On the prosthetic group(s) of component II from nitrogenase

Abstract
The EPR spectrum of the reduced Fe-protein from nitrogenase has been reinvestigated. The dependences on temperature, microwave power, and microwave frequency all suggest that the observed signal represents a magnetically isolated [4Fe-4S]1+(2+;1+) cluster. Also, the signal can be simulated assuming a simple, gstrained S = system. However, the integrated intensity amounts to no more than 0.2 spins per protein molecule. It is, therefore, impossible that Fe-protein preparations contain a single type of [4Fe-4S] cluster.