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

Formation of beta-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 beta-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 beta-hematin. The pH dependence of the process indicates that only the monoprotonated species of hematin is active in forming beta-hematin. The formation of beta-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.

Characterisation of synthetic beta-haematin and effects of the antimalarial drugs quinidine, halofantrine, desbutylhalofantrine and mefloquine on its formation.

Infrared spectroscopy, elemental analysis and X-ray powder diffraction show that the product of 30 min of reaction of haematin in 4.5 M acetate, pH 4.5 at 60 degrees C is identical to beta-haematin prepared in 4.5 M acetic acid at 70 degrees C overnight (pH 2.6). There is no evidence for formation of haem-acetate complex, which could not be isolated, even from 11.4 M acetate solution. The antimalarial drugs quinidine, halofantrine, desbutylhalofantrine and mefloquine were found to inhibit formation of beta-haematin, while 5-, 6- and 8-aminoquinoline and quinoline were found to have no effect. Quinidine was shown to form a complex with ferriprotoporphyrin IX in 40% DMSO with log K = 5.02 +/- 0.03. Log K values for halofantrine and desbutylhalofantrine are 5.29 +/- 0.02 and 5.15 +/- 0.02 respectively (solutions containing 30% acetonitrile in addition to DMSO to solubilise these drugs), which are both stronger than chloroquine under the same conditions (log K = 4.56 +/- 0.02).

Thermodynamic factors controlling the interaction of quinoline antimalarial drugs with ferriprotoporphyrin IX.

The interaction of a variety of quinoline antimalarial drugs as well as other quinoline derivatives with strictly monomeric ferriprotoporphyrin IX [Fe(III)PPIX] has been investigated in 40% aqueous DMSO solution. At an apparent pH of 7.5 and 25 degrees C, log K values for bonding are 5.52 +/- 0.03 (chloroquine), 5.39 +/- 0.04 (amodiaquine), 4.10 +/- 0.02 (quinine), 4.04 +/- 0.03 (9-epiquinine), and 3.90 +/- 0.08 (mefloquine). Primaquine, 8-hydroxyquinoline, 5-aminoquinoline, 6-aminoquinoline, 8-aminoquinoline, and quinoline exhibit no evidence of interaction with Fe(III)PPIX. The enthalpy and entropy changes for the interaction of quinolines with Fe(III)PPIX, as determined from the temperature dependence of the log K values, exhibit a compensation phenomenon that is suggestive of hydrophobic interaction. This is supported by the finding that the interactions of chloroquine and quinine with Fe(III)PPIX are weakened by increasing concentrations of acetonitrile. Interactions of chloroquine, quinine, and 9-epiquinine with Fe(III)PPIX are shown to remain strong at pH 5.6, the approximate pH of the food vacuole of the malaria parasite which is believed to be the locus of drug activity. Implications for the design of antimalarial drugs are briefly discussed.