Bimolecular Electron Transfer in the Marcus Inverted Region

Abstract

The rates for the photoinduced bimolecular reactions of a homologous series of RuII diimines with cytochrome (cyt) c in its oxidized and reduced forms have been measured. The electronic coupling and reorganization energy of the system have been adjusted such that the inverted region may be accessed at reasonable driving forces. The electron transfer (ET) rate constants for *RuII diimine/FeIIcyt c reaction increase monotonically and approach the diffusion limit of 8.8 × 108 M-1 s-1 at ΔG° = −0.7 V. At a higher driving force, which may be accessed with the powerfully oxidizing *Ru(diCF3-bpy)3 2+, the rate for ET is observed to drop off. Similarly, the high driving forces achieved with *RuII diimine/FeIIIcyt c (−ΔG ≥ 1.12 V) are manifested in a decrease of the ET rate constant with increasing exergonicity. The observed ET rates for both systems are well described by a bimolecular model for ET occurring over an equilibrium distribution of reactant separation distances, each having a different formation probability and weighted by the first-order ET rate constant. The unique observation of bimolecular ET in the inverted region is not due to a peculiar reaction pathway engendered by the RuII diimines, which react as do other small-molecule cations at the solvent-exposed edge of the heme. The inherent ET properties of cyt c engender a Marcus curve that is displaced below the diffusion limit and shifted to smaller driving forces.

On the Coherent Description of Diffusion‐Influenced Fluorescence Quenching Experiments II: Early Events

“…(16)], in which I is the fluorescence intensity in the presence of a quencher of concentration c, and I 0 is the fluorescence intensity in the absence of a quencher.…”

Unified Theory of Photochemical Charge Separation

“…Such a system was first discovered and studied experimentally in Ref. 79. This is the photoinduced bimolecular reaction of a homologous series of Ru (11) diimines with cytochrome (cyt) c in its oxidized and reduced forms.…”

Evidence That a Miniature CuI Metalloprotein Undergoes Collisional Electron Transfer in the Inverted Marcus Region

“…Whereas the observation of decreasing ET rates with increasing driving force has long been observed in unimolecular, [7] or otherwise diffusionally restricted donor/acceptor systems, [8,9] such behavior has rarely been seen when the donor and acceptor complexes are allowed to diffuse freely in solution. [10,11] Our group recently reported [5,6] the metal-binding properties of the 32-residue polypeptide C16C19-GGY whose sequence is based on the (IEALEGK) n heptad repeat previously shown to self-assemble into a-helical coiled coils. [12,13] However, the third heptad repeat of C16C19-GGY was modified by substituting cysteine residues at positions 16 and 19 of the sequence to construct a potential metal-binding site within the hydrophobic core of the coiled coil.…”