Equilibrium and kinetic studies of deuteroferrihaem {chloro[dihydrogen-3,7,12,17- tetramethyl-2,18-porphinedipropionato(2–)]iron(III)} dimerization

Abstract

The dimerization constant of deuteroferrihaem (K=[Dimer][H⁺]/[Monomer]²) at 25 °C and I= O.1 M has been redetermined as (3·4 ± 0·2)× 10^–2. Studies of the temperature dependence of the equilibrium (range 5–25 °C) yield ΔH⁰=+44 kJ mol^–1 and ΔS⁰(at 298 K)=+120 J K^–1 mol^–1. The kinetics of monomer-dimer interconversion have been examined by temperature-jump spectrophotometry over a limited pH range at 25 °C and I= 0·1 M. At pH 6·70 the dimerization rate constant k_f= 2·6 × 10⁸ l mol^–1 s^–1 and the dissociation rate constant k_r= 1·1 × 10³ s^–1; at pH 7·05, k_f= 3·9 × 10⁸ l mol^–1 s^–1 and k_r= 0·7 × 10³ s^–1.