Electron Transfer in Binary Hemin-Modified Alkanethiol Self-Assembled Monolayers on Gold: Hemin's Lateral and Interfacial Interactions.

Orientated coupling of redox enzymes to electrodes by their reconstitution onto redox cofactors, such as hemin conjugated to self-assembled monolayers (SAMs) formed on the electrodes, poses the requirements for a SAM design enabling reconstitution. We show that the kinetics of electron transfer (ET) in binary SAMs of alkanethiols on gold composed of in situ hemin-conjugated 11-amino-1-undecanethiol (AUT) and diluting OH-terminated alkanethiols with 11, 6, and 2 methylene groups (MC 11 OH, MC 6 OH, and MC 2 OH) depends on both the SAM composition and surface density of hemin, Γ heme . In AUT/MC 11 OH SAMs composed of equal linker/diluent lengths, the heterogeneous ET rate constant k s decreased with the Γ heme and varied between 70 and 500 s -1 . For shorter diluents, the k s of 245-330 s -1 (C 6 ) and 300-340 s -1 (C 2 ) showed a little (if any) Γ heme dependence. In AUT/MC 11 OH SAMs, the increasing Γ heme resulted in the steric crowding of hemin species and their neighboring lateral interactions in the plane of hemin localization, affecting the potential distribution at the SAM/electrode interface and inducing local electrostatic effects interfering with hemin oxidation. In AUT/MC 6 OH and AUT/MC 2 OH SAMs, hemin discharged at the plane of the closest approach to the gold surface, equal to the diluent length and permeable to electrolyte ions, which lessened those effects. All studied binary SAMs provided steric hindrance for protein reconstitution on the hemin cofactor conjugated to the extended AUT linker. Further use of SAM-modified electrodes with the covalently attached hemin as interfaces for heme proteins' reconstitution should consider SAMs with loosely dispersed redox centers terminating more rigid molecular wires. Such wires place hemin at fixed distances from the electrode surface and thus ensure the interfacial properties required for the effective on-surface reconstitution of proteins and enzymes.