Medium effects on the dimerization of coproporphyrin-I free base

Abstract

Equilibrium constants of dimerization of the tetraanion of coproporphyrin-I free base were determined spectrophotometrically in aqueous solution at pH 9.0 in the presence of nine electrolytes, including salts of alkali metal, tetraalkylammonium and Ca(II) cations. Additions of all electrolytes promoted dimerization and the effect increased in the order tetraalkylammonium ≪ alkali metals ≪ Ca(II) salts. The Effects of alkali metal salts at concentrations below 0.1 M were quantitatively described in terms of Debye–Hückel model assuming an effective charge of the porphyrin monomer of −3. At higher salt concentrations salting-out/in effects must be taken into account. Addition of tetraalkylammonim salts to solutions containing 0.1 M NaCl produced the monomerization effect attributed to strong salting-in behavior of these salts. The effect of Ca(II) was described by complexation-induced dimerization of the porphyrin. Addition of organic co-solvents (methanol, ethanol, acetone, dioxane) to aqueous electrolyte solutions of the porphyrin produced a strong monomerization effect. Logarithms of dimerization constants extrapolated to the zero ionic strength in each mixed solvent roughly correlated with inverse dielectric constant with a negative slope considerably exceeding, however, that expected on the basis of a purely electrostatic model. Correlations with empirical solvent parameters (δH, ETN, SP) were less satisfactory than the correlation with­1/ε. A satisfactory description of solvent effects on dimerization constants was found in terms of a specific solvation model, which takes into account association of porphyrin monomer and dimer with several co-solvent molecules. Logarithms of the association constants were linear functions of the partial molar volume of the hydrocarbon portion of co-solvent molecules. Copyright © 1999 John Wiley & Sons, Ltd.