The reaction of small inorganic redox reagents such as hexaammineruthenium(II), a6Ru2+, and bis(2,2′:6,2″-terpyridine)cobalt(II), Co(terpy)22+, with metalloproteins can be a powerful tool to measure the dynamics of redox metalloproteins. By varying the concentration of a6Ru2+ or Co(terpy)22+, it is possible to move the reduction of the protein from a regime where the reaction is coupled to the conformational change to one where it is limited by the rate of the conformational change. Under optimal conditions, it is possible to extract both the forward and reverse rate constants for the conformational change of the metalloprotein from the data. If there are multiple conformational changes that occur on different time scales, a single experiment can extract data on the dynamics of all processes. We have applied this method to analyze the dynamics of Ω-loop D (residues 70 – 85) of mitochondrial cytochrome c. The dynamics of this surface loop are believed to be important for allowing access to conformations of cytochrome c that permit it to act as a peroxidase in the early stages of apoptosis. To probe the dynamics at different locations within Ω-loop D, we have engineered histidines at various positions in the loop. These variants allow the dynamics of the replacement of the Met80 heme ligand with histidine to be measured. In recent work, we have replaced Pro76 with histidine. Proline at this position would be expected to stiffen the protein backbone slowing dynamics. pH jump and gated electron transfer methods both show that breakage of the heme-Met80 bond is enhanced in the P76H variant of yeast iso-1-cytochrome c, consistent with this interpretation. These results will be discussed in light of results from previous histidine probes in this loop.