Antisymmetric Exchange in [2Fe−2S]1+ Clusters:  EPR of the Rieske Protein from Thermus t hermophilus at pH 14

• Published in: Journal of the American Chemical Society 2004-05, Vol.126 (17), p.5338-5339
• Main Authors: Tiago de Oliveira, Filipe, Bominaar, Emile L, Hirst, Judy, Fee, James A, Münck, Eckard
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja031746a

[2Fe−2S] clusters found in the xanthine oxidase family of proteins exhibit an S = 1/2 EPR feature, called signal II, for which one g-value is significantly above g = 2.0. The g-values of signal II cannot be explained with the standard spin coupling model that has been so successful in describing the g = 1.94 signals of [2Fe−2S] ferredoxins. We have studied the EPR spectra of the Rieske protein from Thermus thermophilus at pH 14 and observed a signal II-type EPR spectrum, with g-values at 1.81, 1.94, and 2.14. It is shown that the g-values of signal II can be explained by including an antisymmetric exchange term, d·S 1 ×S 2 , in the spin Hamiltonian. The presence of this term is sensed by EPR if the isotropic exchange coupling constant J is sufficiently small. For the Rieske protein we determined J = 43 cm-1 which is at least 4 times smaller than the J values reported for [2Fe−2S] clusters that yield standard g = 1.94 signals.