Carbon-13 and oxygen-17 chemical shifts, (O-16/O-18) isotope effects on C-13 chemical shifts, and vibrational frequencies of carbon monoxide in various solvents and of the Fe-C-O unit in carbonmonoxy heme proteins and synthetic model compounds

C-13 shieldings, delta(C-13) O-17 shieldings, delta(O-17), and O-18 isotope effects on C-13 shieldings, (1)Delta(13)C(O-18/O-16), Of carbon monoxide ( 99.7% C-13, 0.9% O-17, and 11.8% O-18 enriched) in a variety of solvents an d of the Fe-C-O unit of several carbonmonoxy hemoprotein models with varyin g polar and steric effects of the distal organic superstructure and constra ints of the proximal side are reported. This enables, first, comparisons wi th hemoproteins, C-O vibrational frequencies, nu(C-O), and X-ray structural data to be made; second, to investigate whether polarizable CO is an adequ ate model for distal ligand effects in carbonmonoxy heme proteins and synth etic model compounds; third, to investigate the effect of electronic pertur bation within the heme pocket and pocket deformation on delta(C-13), delta( O-17), (1)Delta(13)C(O-18/O-16), and nu(C-O). A variety of solvents with va rying dielectric constants and solvation abilities appears to have negligib le effect on delta(O-17), delta(C-13), and (1)Delta(13)C(O-18/O-16) and lit tle direct effect on nu(CO) of dissolved carbon monoxide. On the contrary, C-13 and O-17 shieldings of several carbonmonoxy hemoprotein models vary wi dely and an excellent correlation was found between the infrared C-O vibrat ional frequencies and C-13 shieldings and a reasonable correlation with O-1 8 isotope effects on C-13 shieldings. The C-13 shieldings of heme models co ver a 4.0 ppm range which is extended to 7.0 ppm when several HbCO and MbCO species at different pHs are included. The latter were found to obey a sim ilar linear delta(C-13) vs nu(C-O) relationship. nu(C-O), delta(C-13), and (1)Delta(13)C(O-18/O-16) parameters of heme,model compounds reflect similar interaction which is primarily the modulation of rr back-bonding from Fe d (pi) to CO pi* orbital by the distal pocket polar interactions. Our results suggest that, contrary to earlier claims, polarizable carbon monoxide is n ot an adequate model for distal ligand effects in carbonmonoxy hemoproteins and synthetic model compounds. Very probably this is caused by the large e ffect of the electric field on the back-bonding and the large polarizabilit y of the Jr: subsystem of the Fe-C-O unit. The O-17 shieldings of heme mode ls cover a range of 17 ppm which is extended to 24 ppm when selected heme p roteins are included. The lack of correlation between delta(C-13) and delta (O-17) suggests that the two probes do not reflect a similar type of electr onic and structural perturbation. delta(O-17) is not primarily influenced b y the local distal field interactions and does not correlate with any singl e structural property of the Fe-C-O unit; however, atropisomerism and defor mation of the porphyrin geometry appear to play a significant role.