EPR monitoring of redox states

A redox reaction is a special case of the equilibrium reaction of A + B in Equation 13.1 B is now a reducible group in a biomolecule with an EPR spectrum either in its oxidized or in its reduced state (or both), and A is now an electron or a pair of electrons, that is, reducing equivalents provided by a natural redox partner (a reductive substrate, a coenzyme such as NADH, a protein partner such as cytochrome c), or by a chemical reductant (dithionite), or even by a solid electrode  

Of course free electrons do not exist in a regular laboratory, so Equation 13.6 is a theoretical half reaction of the full equilibrium  

Usually we know the properties of the electron donor, and we want to use EPR spectroscopy to determine those of the acceptor only. So we write down the Nemst equation for a single redox pair (replacing B with X)  

In other words, if we subject a homogeneous solution of A- to an electrochemical potential E, then the amplitude of the EPR spectrum from this (possibly frozen) solution will be given by Equation 13.12. If we make samples for several different values of E, then their collective EPR amplitudes make a graph of /red versus E that will define the value of the unknown E°, the standard reduction potential (biochemists call this the midpoint potential) of the XieA/Xox couple. 

FIGURE 13.1 An EPR-monitored redox titration of two [2Fe-2S] clusters. A 2Fe cluster in a ferredoxin (E° = -170 mV) and one in an oxygenase enzyme (E° = -20 mV) from Pseudomonas maltophilia were each titrated with dithionite in the presence of a mediator mix. Each point is the EPR amplitude from an individual sample drawn at the indicated solution ii-value. The fit is based on Equation 13.12. (Data from Chakraborty et al. 2005.) 

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