Voltammetric Probes of Cytochrome Electroreactivity:  The Effect of the Protein Matrix on Outer-Sphere Reorganization Energy and Electronic Coupling Probed through Comparisons with the Behavior of Porphyrin Complexes

Abstract

Using surface-modified electrodes composed of omega-hydroxyalkanethiols, an experimentally based value for the inner-sphere reorganization energy of the bis(imidazole)iron porphyrin system has been obtained by examining the solvent dependence of the reorganization energy of bis(N-methylimidazole)meso-tetraphenyl iron porphyrin. The value obtained (0.41 +/- 0.06 eV) is remarkably similar to values we have recently reported for the reorganization energy of cytochrome b(5) (0.43 +/- 0.02 eV) and cytochrome c (0.58 +/- 0.06 eV). This strongly suggests that the protein matrix mimics the behavior of a low dielectric solvent and effectively shields the heme from the solvent. The effect of the orientation of the heme relative to the electrode was also explored by sytematically varying the steric bulk of the axial ligands. On the basis of a good linear correlation between the electronic coupling and the cosine of the angle between the heme plane and the surface of the electrode, it is suggested that a parallel orientation of the heme yields a maximum in the electronic coupling. Relevance to interheme protein electron transfer is discussed.

Multifunctional Cytochrome c: Learning New Tricks from an Old Dog

“…A significant increase of λ was also reported by Waldeck, Murgida, and co-workers for a hexacoordinated form of hh-cyt c where the distal M80 ligand is replaced by an exogenous pyridinyl residue and its coexisting pentacoordinated form . The role of the protein matrix in minimizing λ of cyt c as compared to the free porphyrin was early recognized and assessed experimentally . Later studies focused on the effect of disrupting the protein matrix either by addition of denaturing agents or by point mutations .…”

“…448 The role of the protein matrix in minimizing λ of cyt c as compared to the free porphyrin was early recognized 462 and assessed experimentally. 463 Later studies focused on the effect of disrupting the protein matrix either by addition of denaturing agents or by point mutations. 464 It was found, for instance, that the value of λ rises in a sigmoidal fashion with increasing concentrations of urea due to augmented solvent exposure of the metal site in the partially unfolded protein.…”

Kinetics of Intramolecular Electron Transfer in Cytochrome bo3 from Escherichia coli

“…where l is a3 is the inner sphere reorganization associated with heme a 3 , l is a is the reorganizational component associated with heme a, l CuB is the inner sphere reorganization for Cu B , and l os is the outer sphere reorganizational energy attributed to the protein response to the change in the redox states of the two hemes. A recent voltametric study of six-coordinate lowspin hemes suggests that l is for the Fe 31 to Fe 21 transition is on the order of 0.4 eV (Blankman et al, 2000). Thus the value of l is a in bovine heart CcO can also be estimated to be 0.4 eV.…”

Electrochemistry of redox-active self-assembled monolayers

“…Cytochrome c is a small (12 kDa) electron transfer protein containing a heme redox center that has been the subject of extensive study [27,31,188,193,194,215–230]. Modification of electrodes for the immobilization of cytochrome c was first reported by Taniguchi in 1982 [231].…”