The dimerization of ferrihaems. II. Equilibrium and kinetic studies of mesoferrihaem dimerization.

Spectrophotometric data have been determined for mesoferrihaem at several pH values and over a range of concentration covering four orders of magnitude. The data reveal a dimerization process according to the equation 2 monomer in equilibrium dimer + H+, analogous to earlier findings for deuteroferrihaem and protoferrihaem. The value of K (defined as K = [dimer] [H+]/[monomer]2) was found to be 6.92.10(-2). This is close to the value for deuteroferrihaem but much less than that for protoferrihaem. This is interpreted in terms of possible additional bonding between the delocalized electron systems in protoferrihaem dimers relative to those of mesoferrihaem and deuteroferrihaem. Rate constants for dimerization were determined by temperature-jump spectrophotometry. The pH dependence of the rate constants is explained in terms of two distinct pathways for the dimerization process. These involve either direct reaction between two undissociated monomer molecules or alternatively an initial acid dissociation of a monomer molecule followed by reaction between an undissociated and dissociated molecule.