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 relation to fundamental ET theory, and mapped in considerable detail. Adsorption and interfacial electrochemical ET of metalloproteins at metallic surfaces is also broadly important in a range of contexts, and has been addressed by spectroscopic, voltammetric, and thermodynamic methods. 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, sample preparation, mobility, and adsorbate stability are, however, particular problems. We illustrate here the perspectives by recent in situ STM imaging of covalently bound horse heart cytochrome c on polycrystalline platinum, and of chemisorbed Pseudomonas aeruginosa azurin on Au(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 natural unit for gentle linking, facile ET routes through the protein, and tunnel enhancement by the low-lying redox level of the copper atom. The particular electronic-vibrational three-level configuration in in situ STM of metalloproteins, finally, offers a new way of distinction between superexchange, coherent, and sequential ET modes in the long-range ET patterns of metalloproteins. (C) 1998 Elsevier Science Ltd. All rights reserved.