In situ STM and AFM of the copper protein Pseudomonas aeruginosa azurin

Scanning tunnel (STM) and atomic force microscopy (AFM) in the in situ mode under potentiostatic control have opened new perspectives for mapping the two-dimensional organization of surface adsorbates in aqueous solution. In situ STM and AFM, however, also raise recognized problems. In the context of biological macromolecules, sample immobilization and the mechanism of the imaging process are, for example, outstanding issues. We have shown that the blue single-copper redox protein azurin is well suited for gentle surface immobilization and mapping. Azurin has a surface disulphide group which adsorbs to gold and facile electron tunnel routes between this group and the copper atom. Azurin adsorbed on Au(111) can be imaged to molecular resolution by in situ STM and shows regular arrays of individual structures corresponding well to the known molecular size of azurin. The current falls off approximately exponentially with increasing distance with a decay constant of 0.4-0.5 Angstrom(-1). In comparison in situ AFM shows structures laterally convoluted with the tip while the vertical extension is in the same range as the structural size of azurin. The results are of interest in relation to electron tunnel mechanisms of redox metalloproteins and in technological contexts such as electrochemical biosensors, microbial corrosion and broadly for protein adsorption from biological liquids. (C) 1997 Elsevier Science S.A.