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

Fe-57 NMR spectra of several carbonmonoxy hemoprotein models with varying p olar and steric effects of the distal organic superstructure, constraints o f the proximal side, and porphyrin ruffling are reported. Fe-57 shieldings, delta(Fe-57), vary by nearly 900 ppm among various hemes, and an excellent correlation was found between delta(Fe-57) and the absolute crystallograph ic average displacement of the meso carbon atoms, \C-m\, relative to the po rphyrin core mean plane. The great variation of delta(Fe-57) as a function of \C-m\ (similar to 140 ppm/0.1 Angstrom) demonstrates that iron-57 shield ings can be used in structure refinement protocols for the extraction of mo re accurate structures for heme rings in heme model compounds. The excellen t 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 perturbation s, which are primarily due to porphyrin ruffling. The present findings also emphasize the value in predicting 57Fe shieldings in superstructured metal loporphyrins from C-13 shieldings of the meso carbons