The design and characterization of two synthetic peptides that self-assemble into heme-binding proteins are described. The peptides are intended to fold into a four-helix bundle and bind a single heme parallel to the helices in the bundle core using two histidine side chains as ligands. Both proteins bind a single heme in the binding pocket. In one protein there are comparable amounts of low- and high-spin hemes, while in the other low-spin heme predominates. In both proteins, the EPR spectra of the low-spin heme indicate bis-imidazole ligation. The results illustrate that subtle differences in packing, binding pocket flexibility, and ligand orientation can significantly influence the characteristics of functionalized peptides.