METALLOPROTEIN ADSORPTION ON AU(111) AND POLYCRYSTALLINE PLATINUM INVESTIGATED BY IN-SITU SCANNING-TUNNELING-MICROSCOPY WITH MOLECULAR AND SUBMOLECULAR RESOLUTION

Redox metalloproteins exhibit interesting features such as long-range electron transfer (ET), cooperative effects etc, of importance in rela tion to fundamental ET theory, and mapped in considerable detail. Adso rption and interfacial electrochemical ET of metalloproteins at metall ic surfaces is also broadly important in a range of contexts, and has been addressed by spectroscopic, voltammetric, and thermodynamic metho ds. In situ scanning tunneling (STM) and atomic force microscopy (AFM) have opened new perspectives for addressing adsorbed metalloproteins in their natural functional aqueous medium at the molecular level. In addition to broadly recognized problems of in situ STM/AFM imaging, sa mple preparation, mobility, and adsorbate stability are, however, part icular problems. We illustrate here the perspectives by recent in situ STM imaging of covalently bound horse heart cytochrome c on polycryst alline platinum, and of chemisorbed Pseudomonas aeruginosa azurin on A u(111). Molecular resolution is achieved, but azurin gives by far the best images which show, moreover, an interesting submolecular feature. This is likely to be associated with the disulphide group as a natura l unit for gentle linking, facile ET routes through the protein, and t unnel enhancement by the low-lying redox level of the copper atom. The particular electronic-vibrational three-level configuration in in sit u STM of metalloproteins, finally, offers a new way of distinction bet ween superexchange, coherent, and sequential ET modes in the long-rang e ET patterns of metalloproteins. (C) 1998 Elsevier Science Ltd. All r ights reserved.