Iron-57 nuclear shieldings as a quantitative tool for estimating porphyrin ruffling in hexacoordinated carbonmonoxy heme model compounds in solution

⁵⁷Fe NMR spectra of several carbonmonoxy hemoprotein models with varying polar and steric effects of the distal organic superstructure, constraints of the proximal side, and porphyrin ruffling are reported. ⁵⁷Fe shieldings, δ(⁵⁷Fe), vary by nearly 900 ppm among various hemes, and an excellent correlation was found between δ(⁵⁷Fe) and the absolute crystallographic average displacement of the meso carbon atoms, |Cm|, relative to the porphyrin core mean plane. The great variation of δ(⁵⁷Fe) as a function of |C(m)| (~140 ppm/0.1 Å) demonstrates that iron-57 shieldings can be used in structure refinement protocols for the extraction of more accurate structures for heme rings in heme model compounds. The excellent correlation between iron-57 shieldings and the average shieldings of the meso carbons of the porphyrin skeleton of TPP derivatives suggests that the two probes reflect similar types Of electronic and structural perturbations, which are primarily due to porphyrin ruffling. The present findings also emphasize the value in predicting ⁵⁷Fe shieldings in superstructured metalloporphyrins from ¹³C shieldings of the meso carbons.