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 delta(13C) versus nu(C-O) relationship that is primarily due to modulation of pi backbonding from Fe d pi to the CO pi* orbital by the distal pocket polar interactions. There is no direct correlation between delta(13C) and Fe-C-O geometry. The poor monotonic relation between delta(13C) and nu(Fe-C) indicates that the iron-carbon pi bonding is not a primary factor influencing delta(13C) and delta(57Fe). The delta(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.