electrode with polymyxin as co-adsorbate, pH 7.6, temperature 0°C. Conditions (not optimized to reveal couples in clearest detail) are as follows. Top: Azotobacter vinelandii FdI; scan rate 100 mV s ; Center:

Provided interfacial electron exchange is fast, it is possible to make detailed studies of the kinetics and energetics of coupled reactions, and to detect and analyze how electron transfer in a protein may be ‘gated’ by other processes such as proton transfer. Whereas an electron can easily tunnel through 10 Å, the equivalent tunneling distance of a proton at the same energy is less than 0.25 Å [58]. This means that long-range proton transport through proteins, which is of great importance in the conservation of energy, depends on the existence of a chain of closely spaced donors and acceptors (these can be water molecules or amino-acid side chains) whose separation and pK values control the rate. Although redox-driven proton pumps are large membrane-bound enzymes, the use of voltammetry to study proton transfer is illustrated well in recent studies on Azotobacter vinelandii ferredoxin I.