Oxidation-Reduction Reactions in Solution and at Electrodes

Anyone who has worked with the catecholamines knows how readily they are air-oxidized. Indeed, a variety of oxidants other than oxygen will rapidly react with catecholamines (CAs) in the following fashion  

The oxidants O2, Fe(CN)e and I2 all carry out the same reaction with the CA, producing an orthoquinone derivative and releasing two electrons. 

What is the driving force that enables certain oxidants (but not others such as Sn and Cr ) to carry out this electron transfer The answer is, of course, well known. In quantitative terms, the energies of the unfilled electron orbitals of the oxidants lie sufficiently below those of the filled 

These same reactions can be carried out with more finesse using an electrode. The electrode material needs to be some inert electron conductor. The electron energy state in such a material can be influenced by imposing a source of potential on the electrode. If we make an electrode sufficiently positive (by, for example, connecting it to the positive pole of a small battery), it can act as an oxidant and electrons will be transferred to it. The electron current can be measured and is directly proportional to the number of molecules oxidized. By varying the amount of positive potential applied, one has the equivalent of a variable-strength oxidant. 

The electrode has particular advantages for neurochemical applications. It would be cumbersome at best to titrate CAs oxidatively as they pass from the end of a chromatographic column and impossible to do so in a living brain system. But, in principle, we can make an electrode of any size and shape, place it in the flowing eluent from a column or implant it in the CNS, and continuously or intermittently monitor the current and thus the concentration of electro-oxidizable species. Electrons cannot be transferred in solution over distances greater than a few molecular diameters, and this fact means that electroactiVe species must be very close to the surface of an electrode to undergo reaction. This finite spatial resolution is especially useful for in vivo applications. With very small electrodes and short electrolysis times, minute brain regions can be sampled. 

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