FIGURE 8.

A schematic diagram showing the sodium perchlorate induced formation of I_C and I_S in aqueous buffer (a) and in the presence (b) of 3 m urea. The unfolded extended conformer (U) does not have any secondary structure although the cofactor heme is bound to the protein. U has been found to be in rapid equilibrium with a compact conformer (I_C) and the time constant of their interconversion is 50 μs. The addition of sodium perchlorate shifts the equilibrium toward I_C, which is accompanied by a decrease in r_H and an increase in F. The hydrophobic residues, which may be involved in the formation of I_C, are shown by space-filling spheres. The cofactor heme, because it is hydrophobic, may also contribute toward the formation of I_C. I_C has been shown to contain partial secondary structure (presumably at the N and C-terminal helix regions). Sodium perchlorate induced formation of I_C is a sharp cooperative transition with a defined transition state. The formation of the secondary structure (I_S) takes place gradually over an extended range of sodium perchlorate concentration. This is an extended homopolymer-like transition and shown using dotted lines. The formation of the native protein is slow and is not shown. No rapid equilibrium exists between the extended (U) and any I_C-like state in the presence of 3 m urea. A small extent of chain contraction (observed by the decrease in r_H) and an increase in the secondary structure formation occur slowly and simultaneously in the presence of urea. The structure of I_C (or I_S) in the presence of urea is not defined, although these states are more compact than U and more extended than N (or I_C observed without urea).