Engineering Self-Assembling Peptides to Tune the Coordination Environment of Metalloporphyrins The document discusses how self-assembling peptides can be designed to control the coordination environment of bound metalloporphyrins and tune their properties. Three peptides were designed with different metal-binding sites: one with a single histidine, one with two histidines, and one with a histidine and methionine. Spectroscopic analysis showed the single histidine and double histidine peptides bound heme in a low-spin state while the histidine-methionine peptide bound it in a high-spin state. Furthermore, the double histidine peptide could reversibly bind oxygen like natural proteins.