DISTANCE DEPENDENCE OF LONG-RANGE ELECTRON-TRANSFER IN CYTOCHROME-C DERIVATIVES CONTAINING COVALENTLY ATTACHED COBALT CAGE COMPLEXES

Seven horse heart cytochrome c derivatives each with a single Co(diAMsar) cage complex covalently attached by carbodiimide coupling to a specific surface carboxylate side chain have been synthesized. Tryptic peptide mapping combined with Edman degradation and FAB-MS sequencing has identified the attachment sites in the seven derivatives to involve residues D2, E4, E21, E61, E62, E66, and E104. Characterization of these derivatives by UV-visible spectroscopy, circular dichroism spectroscopy, and differential pulse voltammetry suggests that the protein tertiary structure has changed little upon attachment of the Co(diAMsar) probe. Flash photolysis experiments using Ru(bpy)3(2+) as an excited-state reductant or oxidant allow the measurement of the rate constant for intramolecular electron transfer (k(et)) from Co(II)(diAMsar) to the Fe(III) of heme c in each derivative, resulting in values for k(et) ranging from 1.0 to 3.2 s-1 (at 25.0-degrees-C, pH 7.0). The time-averaged Co(diAMsar)-heme c edge-to-edge distances were estimated from solvated molecular dynamics simulations on structural models for the seven derivatives on the basis of crystal structure coordinates for horse heart cytochrome c and Co(diAMsar). These distances range from approximately 14 angstrom (in derivative E66) to approximately 20 angstrom (in derivative D2), which would predict a 200-fold range of rates assuming a simple exponential through-space distance dependence. The molecular dynamics trajectories were also used to calculate time-averaged dominant pathways for electron transfer1,2 in each derivative, resulting in the prediction of a 2000-fold range of rates. The relationship between the experimental intramolecular electron-transfer rate constants and the Co(diAMsar) attachment site is discussed in light of these predictions.