g̃‐tensor calculation for FeIIItriads: Application to the 3Fe clusters in iron–sulfur proteins

The g tensor of a triad of high spin FeIII ions coupled by antiferromagnetic exchange interactions, is calculated in the case where the fine structure terms are not negligible compared to the exchange parameters. Using some simplifying assumptions concerning the relative orientations of the magnetic axes, we obtain the expressions of the g components in closed form. A simple ‘‘g strain’’ model based on these expressions is used to simulate the EPR spectra given by the 3Fe clusters of four different iron sulfur proteins. In each case, a good simulation is only observed if the three exchange parameters are nearly equal, and of the order of −20 cm−1. These conclusions are in good agreement with our earlier estimations which were obtained by an independent method [J. P. Gayda, P. Bertrand, F‐X. Theodule, and J. J. G. Moura, J. Chem. Phys. 77, 3387 (1982)].