Model hemoprotein reduction potentials: the effects of histidine-to-iron coordination equilibrium.

Equilibrium reduction midpoint potentials of natural hemoproteins are influenced by a variety of factors, including the identity,2 alignment,3 and basicity4 of the axial ligands, nonplanar distortions of the heme,5 local charges,6 and proton coupling.7 Heme solvent exposure makes a particularly large contribution.8 Specifically, it has been shown that decreasing solvent exposure correlates with increasing reduction potential (greater ease of reduction) due to the energetic penalty of burying formally charged ferric heme in a low dielectric environment.8a In this report, we provide evidence that the effects of heme Fe(III) and Fe(II) coordination equilibria effectively compete with the observed solvent exposure trend in hemoprotein models having highly solvent-exposed heme. Histidine (His) to Fe(III) coordination in 1+ is accompanied by modest peptide helix induction (to ca. 36% from 0%) at 25 °C in aqueous solution as determined by circular dichroism spectroscopy.9 In 2+, alanine-4 in each peptide chain has been replaced by tryptophan (Trp).10 NMR experiments on the diamagnetic Co(III) analogue of 2+ have shown that the Trp side chains T-stack with the porphyrin,10 thereby shielding a substantial portion of the porphyrin from contact with solvent. One result of these interactions is a markedly higher peptide helix content in 2+ (ca. 74%) at 25 °C.9 The amino acid side chains in 1+ have a smaller hydrophobic surface area than those in 2+ and, hence, the porphyrin environment in 1+ is expected to be more polar. Based purely on heme solvent exposure precedents in the literature,8 we therefore expected that the reduction potential of 2+ would be shifted positive relative to 1+.