Multinuclear ( 13C, 17O, 57Fe) NMR studies of carbonmonoxy heme proteins and synthetic model compounds

Abstract

13C, 17O and 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. Both heme models and heme proteins obey a similar excellent linear δ( 13C) versus ν(C–O) relationship which is primarily due to modulation of π-back-bonding from the Fe d π to CO π * orbital by the distal pocket polar interactions. The lack of correlation between δ( 13C) and δ( 17O) suggests that the two probes do not reflect a similar type of electronic and structural perturbation. δ( 17O) is not primarily influenced by the local distal field interactions and does not correlate with any single structural property of the Fe–C–O unit; however, atropisomerism and deformation of the porphyrin geometry appear to play a significant role. 57Fe shieldings 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, | C m|, relative to the porphyrin core mean plane. 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 a similar type of electronic and structural perturbation which is primarily porphyrin ruffling.