Structural changes in normal-affinity systems

It was proposed earlier that the molecule Fe(PF)(l-MeIm)(02) in the solid or solution state was a fair approximation to the reference gas-phase molecule. The axial base, although not oriented for minimization of contacts with the porphyrin (i.e., </> = 45°), is well-removed from an eclipsing orientation where </> = 0 ± 10°; the Fe atom is centered in the plane of a highly planar porphyrin; the O2 ligand is oriented' for minimization of contacts with the porphyrin, and its geometry is largely unconstrained by distal groups (the pickets); no groups are hydrogen-bonded to the axial base. The major difference from the reference state is that there is a significant attractive interaction between the electronegative dioxygen moiety and the amide groups on the pickets, and a smaller repulsive interaction with the picket t-butyl groups. 176
 

For the CO adduct, contacts with the pickets are all at ideal van der Waals' separations and the Fe-CO moiety is free to assume its normal linear geometry. For CO binding the reference molecule is again the carbonyl adduct of the iron picket-fence porphyrinato molecule. In contrast to O2 binding, there are no specific distal effects, such as hydrogen bonding, by which CO affinity may be increased; there remain many ways, as with O2 binding, by which CO affinities may be reduced. Thus, the CO binder with highest affinity is the iron picketfence porphyrin.  

The O2 affinities of myoglobin, R-state hemoglobin, and the Fe(PF)(l-MeIm) system are similar. However, the means by which this is achieved are different, and this difference is reflected most clearly in the kinetics of binding and release of O2 , which for Mb are much slower. The similarities and differences are summarized in Table 4.9, which is culled from Tables 4.2, 4.4, and 4.5.

For the cobalt-dioxygen derivative, the putative hydrogen bonding between the dioxygen and the amide groups of the pickets assumes greater importance because the coordinated dioxygen is substantially more negative. Again the picketfence porphyrin, being structurally characterized, is the reference system. Although no Co picket-fence porphyrin structures have been determined, the structures may be predicted with confidence from the iron analogues together with related structures of COlI and COllI tetraphenylporphyrinato systems.