GENERAL MAGNETIC DIPOLAR INTERACTION OF SPIN-SPIN COUPLED MOLECULAR DIMERS - APPLICATION TO AN EPR-SPECTRUM OF XANTHINE-OXIDASE

An analysis is given of the general spin-spin dipolar interaction in the presence of a general bilinear superexchange interaction, and its effects on a rigid lattice EPR line shape are calculated. The general magic angle effect is considered, numerically, for specific cases of both single crystal spectra and randomly oriented microcrystallite samples. Analysis of the number of determinable spin-spin coupling coefficients suggests that a component analysis can be developed which can yield useful information about the geometrical arrangement of two interacting spin = 1/2 molecules, as well as covalency information about the interaction. This analysis is applied to a Mo(V)-iron sulfur interaction in xanthine oxidase and xanthine dehydrogenase, which has been described by Lowe and Bray. Line shape calculations show that the observed small amount of dipolar anisotropy is consistent with a separation of the Mo(V) and Fe/S groups of 14 Å or less. Considerations of the long-range nature of superexchange, from existing experimental data on superexchange, are consistent with this estimate. © 1979 American Chemical Society.