Dynamic Ionic Strength Effects in Fast Bimolecular Electron Transfer between a Redox Metalloprotein of High Electrostatic Charge and an Inorganic Reaction Partner

The ionic strength dependence of the bimolecular electron transfer (ET) reaction between Clostridium pasteurianum rubredoxin(III) and photochemically generated [Ru(bpy)_3]^+ has been investigated. The reaction is fast and close to the diffusion-controlled limit. Dynamic ionic strength effects on all kinetics parameters (work terms, driving force, and reorganization free energy) have been incorporated in the data analysis; the effects on the intermolecular work terms are large and dominate in the driving force region close to the activationless limit. The variation in ET rate constant over the 0.005−2.0 M ionic strength range is quantitatively consistent with the high negative charge (−9e where e is the unit electric charge) and size (crystallographic radius ≈ 12 Å) of rubredoxin(III). If low ionic strength (0.005−0.01 M) data are omitted, the variation in rates suggests a somewhat smaller charge on the protein (−4e to −5e).