Analysis of strain-induced EPR-line shapes and anisotropic spin-lattice relaxation in a [2Fe-2S] ferredoxin.

The electron paramagnetic resonance spectrum of the [2Fe-2S]1+(2+;1+) cluster in spinach-leaf ferredoxin has been measured at four microwave frequencies from 1 to 35 GHz. Using a modified g-strain formula, the asymmetrical spectrum has been simulated in detail without the assumption of signal multiplicity. In all but the lowest frequency bands the line width is dominated by an extremely anisotropic g-shift distribution, caused by a statistical distribution in dislocation strains. The crossover point of domination by unresolved proton splittings is around 2 GHz. The angle-dependent elasticity of the cluster can be related to an anisotropy in the spin-lattice relaxation rate. Intensity behaviour under continuous saturation, at temperatures in the two-phonon region, is in qualitative agreement with elementary theory. On the basis of these results it is argued that biochemists should be aware of the questionable nature of some ad hoc assumptions commonly made to interpret EPR of metalloproteins. Specifically, a physically meaningful determination of the number and stoicheiometry of distinguishable compounds, represented in a complex spectrum, may well require more advanced theoretical tools than the frequently employed deconvolution in symmetrical Gaussians with associated unique relaxation times.