Acid–base behaviour of mesoporphyrin IX bis(methyl ester) and coproporphyrin I tetrakis(methyl ester) in 95%(v/v) dimethyl sulphoxide–water: kinetic and equilibrium studies

Abstract

Individual equilibrium constants for ionisation of both protons from diprotonated mesoporphyrin IX bis(methyl ester) in 90 and 95%(v/v) Me₂SO–H₂O containing hydrochloric acid have been obtained despite the fact that the first dissociation constant (K₁) is smaller than the second (K₂). The values in 95%(v/v) Me₂SO–H₂O are K₁ 0.007 and K₂ 0.06 mol l^–1 which means that the monoprotonated porphyrin (BH⁺) is always present in deficit compared with the free base (B) and diprotonated species (BH₂²⁺). Relaxation times for the equilibration between B and BH₂²⁺ were measured using the temperature-jump method and forward and reverse rate coefficients for inter-conversion of BH₂²⁺ and BH⁺ have been obtained. The thermodynamically favourable protonation by hydronium ion of a pyrrole nitrogen atom in BH⁺ occurs slowly and possible reasons for this are discussed.