Proposal for direct measurement of reorganization energies in electron-transfer proteins by the V–I characteristics of photoinduced STM currents via large adsorbates with a redox active center

When an electron-transfer protein with a redox active center is adsorbed on the substrate, the STM current rises steplike at a threshold bias at low temperatures. This threshold is determined not only by the redox potential of the active center, but also by the energy λ of reorganization in the protein matrix upon the redox change. Under photoirradiation raising the active center to its excited state, the current will rise also at a smaller threshold bias. Being proportional to λ, the threshold difference enables us to directly obtain λ, which plays important roles in electron-transfer rates to or from the protein.