When p 0 the axial base eclipses a pair of m n p

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When <P = 0, the axial base eclipses a pair of M-N p bonds; contacts with the porphyrin are maximized. When <p = 45°, contacts are minimized. Unless the axial base has a 2-substituent, however, the contacts are not excessively close for any value of <p. With a 2-methyl substituent, the contacts are suffi- ciently severe that the M-N L1 vector is no longer perpendicular to the porphyrin plane, and the imidazole group is rotated so that the M-N L1 vector no longer
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228 4 / BIOLOGICAL AND SYNTHETIC DIOXYGEN CARRIERS approximately bisects the imidazole C-N-C bond angle, as illustrated III Figure 4.25. 110 ,172 2. Distal effects Distal effects arise from noncovalent interactions of the coordinated dioxy- gen, carbon monoxide, or other ligand with its surroundings. The protein ma- trix, the pickets, and the caps are functionally equivalent to an anisotropic sol- vent matrix that contains a variety of solutes. The limits of this simplification are illustrated in the following example. The electronically similar cobalt meso-, deutero-, and protoporphyrin IX complexes bind dioxygen with similar affinities under identical solvent conditions. When they are embedded in globin, larger differences in affinity and changes in cooperativity are observed. 170 These ef- fects are attributed to the slightly different nestling of the porphyrin molecules in the cleft in hemoglobin or, in the generalization introduced, to slightly dif- ferent solvation effects. Interaction of the coordinated O 2 or CO molecule with solvent molecules or with the protein has a profound influence on kinetics and thermodynamics (see Figure 4.24, and Tables 4.2 and 4.5). As discussed earlier, there is accumula- tion of negative charge on the dioxygen ligand. The possibility then arises for stabilization of coordination through hydrogen bonding or dipolar interactions with solute molecules,175 porphyrin substituents (such as amide groups in the picket-fence porphyrins 176 and some species of strapped porphyrins 161), or with protein residues* (such as histidine). 167.177-179 Destabilization of coordinated ligands and lowered affinity can result if the coordinated ligand is unable, through steric clash, to achieve its optimum ster- eochemistry or if the closest neighboring groups are electronegative, as are the ether and ester linkages on capped porphyrins. 31 ,180 We will describe in detail in the next subsection (III. C) the fascinating variety of means by which ligand binding is modulated by distal amino-acid residues. 3. Approximate contributions of proximal and distal effects to ligand affinity Dissimilar systems may show similar affinities for a ligand as a result of a different mix of the proximal and distal effects enumerated above. These effects are not all of equal magnitude, and an attempt is made here to show the incre- ment in free energy that occurs if the effect is manifest in the deoxy or liganded state of Figure 4.3. Increasing the free energy of the deoxy state while holding that of the liganded state constant leads to an increase in affinity. The reference state is gaseous Fe(TPP)(I-Me 1m) . The magnitude and sign
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