited even under conditions where downhill folding is predicted to occur. |
