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 increment 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-Me1m). The magnitude and sign of these effects are shown in Figure 4.27. For the coordination of alkylisocyanide molecules to

hemoglobin, the steric effects of different alkyl groups have been quantified. 35 Lowered affinity occurs with increasing alkyl chain length, with the exception of methyl isocyanide.