Optical Triggers of Protein Folding

In our report about the electron-transfer (ET)-initiated folding of ferrocytochrome c (cyt c^(II)) (1), we noted rapid changes in the visible absorption spectrum corresponding to a process with a time constant of ~40 µs. We indicated that this observation was consistent with studies of cyt c^(ll) folding initiated by CO dissociation where the fast dynamics were attributed to changes in heme ligation (2). We also suggested that these dynamics might correspond to the collapse of the protein into a compact denatured state, as has been proposed for apomyoglobinon the basis of laser-temperature-jump measurements (3). Our transient absorption data could not distinguish between the two possibilities, and these fast folding dynamics were a relatively minor component of the study described in our report. Chan et al., with the use of tryptophan (Trp) fluorescence as a probe, found no significant collapse of the protein on the submillisecond time scale following dissociation of CO from unfolded cyt c^(II). Clearly, multiples pectroscopic probes must be employed to study protein folding; accordingly, we are currently developing time-resolved Trp fluorescence as a probe for ET-initiated folding of cyt c^(II)