Direct electrochemistry of cytochrome c on a phosphonic acid terminated self-assembled monolayers

The direct electrochemistry of cytochrome c (cyt c) on a gold electrode modified with 3-mercaptopropylphosphonic acid [HS-(CH(2))(3)-PO(3)H(2), MPPA] self-assembled monolayers (SAMs) was for the first time investigated. Electrochemical measurements and surface-enhanced infrared absorption spectroscopic reveal that the adsorption kinetics of cyt c on the MPPA-SAMs is very fast (saturation adsorption is completed within 5s) and the immobilized cyt c molecules retain their native secondary protein structure. The nature of interaction between cyt c and -PO(3)H(2) groups is mainly the electrostatic interaction. The direct electrochemistry of the immobilized cyt c on the -PO(3)H(2) terminated SAMs with short chain is nearly reversible. Its formal potential (E(0)'=18+/-3 mV vs. SCE) is very close to that of cyt c in an aqueous solution (E(0)'=18-22 mV vs. SCE). In addition, the electron transfer rate of cyt c immobilized on -PO(3)H(2) terminated SAMs is relatively slow as compared to -SO(3)H and -COOH terminated SAMs, indicating excess negative charge density on the SAMs surface will decrease the electron transfer rate of cyt c.