Cyclic and Hairpin Peptide Complexes of Heme

We have synthesized and characterized a new class of heme−peptide complexes using disulfide-linked hairpin-turn and cyclic peptides and compared these to their linear analogues. The binding affinities, helicities, and mechanism of binding of linear, hairpin, and cyclic peptides to [FeIII(coproporphyrin-I)]+ have been determined. In a minimalist approach, we utilize amphiphilic peptide sequences (15-mers), where a central histidine provides heme ligation, and the hydrophobic effect is used to optimize heme−peptide complex stability. We have incorporated disulfide bridges between amphiphilic peptides to make hairpin and even cyclic peptides that bind heme extremely well, roughly 5 × 106 times more strongly than histidine itself. CD studies show that the cyclic peptide heme complexes are completely α-helical. NMR spectra of paramagnetic complexes of the peptides show that the 15-mer peptides bind sequentially, with an observable monopeptide, high-spin intermediate. In contrast, the cyclic peptide complexes ligate both imidazoles cooperatively to the heme, producing only a low-spin complex. Electrochemical measurements of the E 1/2 of the FeIII(coproporphyrin-I)+ complexes of these peptides are all at fairly low potentials, ranging from −215 to −252 mV versus NHE at pH 7.