Emulsions of monopalmitoylglycerol (MPG) and of a neutral
lipid blend (NLB), consisting of MPG, monostearoylglycerol, dipalmitoylglycerol,
dioleoylglycerol and dilineoylglycerol (4:2:1:1:1), the composition associated with
hemozoin from the
malaria parasite Plasmodium falciparum, have been used to mediate the formation of β-
hematin microcrystals. Transmission electron microscopy (TEM), electron diffraction and electron spectroscopic imaging/electron energy loss spectroscopy (ESI/EELS) have been used to characterize both the
lipid emulsion and β-
hematin crystals. The latter have been compared with β-
hematin formed at a pentanol/aqueous interface and with
hemozoin both within P. falciparum parasites and extracted from the parasites. When
lipid and
ferriprotoporphyrin IX solutions in 1:9 v/v
acetone/
methanol were thoroughly pre-mixed either using an extruder or ultrasound, β-
hematin crystals were found formed in intimate association with the lipid droplets. These crystals resembled
hemozoin crystals, with prominent {100} faces. Lattice fringes in TEM indicated that these faces made contact with the
lipid surface. The average length of these crystals was 0.62 times the average diameter of NLB droplets and their size distributions were statistically equivalent after 10 min incubation, suggesting that the lipid droplets also controlled the sizes of the crystals. This most closely resembles
hemozoin formation in the helminth worm Schistosoma mansoni, while in P. falciparum, crystal formation appears to be associated with the much more gently curved digestive vacuole membrane which apparently leads to formation of much larger
hemozoin crystals, similar to those formed at the flat pentanol-water interface.