EXCHANGE COUPLING AND RESONANCE DELOCALIZATION IN REDUCED [FE4S4]+ AND [FE4SE4]+ CLUSTERS .1. BASIC THEORY OF SPIN-STATE ENERGIES AND EPR AND HYPERFINE PROPERTIES

We develop a theoretical spin-coupling model to describe the energies and properties of the spin eigenstates in reduced [Fe4S4]+ and [Fe4Se4]+ proteins and their synthetic analogues. Appropriate ranges for the resonance B parameter and the two Heisenberg J parameters can be estimated from our previous quantitative work on related [ZnFe3S4]+ systems. This phenomenological model provides a resonably good account of the EPR and hyperfine properties of typical S = 1/2 states of reduced clusters, and we show that it is superior to a Heisenberg model in accounting for these properties and site equivalence patterns. The theory is also successful in describing the coexistence of S = 1/2 and 7/2 states in reduced Se ferredoxin, including the 3:1 site localization pattern and the corresponding hyperfine properties of the S = 7/2 state, as well as the delocalized S = 1/2 state. There is a competition between resonance delocalization (governed by the B parameter) and external localizing forces. The S = 7/2 state becomes favored over S = 1/2 when the ratio between the smaller Fe2+-Fe2+ J parameter and the larger Fe2+-Fe3+ J parameter is sufficiently small. There is a corresponding spin-state crossing point involving S = 1/2, 3/2, 5/2, and 7/2 in parameter space. Properties of S = 3/2 states are not well described by the present model; these are more accurately given by the more general model described in a companion paper. Nevertheless, the model outlined here provides a simple phenomenological framework for understanding many features of reduced iron-sulfur and iron-selenium clusters.