Diiron and dimanganese proteins catalyze a wide range of hydrolytic and oxygen-dependent reactions. To probe the mechanisms by which individual members of this class of proteins are able to catalyze such a wide range of reactions, we have prepared a model four-helix bundle with a diiron site located near the center of the bundle. The four-helix bundle is constructed by the noncovalent self-assembly of three different chains (A(a), A(b), and B) that self-assemble into the desired heterotetramer when mixed in a 1:1:2 molar ratio. On addition of ferrous ions and oxygen, the protein forms a complex with a UV-visible spectrum closely resembling that of peroxo-bridged diferric species in natural proteins and model compounds. By combining a small collection of n variants of these peptides, it should now be possible to prepare an n(3) member library, which will allow systematic exploration of the features giving rise to the catalytic properties of this class of proteins.