The dimerization of ferrihaems: I. The effect of buffer ions and specific cations on deuteroferrihaem dimerization

The dimerization of deuteroferrihaem in aqueous solution has been investigated using a parameter, named the dimerization index (R_obs). This is defined as the ratio of extinction coefficients at wavelengths corresponding to Soret band maxima for the monomeric and dimeric species, respectively. For solutions containing mainly monomeric species, R_obs > 2, whereas for solutions containing mainly dimeric species R_obs < 1.A computer programme has been applied to determine values of the dimerization constant, K, defined as: $\text{K =}\text{dimer}\text{]}\text{H}{}^{\mathrm{+}}\text{]}\text{monomer}\text{]}{}^2$.Phosphate buffer anions and Tris · HCl buffer enhanced dimerization. Monovalent and divalent cations also increased dimerization, but in a specific manner. The magnitudes of their effects increased in the order $\text{K}{}^{\mathrm{+}}\text{< Na}{}^{\mathrm{+}}\text{< Li}{}^{\mathrm{+}}\text{<}\text{Sr}{}^{\mathrm{2+}}\text{< Mg}{}^{\mathrm{2+}}\text{? Ca}{}^{\mathrm{2+}}$. Values of K were determined for several concentrations of Na⁺ and Sr²⁺. These data are interpreted in terms of a stabilization of the ferrihaem dimer by the formation of ion triplets with the added cation ‘sandwiched’ between carboxyl residues of the adjacent ferrihaem monomeric units. General guidelines are recommended for the choice of conditions which minimize dimerization.