¹³C- and ⁵⁷Fe-NMR studies of the Fe-C-O unit of heme proteins and synthetic model compounds in solution: Comparison with IR vibrational frequencies and X-ray structural data

¹³C- and ⁵⁷Fe-NMR spectra of several carbon monoxide hemoprotein models with varying polar and steric effects of the distal organic superstructure, constraints of the proximal side, and solvent polarity are reported. The ¹³C shieldings of heme models cover a 4.0 ppm range that is extended to 7.0 ppm when several hemoglobin CO and myoglobin CO species at different pHs are included. Both heme models and heme proteins obey a similar excellent linear δ(¹³C) versus ν(C-O) relationship that is primarily due to modulation of π backbonding from Fe d_π to the CO π* orbital by the distal pocket polar interactions. There is no direct correlation between δ(¹³C) and Fe-C-O geometry. The poor monotonic relation between δ(¹³C) and ν(Fe-C) indicates that the iron-carbon π bonding is not a primary factor influencing δ(¹³C) and δ(⁵⁷Fe). The δ(⁵⁷Fe) was found to be extremely sensitive to deformation of the porphyrin geometry, and increased shielding by more than 600 ppm with increased ruffling was observed for various heme models of known X-ray structures.