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 2J(th)s(1) . s(h) results in the induced paramagnetism of individual ions of the Fe/S tetramer which provides non-zero effective hyperfine constants A(1,2) = - A(3,4) = - 33/8(J(th)/J)a(i), where J is the parameter of the Heisenberg pair-pair interaction in the Fe/S tetramer and a(i) is the local hyperfine constant. The siroheme g-factor contributes dominantly to the molecular g-factor, the tetramer contribution is small (approximate to(J(th)/J)(2), J(th)/J << 1). The model explains the observed paramagnetism of the Fe/S component of the active site in Mossbauer, ENDOR and H-1 NMR experiments and concurrently the diamagnetism of this moiety in magnetic and EPR measurements.