Helix-loop-helix peptides containing 63 residues (HC4H2, HC4HC, HC5H), designated by their sequence and content of histidyl (H) and cysteinyl (C) residues, have been previously synthesized for the purpose of stabilizing certain bridged metal sites in proteins. These peptides bind one Fe4S4 cluster by means of a ferredoxin tricysteinyl consensus sequence and an additional Cys residue, and one Ni(II) atom (HC4H2, HC5H) in predesigned binding sites. In this investigation, the apopeptides and their Fe4S4 derivatives are shown to be relatively stable to unfolding by guanidine hydrochloride, indicating stability of secondary structure. With this property demonstrated, Ni(II) binding equilibria have been evaluated in the terms of site-specific (Scatchard model) and stepwise (stoichiometric) binding constants. Two peptides were designed to have preformed CysHis3 (HC4H2) and Cys2His2 (HC5H) binding sites. The data indicate one strong binding site in each peptide with preferred binding constants k1=4.4×105 M–1 (HC4H2) and 2.7×105 M–1 (HC5H). Based on X-ray absorption spectroscopic data, these binding steps are associated with the formation of the desired coordination units NiIICysHis3 and NiIICys2His2. For peptide HC4HC, k1=2.5×105 M–1, but the binding site could not be fully identified. Collective evidence from this and prior investigations supports the presence of the bridged assemblies NiII-(µ2-S·Cys)-[Fe4S4], stabilized by a scaffolding effect in peptides HC4H2 and HC5H. The assembly NiII-X-[Fe4S4] is the minimal structure of the A-Cluster of carbon monoxide dehydrogenase adduced from spectroscopic evidence; bridge X is currently unidentified. These results suggest that de novo designed peptides may serve as scaffolds for the construction of native bridged sites in proteins. Electronic supplementary material to this paper can be obtained by using the Springer Link server located at http://dx.doi.org/10.1007/s00775-001-0320-4.