Electroreflectance spectroscopic study of the electron transfer rate of cytochrome c electrostatically immobilized on the ω-carboxyl alkanethiol monolayer modified gold electrode

Horse heart cytochrome c was immobilized electrostatically on self-assembled monolayers (SAMs) of carboxylic-acid-terminated alkanethiols, HS(CH₂)_nCOOH (n = 2, 10) on gold electrodes, The rate constants of the electrode reaction of cytochrome c at the SAM-modified gold electrodes were determined by the frequency dependence of a.c. modulated UV-visible electroreflectance signals. The apparent standard electrode reaction rate constants of cytochrome c were 880 s^-1 and 72 s^-1 at the HS(CH₂)₂COOH and HS(CH₂)₁₀COOH SAM modified gold electrodes respectively. The rate constant at HS(CH₂)₂COOH was much smaller than expected from Marcus theory. However, the rate constant at HS(CH₂)₁₀COOH could be explained in terms of the through-bond tunnelling mechanism with tunnelling parameter β = 8.2 nm^-1.