KINETICS AND MECHANISMS OF CARBON-MONOXIDE DISSOCIATION FROM CHELATED HEME-CO COMPLEXES - MODELS FOR HEMOPROTEIN REACTIONS

Rates of carbon monoxide dissociation from a series of chelated heme compounds have been measured. Unstrained chelated hemes, having attached proximal bases, remain five coordinated and therefore react reversibly with carbon monoxide by a simple one-step mechanism. Both the rates and activation enthalpies of dissociation are similar to those of fully ligated (R-state) hemoglobin, indicating that chelated protoheme accurately models hemoglobin in its R state. This leads to the conclusion that the iron-carbon-oxygen bending observed in carboxyhemoglobin does not appreciably affect the CO off rates or affinities. Variations in side-chain electron donation or in proximal basicity cause no appreciable change in the CO dissociation rate. Solvent polarity is also without substantial effect. However, introducing steric strain in the proximal base chelating arm causes the mechanism of dissociation to change from direct dissociation to base elimination. © 1979, American Chemical Society. All rights reserved.