Antiferromagnetic coupling, irons

The latter compound attracts special interest because it forms more rapidly in the absence of substrates (k = 1.2 s 1) than it autodecays (k = 0.05 s 1) and, therefore, can be directly investigated by physicochemical methods. The Mossbauer spectrum of compound Q from M. trichosporium indicates that the diiron center consists of two high-spin antiferromagnetically-coupled iron atoms, each in the Fe(IV) state bridged by oxygen atom. Compound Q reacts very quickly with methane and other substrates with the formation of compound T. The latter releases a product and is transformed to diferric MMOH. 

Two-iron two-sulfur (Fc2S2) clusters are present in several, ferredoxins. In the oxidized state they contain two high spin antiferromagnetically coupled iron (III)... 

The diiron center of nigerythrin has been studied by absorption and EPR spectroscopy [414], The low-temperature EPR spectrum showed three resonances at g < 2 (gav = 1-74), consistent with two antiferromagnetically coupled irons (Fe " with S = 5/2 and Fe with S = 2), yielding an Stat = V2 spin ground state. The spectram is reminiscent of those observed for mixed-valent MMO ( 2) and hemerythrin ( 8) [80,81]. EPR spectroscopy was also used to determine the redox potentials of the non-heme diiron center and the mononuclear iron of the FeS4 site in nigerythrin, which were found to be > -1-200 mV. 

The two iron ions of the Rieske cluster are antiferromagnetically coupled therefore, the ground state has a spin S = while excited states of the spin ladder S = I, i, I, and, are at energies -3J, 8J,... 

For all known cases of iron-sulfur proteins, J > 0, meaning that the system is antiferromagnetically coupled through the Fe-S-Fe moiety. Equation (4) produces a series of levels, each characterized by a total spin S, with an associated energy, which are populated according to the Boltzmann distribution. Note that for each S level there is in principle an electron relaxation time. For most purposes it is convenient to refer to an effective relaxation time for the whole cluster. 

Strongly supporting this spectroscopic data, Mossbauer spectroscopy of the as-isolated Rr shows the presence of two types of iron centers a magnetic component that can be well simulated by the parameters of Rd, and a diamagnetic component attributed to the diiron-oxo cluster and resulting from the antiferromagnetic coupling of the two irons. 

Mossbauer spectroscopy gives information on oxidation and spin states of iron-containing species and may diagnose antiferromagnetic coupling. 

---