Induced paramagnetism and hyperfine interactions in the {[Fe4S4]-Fe} active site of Escherichia coli sulfite reductase

The spin-coupling model of the {[Fe4S4]-siroheme} center of the active site of sulfite reductase is developed. The tetramer-siroheme coupling 2Jths1·sh results in the induced paramagnetism of individual ions of the Fe/S tetramer which provides non-zero effective hyperfine constants A1,2 = −A3,4 = −338(Jth/J)ai, where J is the parameter of the Heisenberg pair-pair interaction in the Fe/S tetramer and ai is the local hyperfine constant. The siroheme g-factor contributes dominantly to the molecular g-factor, the tetramer contribution is small (≈(Jth/J)2, Jth/J ⪡ 1). The model explains the observed paramagnetism of the Fe/S component of the active site in Mössbauer, ENDOR and 1H NMR experiments and concurrently the diagmagnetism of this moiety in magnetic and EPR measurements.