Nonnatural amino acid ligands in heme protein design.

The de novo design, synthesis, and characterization of a four-alpha-helix bundle scaffold containing heme ligated by 4-beta-(pyridyl)-l-alanine (Pal) is presented. The protein scaffold is highly helical, stable, and conformationally specific in the apo-state. Incorporation of heme using the designed bis-Pal axial ligation is shown using UV-visible and EPR spectroscopies. The observed heme midpoint reduction potential, +58 mV versus SHE, is 287 mV (6.8 kcal/mol) higher than the analogous bis-histidine-ligated heme protein.