An approach to long-range electron transfer mechanisms in metalloproteins:In situ scanning tunneling microscopy with submolecular resolution

In situ scanning tunneling microscopy (STM) of redox molecules, in aqueous solution, shows interesting analogies and differences compared with interfa cial electrochemical electron transfer (ET) and ET in homogeneous solution. This is because the redox level represents a deep indentation in the tunne l barrier, with possible temporary electronic population. Particular perspe ctives are that both the bias voltage and the overvoltage relative to a ref erence electrode can be controlled, reflected in spectroscopic features whe n the potential variation brings the redox level to cross the Fermi levels of the substrate and tip. The blue copper protein azurin adsorbs on gold(11 1) via a surface disulfide group. Well resolved in situ STM images show arr ays of molecules on the triangular gold(111) terraces. This points to the f easibility of in situ STM of redox metalloproteins directly in their natura l aqueous medium. Each structure also shows a central brighter contrast in the constant current mode, indicative of 2- to 4-fold current enhancement c ompared with the peripheral parts. This supports the notion of tunneling vi a the redox level of the copper atom and of in situ STM as a new approach t o long-range electron tunneling in metalloproteins.