Temperature dependence of the electronic spin-lattice relaxation time in a 2-iron-2-sulphur model complex.

The complex [Fe2S2(S2-o-xylyl)2]2- in DMF (dimethylformamide), DMSO (dimethylsulphoxide) or a 1:1 DMF/DMSO mixture, a model for the chromophore in the 2Fe-2S proteins (ferredoxins), has been reduced and studied by conventional EPR over a temperature range. The low-field feature of the spectrum, Hz, has been computer simulated in order to analyse the lineshape in terms of a convolution product of Lorentzian and Gaussian distributions. The Gaussian contribution to the linewidth and a fixed part of the Lorentzian contribution, which is a function of the solvent and the way it freezes, were measured at a low temperature (less than or equal to 100 K) and subtracted from the linewidths in the higher-temperature range (130-200 K). The variable Lorentzian contribution thus obtained was related to spin-lattice relaxation times. The spin-lattice relaxation times of the sample having 1:1 DMSO/DMF solvent were measured in the range 6 to 11 K by the saturating pulse technique and in the range 10 to 65 K by continuous saturation methods. Up to 65 K the results follow the law 1/T1 alpha T4.5, a relationship which is not readily interpreted in terms of a simple Debye model. At higher temperatures the results may be interpreted in terms either of a dominant Orbach mechanism involving excited states at approx. 900 +/- 50 cm-1 (DMSO, DMF) or 770 +/- 50 cm-1 (1:1 DMSO/DMF), or of a Raman process in which 1/T1 alpha T7.5. The former is compatible with the two-phonon process already described in some ferredoxins, especially those with little anisotropy (gy - gx approximately 0.0) which have characteristically high [J] values.