Marcus parabola and reorganization energies associated with redox change of electron-transfer proteins, detected by V–I characteristics of STM currents

An electron-transfer protein (ETP) was theoretically investigated by the V–I characteristics in the scanning tunneling microscopy (STM) where the ETP is placed between the metallic tip and substrate. The STM current rises nearly steplike at a threshold value of the tip bias potential ϕ in each of the positive and the negative region of ϕ. The ratio of these values enables us to determine the relative height of the active center (AC) within the ETP between the tip and the substrate. The differential conductance of the current makes a peak at these threshold values. Each shape of these peaks as a function of ϕ corresponds to the Marcus parabola measured electrochemically. Under photo-illumination, additional steplike rises of the STM current take place earlier than that mentioned above in each region of ϕ. In the case that the current is mediated by the reduced state AC− of the AC, a difference in ϕ between the threshold values for the steplike rises with and without photo-illumination enables us to directly measure the energy of reorganization associated with the redox change of AC−/AC. The free energy for an electron in the AC− can also be obtained. Measurable in the case that the current is mediated by the oxidized state AC+ are the reorganization energy associated with the redox change of AC+/AC and the free energy for a positive hole in the AC+.