613Cofactor 430 of methyl-coenzyme M reductase from Methanobacterium thermoautotrophicum was studied in both the extracted form in aqueous solution and protein-bound by using low-temperature magneticcircular-dichroism spectroscopy. In both forms the nickel was present as high-spin paramagnetic nickel(II), spin S = 1, subject to almost equal zero-field splitting (cofactor F430, D = + 9.0 cm-1, EID = 0; methylcoenzyme M reductase, D = + 8.5 cm-', IE/DI = 0.2). This suggests identical axial co-ordination by oxygen ligand(s) both in aqueous cofactor F430 and in the investigated state of the protein.