The dimerization of ferrihaems. III. Equilibrium and kinetic studies on the dimerization of coproferrihaem.

Spectrophotometric studies on the behaviour of coproferrihaem in aqueous solution showed that, in the pH range 6.66--8.04, a dimerization process occurs according to the equation 2 monomer K in equilibrium dimer + H+ The value of K, the pH-independent dimerization constant, was found to be 2.10 . 10(-3), signifying that coproferrihaem shows the least tendency to dimerize of any ferrihaem so far investigated. Forward and reverse rate constants for the dimerization process have been determined by the temperature-jump method. The results suggest that the cation-bridging between carboxyl residues, postulated for the dimers of the dicarboxylic ferrihaems, cannot occur between the additional carboxyl residues of coproferrihaem and that the increased negative charge may cause destabiliztion of the coproferrihaem dimer by repulsion effects.