Ultrafast signals in protein folding and the polypeptide contracted state

To test the significance of ultrafast protein folding signals (much less than 1 msec), we studied cytochrome c (Cyt c) and two Cyt c fragments with major C-terminal segments deleted, The fragments remain unfolded under all conditions and so could be used to define the unfolded baselines for protein fluorescence and circular dichroism (CD) as a function of denaturant concentration, When diluted from high to low denaturant in kinetic folding experiments, the fragments readjust to their new baseline values in a ''burst phase'' within the mixing dead time, The fragment burst phase reflects a contraction of the polypeptide from a more extended unfolded condition at high denaturant to a more contracted unfolded condition in the poorer, low denaturant solvent, Hole Cyt c exhibits fluorescence and CD burst phase signals that are essentially identical to the fragment signals over the whole range of final denaturant concentrations, evidently reflecting the same solvent-dependent, relatively nonspecific contraction and not the formation of a specific folding intermediate, The significance of fast folding signals in Cyt c and other proteins is discussed in relation to the hypothesis of an initial rate-limiting search-nucleation-collapse step in protein folding [Sosnick, T, R,, Mayne, L, & Englander, S, W. (1996) Proteins Struct. Funct. Genet. 24, 413-426].