How fast can a protein fold? The rate of initial collapse from the unfolded state to a compact structure provides one upper limit to the folding rate. Although hydrophobic collapse of heteropolymers is not well understood, its rate may be controlled by the rate at which contacts form between distant parts of an unfolded polypeptide chain. The rate of this intrachain diffusion has not been measured directly. However, information about that time scale is contained in the experimental results of Jones et al. (Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 11860), who triggered the folding of reduced cytochrome c by nanosecond photolysis of the carbon monoxide complex. Jones et al. found that the methionine residues at positions 65 and 80 bind to the heme at position 18 at a rate of (40 mu s)(-1), while the histidine residues at positions 33 and 26 bind at a rate of (400 mu s)(-1). To identify the separate contributions of chain dynamics and chemical bond formation (i.e, the geminate binding rates) to the observed rates, we have used nanosecond-resolved absorption spectroscopy to determine the bimolecular and geminate rates for free methionine and histidine binding to the hemepeptide of cytochrome c under the solvent conditions of Jones et al. The rate of his33 (and his26) binding to the heme of the intact polypeptide appears to be limited by a slow geminate rate and by the equilibrium probability that the required loop will form. spontaneously. In contrast, the binding of met65 and met80 is rate limited only by the diffusion of the polypeptide chain to form an encounter complex. The (40 mu s)(-1) rate observed by Jones et al. therefore allows us to calculate the met80-his18 intrachain diffusion rate k(D+) approximate to (35-40 mu s)(-1). From this result we estimate that the smallest intrachain loops in a polypeptide will form in no less than similar to 1 mu s. This may set a limit of similar to 10(6) s(-1) on the rate of collapse of the polypeptide chain under folding conditions. We also use the theory of Szabo et al. (J. Chem. Phys. 1980, 72, 4350) to calculate the relative diffusion constant D of the heme and methionine residues, obtaining a value D approximate to 4 x 10(-7) cm(2)/s.