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

Area of Science:

• Electrochemistry
• Surface Science
• Biotechnology

Background:

• Oriented coupling of redox enzymes to electrodes is crucial for biosensor development.
• This coupling often involves reconstituting enzymes onto redox cofactors like hemin, which are conjugated to self-assembled monolayers (SAMs) on electrode surfaces.
• Designing effective SAMs that facilitate this reconstitution is a key challenge.

Purpose of the Study:

• To investigate how the composition and hemin surface density of binary SAMs affect electron transfer (ET) kinetics.
• To understand the relationship between SAM structure and the efficiency of hemin cofactor accessibility for enzyme reconstitution.
• To provide design principles for SAMs that enable effective on-surface reconstitution of heme proteins and enzymes.

Main Methods:

• Fabrication of binary SAMs on gold electrodes using 11-amino-1-undecanethiol (AUT) conjugated with hemin and diluting alkanethiols of varying lengths (MC11OH, MC6OH, MC2OH).
• Electrochemical characterization to determine heterogeneous electron transfer rate constants (ks) at different hemin surface densities (Γheme).
• Analysis of SAM structure-property relationships, including steric effects and electrostatic interactions, influencing ET and protein reconstitution.

Main Results:

• In AUT/MC11OH SAMs, electron transfer rate constants (ks) decreased with increasing hemin density (Γheme) due to steric crowding and lateral interactions.
• Shorter diluents (MC6OH, MC2OH) in binary SAMs resulted in higher and less Γheme-dependent ks values, indicating improved electron transfer.
• All studied SAMs exhibited steric hindrance for protein reconstitution due to the extended AUT linker; shorter diluents mitigated hemin oxidation interference.

Conclusions:

• SAM design significantly impacts electron transfer kinetics and the potential for enzyme reconstitution.
• Shorter, more rigid molecular wires with loosely dispersed redox centers are recommended for improved hemin cofactor accessibility and protein reconstitution.
• Optimized SAMs are essential for developing efficient electrode interfaces for heme protein-based biosensors and biocatalysts.