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Metalloproteins can be self-assembled in molecularly ordered, electrochemically addressable arrays. We report here on a study of the transport characteristics of the blue copper protein, azurin, from Pseudomonas aeruginosa, by a combination of electrochemical and scanning probe techniques (scanning tunnelling microscopy and conducting atomic force microscopy). Redox-switchable chemisorbed molecular arrays can be formed from both wild-type and mutant proteins using the strong affinity of cysteine residue thiolates for pristine gold surfaces. The molecular transconductance of single protein molecules within these arrays has been studied under controllable conditions where it has been additionally possible to resolve the effects of protein mechanical perturbation. Although tunnelling appears to be non-resonant and adequately explained through the use of a square barrier model, under some conditions the contribution of the redox-active copper centre to conductance is resolvable.

The interfacing of man-made electronic components with specifically-folded biomacromolecules lies central not only to the development of sensory interfaces and potential new molecular-scale devices, but also enables us to analyse processes of great biological importance in a refined and controllable manner. Recent advances in both available technology, most notably optical and scanning probes in nature, and our understanding of suitable methodologies, have led us to the point where the characteristics of single biological molecules can be interrogated with good levels of reproducibility. We review here the application of scanning probe microscopy to the analysis of and experimentation on biological redox systems. Within this paper the tunnel transport characteristics, as assayed by both scanning tunnelling microscopy (STM) and conducting probe atomic force microscopy (AFM), of single metalloproteins are discussed. In a specific case study the electron transfer characteristics of the blue copper metalloprotein, azurin, are reported. The modulation of these properties under the influence of calibratable compressional force has also been examined in some detail. Work such as this enables one to reproducibly establish the conductance, barrier height, environmental sensitivity and electromechanical properties of these molecules.