The mechanism of β-hematin formation in acetate solution. Parallels between hemozoin formation and biomineralization processes

Formation of β-hematin in acidic acetate solution has been investigated using quantitative infrared spectroscopy, X-ray diffraction, and scanning and transmission electron microscopy. The process occurs via rapid precipitation of amorphous (or possibly nanocrystalline) hematin, followed by slow conversion to crystalline β-hematin. Definitive evidence that the reaction occurs during incubation in acetate medium, rather than during the drying stage, is provided by X-ray diffraction and infrared spectroscopy of the wet material. The reaction follows a sigmoidal function indicative of a process of nucleation and growth and was modeled using the Avrami equation. Reaction rates and the dimensionality of growth (as indicated by the value of the Avrami constant) are strongly influenced by stirring rate. The reaction follows Arrhenius behavior, and there is a strong dependence of both the rate constant and the Avrami constant on acetate concentration. Acetate may act as a phase transfer catalyst, solubilizing hematin and facilitating its redeposition as β-hematin. The pH dependence of the process indicates that only the monoprotonated species of hematin is active in forming β-hematin. The formation of β-hematin closely parallels many mineralization processes, and this suggests that hemozoin formation may be a unique biomineralization process. Inferences are drawn with respect to the formation of hemozoin in vivo.