A Mössbauer study of the three spin states of the reduced selenium-substituted 2[4Fe-4Se]+ ferredoxin from Clostridium pasteurianum was carried out at T = 1.6 K, in perpendicular and parallel applied magnetic fields of up to 10 T. In low (0.1 T) applied fields, the spectra of the classical S = 1/2 state exhibit magnetic hyperfine patterns arising from two pairs of iron atoms with opposite hyperfine fields. The S = 7/2 state is in the slow relaxation limit at T less than or equal to 4 K in the absence of applied field. The corresponding Mössbauer spectrum can be understood in terms of anti-parallel coupling between one high-spin Fe3+ ion (Hhf = +21.2 T) and three high-spin Fe2+ ions (Hhf, xy = -25.5 T, Hhf,z = -28.5 T). For the S = 3/2 spin state, the use of high (8-10 T) applied fields was necessary to ensure the validity of the high-field approximation and overcome the intercluster spin-spin interaction. The spectral data obtained under such conditions allowed the determination of the hyperfine field (-4.2 T) and a tentative estimation of the zero-field splitting (D approximately less than 3 cm-1).

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