¹H and ¹³C NMR investigation of the influence of nonligated residue contacts on the heme electronic structure in cyanometmyoglobin complexes reconstituted with centro- and pseudocentrosymmetric hemins

The ¹H and ¹³C chemical shifts for the heme methyls of low-spin, ferric sperm whale cyanometmyoglobin reconstituted with a variety of centrosymmetric and pseudocentrosymmetric hemins have been recorded and analyzed to shed light on the nature of heme-protein contacts, other than that of the axial His, that modulate the rhombic perturbation to the heme's in-plane electronic asymmetry. The very similar ¹H dipolar shifts for heme pocket residues in all complexes yield essentially the same magnetic axes as in wild type, and the resultant dipolar shifts allow the direct determination of the heme methyl proton and ¹³C contact shifts in all complexes. It is demonstrated that, even when the magnetic axes and anisotropies are known, the intrinsic uncertainties in the orientational parameters lead to a sufficiently large uncertainty in dipolar shift that the methyl proton contact shifts are inherently significantly less reliable indicators of the unpaired electron spin distribution than the methyl ¹³C contact shifts. The pattern of the noninversion symmetry in ¹³C contact shifts in the centro- or pseudocentrosymmetric hemes is shown to correlate with the positions of aromatic rings of Phe43(CD1) and His97(FG3) parallel to, and in contact with, the heme. These results indicate that such π-π interactions significantly perturb the in-plane asymmetry of the heme π spin distribution and cannot be ignored in a quantitative interpretation of the heme methyl ¹³C contact shifts in terms of the axial His orientation in b-type hemoproteins.