1H NMR investigation of the role of intrinsic heme versus protein-induced rhombic perturbations on the electronic structure of low-spin ferrihemoproteins:: Effect of heme substituents on heme orientation in myoglobin

Solution H-1 NMR spectroscopy has been used to characterize the cyanomet myoglobin complexes of a variety of chemically modified hemins in order to elucidate the importance of hemin peripheral electronic, relative to axial His imidazole-induced, rhombic perturbations in raising the orbital degeneracy of the pi-bonding d(xz),d(yz) orbitals. Variation of the hemin 2- and/or 4-position substituents among hydrogen, ethyl, vinyl, acetyl, and formyl groups leads to conserved molecular structure of the heme pocket and orientation of the major magnetic axis for the heme iron, but systematically perturbed heme methyl contact shift patterns. Two strongly rhombically perturbed hemins with single acetyl groups on either pyrrole I or II exhibit heme methyl contact shift patterns and characteristic deviations from Curie law that are very similar to that induced in pseudosymmetric hemins upon incorporation into metMbCN in the alternate orientations about the alpha,gamma-meso axis. The perturbation due to the 4-acetyl group and the axial His bond leads to increased contact shift spread and stronger deviations from Curie behavior compared to WT, indicative of an increased d(xz)/d(yz) spacing relative to WT. In contrast, the perturbation due to the 2-acetyl group and axial His nearly cancel, leading to a highly compressed methyl contact shift spread and weaker deviations from Curie behavior than WT. It is shown, moreover, that the larger d(xz)/d(yz) splitting with 4-acetylhemin, and the smaller splitting with 2-acetylhemin, relative to WT, result in the expected increase and decrease, respectively, for the axial His contact shift relative to WT. Comparison of the methyl shifts for 16 peripherally modified hemins as model compounds and incorporated into metMbCN shows that the rhombic influences are additive in each of the complexes. Thus, the present results show that chemical functionality of the heme periphery contributes to raising the orbital degeneracy of the heme iron and that such influences can account for orbital ground states that are not necessarily aligned with the axial His orientation. The range of variant 2- and/or 4-substitutions have led to equilibrium heme orientations that are largely the same as found in WT Mb, except for a 4-ethyl group, which favors the reversed heme orientation by 2:1.