Fig. 8.

Representations of the folding energy landscapes for cytochrome c. The height corresponds roughly to the structural similarity to the native state, whereas the width corresponds to entropy. In all cases, the folding energy landscape is highly funneled, even in the denatured region. Nevertheless, the region of the funnel that is populated, shown in orange, varies significantly with solvent conditions. (A) The GuHCl-denatured ensemble at pH 7 with imidazole to inhibit misligation. GuHCl acts to destabilize native state contacts causing the ensemble to populate higher regions of the funnel. Electrostatic forces are highly screened and do not cause significant traps, allowing the funnel to be fairly smooth. (B) The GuHCl-denatured ensemble at pH 7 with several large traps due to misligation by histidine residues. These misligated intermediates can be structurally similar to the native state as indicated by the depth of their minima in the funnel. (C) The acid-induced unfolded ensemble with modest traps caused by chemical frustration. These frustrated traps are due to nonnative electrostatic interactions that result from protonation of residues at low pH.