Ing its optimum geometry with the iron at the center

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ing its optimum geometry with the iron at the center of the porphyrin hole, as seen in the structure of Fe(PF)(l-MeIm)(02)' Thus, the sterically active 2-methyl substituent leads to lowered O 2 (and CO) affinity relative to the I-methyl ana- logue. In metrical terms the lowered affinity is reflected in an increase in the sum of the axial bond lengths from 1.75 + 2.07 = 3.82 A to 1.90 + 2.11 = 4.01 A. In the crystal structure of Fe(C 2 Cap)(l-MeIm)(CO) the cap is about 5.6 A from the porphryin plane. I2Id Hence, in the crystal structures of the free base H 2 (C 2 Cap)201a and FeCI(C 2 Cap)20Ib species, in which the cap is screwed down to approximately 4.0 A from the porphyrin plane, considerable conformational rearrangement of the cap and the four chains attaching it to the porphyrin is needed to provide room for a small ligand such as CO. This is even more pronounced in a Co(C 3 Cap) complex where the cap is only 3.49 A from the mean porphyrin plane. 202 Thus not only is affinity for CO lowered, but some additional discrimination against it is induced, since a linear, perpendicular co- ordination creates considerable strain energy elsewhere in the molecule. For the pocket porphyrin (Figure 4.23), structural data are available on the * For CoHb0 2 , single-crystal EPR spectra have been interpreted in terms of a nearly triangularly coordi- nated O 2 ,'97 although a crystal structure of CoMb0 2 shows a bent CoOO group.'98 There is no precedent for this triangular arrangement in any Colll-superoxo (0 2 ' ) system, whereas there are many for angularly coor- dinated O 2 in electronically not dissimilar square-planar Schiff-base systems 66 Regardless of geometry, the picket amide ... O 2 contacts do not change substantially. t Shortly (10- 9 _10- 12 s) after a ligand dissociates, a large difference in v(Fe-N ,m ) between Rand T structures is observed, prior to relaxation to the equilibrium Rand T conformations. 199
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III. STRUCTURAL BASIS OF LIGAND AFFINITIES OF OXYGEN CARRIERS 241 carbonyl adduct. 12lb The CO ligand is unable to achieve the linear perpendicular geometry seen in the high-affinity picket-fence porphyrin derivative, Fe(PF)(l- MeIm)(CO), IIO and distortion of the porphyrin core is greater. In the pocket- porphyrin system, O 2 affinity is unaffected, but CO affinity is lowered. The crystal structure of partially oxygenated hemoglobin, [a- Fe0 2h[t3-Feh, 191a reveals that the quaternary structure, except in the immediate vicinity of the a hemes, which have O 2 coordinated, resembles that of T-state deoxyhemoglobin rather than R-state liganded hemoglobin. In accord with the low affinity of T- state hemoglobin, the Fe--N lm bonds for the six-coordinate a-hemes at 2.37 A are significantly longer than those in fully oxygenated R-state oxyhemoglobin, [a-Fe02h[l3-Fe02h in the notation above, (1.94 (a-hemes) and 2.06 A (13 hemes» and that found in oxymyoglobin (2.07 A). In contrast to the R-state structure and oxyMb, the a-hemes are folded as seen in the deoxy parent, leaving the Fe still substantially displaced (0.2 A) from the plane of the four pyrrole nitrogen atoms. The deoxyhemoglobin T-state
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