On Biomimetics
376
- Structure, physical properties and spectroscopy
2.1 Stucture
When Fitch and Kanjananggulpan first isolated purified haemozoin in 1987 (Fitch &
Kanjananggulpan, 1987), they suggested that it was identical to a synthetic Fe(III)PPIX
precipitate obtained from acidic solutions known since the 1930s as -haematin (Hamsik,
1936). However, at the time little was known about the structure of either. In 1991 Slater et al.
demonstrated that -haematin prepared by heating haemin (Cl-Fe(III)PPIX) in 4.5 M acetic
acid at pH 4.5 and 70 C is identical to haemozoin with respect to its infrared spectrum, X-ray
powder diffraction pattern and solubility in DMSO and alkaline aqueous solution. Using
extended X-ray absorption fine structure (EXAFS) spectroscopy they demonstrated that the
Fe(III)PPIX molecules are linked via coordination of the haem-propionate group of one
Fe(III)PPIX to the Fe(III) centre of another (Slater et al, 1991). While this model of the
immediate bonding environment around the Fe(III) centre has proved to be correct, their
suggestion that haemozoin is polymeric (Figure 1) was later found to be wrong. Nonetheless,
this is a sentinel study, because it opened the way to routinely prepare synthetic haemozoin (-
haematin) on which much of our subsequent understanding is based.
Any lingering doubts that the structure of haemozoin may be different from that of -
haematin, or that -haematin might be produced only during the extraction procedure was
removed when Bohle et al. showed that the high resolution X-ray powder diffraction pattern
of freeze-dried whole parasitized red blood cells obtained using synchrotron radiation is
identical to that of -haematin prepared by dehydrohalogenation of haemin with 2,6-lutidine
in rigorously dry methanol (Bohle et al. 1997). In this study the -haematin crystal was found
to belong to the centrosymmetric space group P1 and the structure was proposed to consist of
two anti-parallel polymer chains linked by hydrogen bonds (Figure 1). Finally, in 2000 the
same group solved the structure of -haematin from the powder diffraction pattern by
Rietveld refinement (Pagola et al. 2000). This showed that it is not in fact a polymer, but rather
a crystal formed from cyclic dimers of Fe(III)PPIX each linked via coordination of the
propionate group of one porphyrin to the Fe(III) centre of the other and vice versa. These
dimers are linked to neighbouring dimers by hydrogen bonds between the remaining
uncoordinated and protonated propionic acid groups (Figure 1). The structure demonstrates
that -haematin is really a haematin anhydride and indeed this change in nomenclature has
recently been proposed (Bellemare et al., 2009; Walczak et al., 2010 and 2011).
a b c
Fig. 1. Knowledge of the structure of haemozoin has progressed over time. (a) A
coordination polymer proposed by Slater et al. in 1991, (b) antiparallel coordination polymer
chains proposed by Bohle et al. in 1997 and (c) the structure determined by Rietveld
refinement of the X-ray powder pattern (Pagola et al., 2000).