C-13-NMR AND FE-57-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

C-13- and Fe-57-NMR spectra of several carbon monoxide hemoprotein mod els with varying polar and steric effects of the distal organic supers tructure, constraints of the proximal side, and solvent polarity are r eported. The C-13 shieldings of heme models cover a 4.0 ppm range that is extended to 7.0 ppm when several hemoglobin CO and myoglobin CO sp ecies at different pHs are included. Both heme models and heme protein s obey a similar excellent linear delta(C-13) versus nu(C-O) relations hip 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(C-13) and Fe-C-O geometry. The poor monotonic relation between delta(C-13) and nu(Fe-C) indicates th at the iron-carbon n bonding is not a primary factor influencing delta (C-13) and delta(Fe-57). The delta(Fe-57) was found to be extremely se nsitive to deformation of the porphyrin geometry, and increased shield ing by more than 600 ppm with increased ruffling was observed for vari ous heme models of known X-ray structures. (C) 1998 John Wiley & Sons, Inc.