The molecular structure of cytochrome c3 isolated from Desulfovibrio desulfuricans has been solved on the basis of its crystallographic determination at 2.5 A resolution and of an essentially complete sequence. The molecule consists of a single polypeptide chain wrapped around a very compact core of four non-parallel haems which present a relatively high degree of exposure to the solvent. Alignment of the amino acid sequences of cytochrome c3 from several species suggests that the structure reported here is characteristic of the cytochrome c3 group.

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Two new dinuclear complexes of formula [M 2 (dcpz) 2 (H 2 O) 4 ] in which M is nickel(II) (1) or cobalt(II) (2) and dcpz is the bridging ligand 1,4-dicarboxylatopyridazine have been synthesized and characterized. The crystal structure for the nickel compound has been solved. Complex 1 crystallizes in the orthorombic system, space group Pbca(61), with FW ) 521.7, a ) 14.501(3) Å, b ) 13.169(3) Å, c ) 8.774(2) Å, V ) 1675(2) Å 3 , Z ) 4, and R ) 0.022. The cobalt compound is isomorphous to 1. The pyridazine ligand acts as a double bridge between the two paramagnetic centers. Magnetic measurements show moderate antiferromagnetic coupling, with J values of -33.6 cm -1 for 1 and -11.5 cm -1 for 2. Magnetostructural correlations have been obtained by extended-Hu ¨ckel MO calculations, and several electronic or structural factors that influence the coupling were analyzed.