Crystallization of synthetic haemozoin (β-haematin) nucleated at the surface of lipid particles

The mechanism of formation of haemozoin, a detoxification by-product of several blood-feeding organisms including malaria parasites, has been a subject of debate; however, recent studies suggest that neutral lipids may serve as a catalyst. In this study, a model system consisting of an emulsion of neutral lipid particles was employed to investigate the formation of β-haematin, the synthetic counterpart of haemozoin, at the lipid–water interface. A solution of monoglyceride, either monostearoylglycerol (MSG) or monopalmitoylglycerol (MPG), dissolved in acetone and methanol was introduced to an aqueous surface. Fluorescence, confocal and transmission electron microscopic (TEM) imaging and dynamic light scattering analysis of samples obtained from beneath the surface confirmed the presence of homogeneous lipid particles existing in two major populations: one in the low micrometre size range and the other in the hundred nanometre range. The introduction of haem (Fe(III)PPIX) to this lipid particle system under biomimetic conditions (37 °C, pH 4.8) produced β-haematin with apparent first-order kinetics and an average half life of 0.5 min. TEM of monoglycerides (MSG or MPG) extruded through a 200 nm filter with haem produced β-haematin crystals aligned and parallel to the lipid–water interface. These TEM data, together with a model system replacing the lipid with an aqueous organic solvent interface using either methyl laurate or docosane demonstrated that the OH and CO groups are apparently necessary for efficient nucleation. This suggests that β-haematin crystallizes via epitaxial nucleation at the lipid–water interface through interaction of Fe(III)PPIX with the polar head group. Once nucleated, the crystal grows parallel to the interface until growth is terminated by the curvature of the lipid particle. The hydrophobic nature of the mature crystal favours an interior transport resulting in crystals aligned parallel to the lipid–water interface and each other, strikingly similar to that seen in malaria parasites.