| Abstract
| - The early dynamics of reduced cytochrome c (cytc) folding initiated by a photoinduced electron-transfer reaction are studied using time-resolved circular dichroism spectroscopy. At 3.5 M GdnHCl oxidizedcytc is partly unfolded, whereas the reduced form is folded. Thus, under these conditions, rapid electron injectioninto unfolded, oxidized cytc triggers folding. The kinetics of secondary structure formation in reduced cytcoccurs in two major phases. The earliest detection of reduction is accompanied by the appearance of 20% ofthe secondary structure within 5 μs. From time-resolved absorption and circular dichroism studies this rapidfolding is ascribed to a subpopulation of unfolded protein molecules that have a structural tendency to forma native His18-Fe(II)-Met80 ligation. Thereafter, almost no change in the secondary structure is observeduntil the CD signal starts to decrease in magnitude between 16 μs and 1 ms. This “unfolding” phase (τ ≈ 180μs) is followed by a small increase in the magnitude of the CD signal (τ = 6 ms), forming 30% of the nativesecondary structure. It appears that a second subpopulation that is initially trapped by a His18-Fe(II)-His26/33non-native ligation slows down folding until His is displaced by the Met ligand (τ = 110 ms). Formation of90−95% of the native reduced cytc secondary structure is then detected by ∼320 ms.
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