RUBREDOXIN, A SIMPLE IRON-SULFUR PROTEIN : ITS SPIN HAMILTOMAN AND HYPERFINE PARAMETERS

Rubredoxin (Rd) from Clostridium pasteurianum is a protein that contains, in its redox active center, a high-spin iron atom coordinated to four cysteine sulfurs of the polypeptide chain. The roughly tetrahedral FeS4 complex of Rd is the basic structural unit of the active centers in the more complicated iron-sulfur proteins that contain linked clusters of two and four of these tetrahedra. Under suitable conditions Rd shows Mossbauer spectra with well resolved magnetic and electric hyperfine splittings both in the oxidized and reduced state. These spectra have been parametrized using the spin Hamiltonian X = D(S} — S (S + l)/3 + X (Si — Si)) + p S.J.H + S.I.I + X Q. At low temperatures the Mossbauer spectra of oxidized Rd, S = 5/2, consist of a superposition of three resolved component spectra due to the individual Kramers doublets. Only minor relaxation effects are observed in frozen solution at temperatures up to 20 K. The parameters D = 2.7 K, X = 0.23, g = 2, A = — 22.5 MHz, A = — 0.5 mm/s, n = 0.2 provide a first approximation of the spectra ; further corrections are discussed. The same Hamiltonian reproduces the 4.2 K, high- field spectra of reduced Rd, S = 2, with D = 10.9 K, X = 0.28, g = (2.1, 2.2, 2), A = — (27.6, 11.4, 41.3 )MHz, A = — 3.25 mm/s, n = 0.65, in the slow relaxation limit.