Conductive atomic force microscopy study of plastocyanin molecules adsorbed on gold electrode

The electrical conduction of mutant plastocyanin molecules self-assembled on gold was measured by conductive atomic force microscopy. The copper protein molecules, able to bind to gold through an engineered S-S group, were immobilised on Au(1 1 1) substrates to form a dense monolayer, which was contacted by a conductive atomic force microscopy tip at controlled force. Specifically, the electronic conduction of the gold coated tip-plastocyanin-Au(1 1 1) junction was well characterised between +1 and −1 V for increasing compressional forces. The conduction varies slightly within the force range of 3-9 nN, while it rapidly increases above these force values. The occurrence of a jump in mutant plastocyanin conduction at a critical force value, suggests that the current transport mechanism through the bioelectronic junction can be dominated by protein mechanical characteristics and/or by considerable variations in the protein conduction upon molecular deformation. By operating in imaging mode, we have obtained good topographic images of the protein self-assembled on Au(1 1 1) surface and for the first time simultaneous current images were recorded. Remarkably, a correspondence between the biomolecules observed in the topography images and the current spots was found for negative bias.