Except when radiation participates, all biological activities involve contact interactions between constituent reactants. At the molecular level, if one of the participants is smaller than its complementary partner, the former is usually designated the ‘ligand’ and the latter the ‘receptor’. Thus in an enzyme–substrate complex, the substrate is the ligand, the enzyme is the receptor. In immunological interactions, the ligand is the hapten or antigen, the receptor is the immunoglobulin. Neurotransmitters or hormones are effector ligands when they are bound to receptor sites at synapses or on cell membranes.

Studies of electron-transfer reactions of redox proteins have, in recent years, attracted widespread interest and attention. Progress has been evident from both physical and biological standpoints, with the increasing availability of three-dimensional structural data for many small electron-transfer proteins prompting a variety of systematic investigations (Isied, 1985). Most recently, attention has been directed towards questions concerning the elementary transfer of electrons between spatially remote redox sites, and the nature of protein–protein interactions which, for intermolecular processes, stabilize specific precursor complexes which may be optimally juxtaposed for electron-transfer. These and other issues, including the necessary reversibility of protein interfacial interactions and the dynamic properties of proteins as carriers of electrons in biological electron-transport systems, are now being addressed in the rapidly emerging field of direct (unmediated) protein electrochemistry. It is our intention in this article to discuss developments made in this area and highlight points which we believe to have the most bearing on our current understanding of diffusion-dominated, protein-mediated electron transport at electrode surfaces. First we shall outline some basic considerations which are best considered with reference to homogeneous systems.