Study of the exchange rates of ethanol and water on ferriprotoporphyrin IX by n.m.r. line broadening

Abstract

The nuclear magnetic resonance (n.m.r.) line broadening technique has been used to determine the rate of solvent exchange from the first coordination sphere of ferriprotoporphyrin IX (hemin) in aqueous ethanol. The line broadenings of the methyl and methylene protons of ethanol and the water protons have been studied as a function of solvent composition at 35° and of temperature in 48.8 mole % ethanol.The average values of the kinetic parameters for ethanol molecule exchange, taken from the CH3 and CH2 proton broadening are ΔH≠ = 6.2 ± 1.0 kcal mole−1 and ΔS≠ = −9.1 ± 4.4 cal mole−1 deg−1. Both proton transfer and water molecule exchange contribute to the water proton line broadening with a net value of ΔH≠ = 6.3 ± 0.5 kcal mole−1 and ΔS≠ = −3.8 ± 2.0 cal mole−1 deg−1.From the n.m.r. study at varying solvent composition, it is concluded that monomeric and dimeric hemin undergo solvent exchange at the same rate. The equilibrium constant for formation of the monoethanol solvated hemin from hydroxyhemin is determined as 8.7 at 35° from this study.A comparison of the rate of ethanol exchange (1.8 × 106 s−1 at 25°) and the rate of imidazole binding to hemin (from a separate temperature jump study) indicates that the imidazole substitution occurs through an SN1 ion pair mechanism.