Dioxygen and Carbon Monoxide Binding to Apolar Cyclophane Hemes: Durene-Capped Hemes
David, S., James, B.R., David Dolphin, Traylor, T.G., and Lopez, M.
J. Am. Chem. Soc.
Detailed solution kinetic and equilibrium data (mainly in toluene) are presented for the reversible binding of CO and O2 to the five-coordinate hemes Fe(por)B, where B is 1,5-dicyclohexylimidazole or 1,2-dimethylimidazole (chosen to mimic the R- and T-states.) and por = the dianion of some durene-capped porphyrins with variable length linking methylene straps on either side of the durene moiety (4/4, 5/5, or 7/7 methylenes). Use of spectrophotometric equilibrium titrations from 30 to -50oC, stopped-flow data, and laser flash photolysis under either CO or CO/O2 mixtures, has allowed for determination of on and off rates, equilibrium constants, and, in the case of the 4/4-system, thermodynamic constants for the binding. Increasing steric hindrance provided by the durene cap, in the order 7/7 < 5/5 < 4/4, is generally less than expected from studies with other heme derivatives; in combination with the complete absence of polarity effects, as within nonpolar distal sites, the durene hemes exhibit poor differentiation between CO and O2. However, the distorted 4/4-deriv. discriminates between CO and O2 in a novel way through a "proximal effect" associated with deformation of the porphyrin skeleton from planarity, the effect being largely reflected by an increased CO dissociation rate.