The low temperature magnetic circular dichroism spectra of iron-sulphur proteins. I. Oxidised rubredoxin.

Variable temperature magnetic circular dichroism spectra have been measured on oxidised Clostridium pasteurianum rubredoxin. Evidence has been obtained for the presence of two one-electron charge-transfer transitions, sulphur to ferric ion, in the region 15 000 to 28 000 cm-1. The first moment of the lower energy band is consistent with it being the orbital transition t1 non-bonding sulphur orbital, to the 2 e ferric d-orbital. The magnitude of the spin-orbit coupling constant in the lower excited state has been determined and shown to be small compared with the axial distortion. The splitting of the low energy band observed in the absorption spectrum can therefore be equated directly with the axial distortion of the lowest excited charge-transfer state. Finally, the potential utility of making saturation experiments at very low temperatures has been examined.