Promotion of direct electron transfer to cytochrome c by functionalized thiophene-based conducting polymers

Controlling direct electron transfer (DET) to redox proteins is of great interest for fundamental studies on biochemical processes and the development of biotechnological devices, such as biosensors or enzymatic fuel cells. Cytochrome c is a classical model protein for studying DET reactions that plays a key role in the onset of cellular apoptosis and the mitochondrial respiratory chain. In this contribution, we explored DET between cyt c and conducting polymers bearing the chemical structure of thiophene, specifically PEDOT, and its OH-containing derivative, PHMeEDOT. The combination of electrochemistry and in situ FTIR spectroscopy allowed us to gain more insight into the inner mechanism of DET at physiological pH. Hydrophilic interactions favour the correct orientation of the heme crevice of cytochrome c towards the polymer surface. When a positive charge is injected into the conducting polymer, the increasing electrostatic repulsion between protein and surface induces the desorption of lysine residues near the heme group and stimulates protein flipping. This effect was more pronounced at PEDOT- than PHMeEDOT-modified electrodes since the latter shows stronger interactions with lysine residues, partially hindering protein rotation at moderate potential. The potential-induced reorientation process was similar on both polymer surfaces, only at high positive potentials.