The dimerization of ferrihaems. IV. Studies on haematoferrihaem and a general appraisal of the nature and implications of dimerization.

The dimerization of haematoferrihaem was studied in phosphate buffer in the pH range 7.02--8.14. The absorbance of dilute solutions decreased over a period of several hours due to adsorption of haematoferrihaem to glass vessels. This problem was overcome by using dilute solutions within 10 min of preparation. Spectrophotometric data were consistent with a dimerization process according to the equation 2 monomer in equilibrium dimer + H+ as found earlier for other ferrihaems studied. The value of K, defined as K = [dimer] [H+]/[monomer]2, was found to be 1.00 . 10(-2). Rate constants for the forward and reverse steps in dimerization were determined at pH values of 6.63, 7.01 and 7.44, using the temperature-jump technique. The reaction pathway for dimerization was found to be similar to those of deuteroferrihaem and mesoferrihaem, but different from that of coproferrihaem. A general appraisal of the factors influencing dimerization is attempted in the light of accumulated data on various ferrihaems. With the exception of protoferrihaem, it is suggested that dimerzation increases with the hydrophobicity of the 2,4 substituents. The additional stability of the protoferrihaem dimer is explained in terms of increased interaction due to conjugation of the vinyl groups with the porphyrin macrocycle.