Electron-transfer pathways between redox enzymes and electrode surfaces: Reagentless biosensors based on thiol-monolayer-bound and polypyrrole-entrapped enzymes

Based on previous results which showed that quinohemo-protein alcohol dehyd rogenase (QH-ADH) entrapped within polypyrrole is able to directly transfer electrons via the conducting polymer to the electrode surface, the electro n-transfer properties of this multi-cofactor enzyme adsorbed and covalently -bound to self-assembled thiol monolayers and bare electrode surfaces has b een investigated more closely. While the dissolved enzyme is able to transf er electrons to the electrode via heme c as well as via the more deeply bur ied PQQ (fast adsorption-chemical reaction-desorption mechanism), an orient ation of adsorbed QH-ADH on hydrophobic electrode surfaces, as well as of a dsorbed and covalently bound QH-ADH on negatively-charged thiol monolayers could be observed. In these cases the heme c units are pointing towards the electrode surfaces resulting in an optimised direct ET rate.