The heme detoxification pathway in the digestive vacuole in the intra-red blood cell stage of malaria is a unique biochemistry in a unique organelle [[1], [2], [3], [4]]. Unlike its host, the malaria parasite does not use a typical heme oxygenase [5] to recover the components of heme and instead it crystallizes the iron(III)protoporphyrin-IX as an insoluble crystalline solid termed malaria pigment or hemozoin [6]. This biochemistry is the target for the quinoline family of antimalarial drugs, and considerable effort has been dedicated to the elucidation of the target structure HA [7,8,[9], [10], [11], [12], [13]] Fig. 1 (with synthetic analogs in Fig. 2, Fig. 3, Fig. 4) and the possible mechanisms of quinoline antimalarial drug action [2,[14], [15], [16], [17], [18]]. Synthetic equivalents of malaria pigment, also termed hemozoin, or hematin anhydride (HA) or β-hematin, are crystallographically identical but can vary from the natural product in mosaicity and morphology in a synthesis dependent manner [19].

Perhaps the most defining chemical and biological characteristic of malaria pigment is its profound insolubility. This insolubility leads to its accumulation in the vasculature and organs of of its victims, to the extent that it is often visible with the naked eye during autopsy [20]. Malaria pigment's insolubility has chemical, physiological, bioanalytical, and clinical consequences which include it being a broad band semiconductor [21], a diagnostic target [22], and an immunomodulator [23], and is associated with malaria linked anemia [24], This insolubility also contributes to the uncertainty in defining drug binding sites, and thus the difficulty in designing new antimalarial drugs. In particular, UV–visible and NMR spectroscopic methods cannot be readily applied to determine binding constants, orientations, and stoichiometries for drug/dimer interactions. Successors to chloroquine which can also inhibit the heme detoxification pathway, and ultimately the formation of malaria pigment, are still sought. With the rise of chloroquine- [25], and now artemisinin-resistant malarial strains [26], the need for new drug therapies is more urgent than ever.

Another characteristic of these condensed heme phases is their relative inertness. Reactive solvents/reagents such as trifluoroacetic acid, 2-mercaptoethanol, or 1 M NaOH are required to solubilize this material. These twin traits of insolubility and inertness led to an early incorrect hypothesis that malaria pigment is composed of heme coordination polymers [11,27], but X-ray diffraction clearly demonstrates that both the synthetic and the natural materials are weakly linked hydrogen bonded η1 propionate bound dimers, Figure 1 [9]. Our understanding of the nature of the drug/substrate interaction has been considerably hindered by hemozoin insolubility and this has limited our ability to use spectroscopic methods to determine the structures of the heme(crystal)/quinoline solution interactions. Atomic force microscopy has recently enabled insights into these solid state surface interactions [28]. In prior work we have described [29,30] two solutions to this impasse. In the first instance, by employing closely related porphyrins with either ethyl (mesoporphyrin, Fig. 2) or protio (deuteroporphyrin) substitutents for the vinyls in protoporphyrin-IX, Figure 3 [31]. In the second instance solubility is attained with the use of gallium(III) protoporphyrin [32]. Herein we describe in detail the spectroscopic and structural studies of the first strategy with new soluble iron based analogs of malaria pigment.