Ligand Binding by Metalloporphyrins. II. The Effect of Solvent on the Thermodynamic Functions

Abstract

The study of free energy, enthalpy, and entropy changes accompanying the addition of substituted pyridines to iron(II) porphyrins has been extended to follow the effect of solvent change on these thermodynamic functions. Free energy changes are an unreliable index of strength of binding, but very large solvent effects are observed for enthalpy and entropy changes. Enthalpy effects were most favorable and entropy effects least favorable in carbon tetrachloride with decreasing magnitudes in chloroform and benzene. Statistical treatment of the results (62 data pairs) suggests that the contributions to these thermodynamic functions from different solvents, ligands, and metalloporphyrins operate independently and are additive. A linear model is proposed which simulates these effects. The thermodynamic data may be rationalized in terms of a relaxation in the binding of solvent molecules by heme when ligands are attached. The types of heme-solvent interaction occurring in the three solvents are different and solva.tion energies appear to decrease in the order benzene < chloroform < carbon tetrachloride. The associated large entropy changes follow the same order.