Spin lattice relaxation and exchange interaction in a 2-iron, 2-sulphur protein.

A two-iron-two-sulphur non-haem iron protein, the ferredoxin from Spirulina maxima, has been studied by means of electron paramagnetic resonance (EPR) in the range where the spectrum loses resolution with increasing temperature. The spin-lattice relaxation times were deduced from linewidths measured by spectral simulation and their variation as a function of temperature is interpreted in terms of an Orbach mechanism. On this basis, the exchange integral between the two iron atoms, assuming as antiferromagnetic interaction between them, is estimated to be - 83 cm-1.