Distinct Physical Properties of β-Hematin in Two Synthetic Media: Compelling Evidence

It is now widely accepted that detailed knowledge of the physicochemical characteristics of the β-hematin crystals, i.e., the synthetic versions of the natural hemozoin crystals, is important for understanding their formation, the design of antimalarial medicines, and malarial diagnosis. We report that the overall physical properties exhibited by β-hematins greatly depend on the synthetic media. Here, we synthesize β-hematin from hemin in aqueous-acetate and in aqueous-oily media and characterize their properties by several techniques. Infrared spectra clearly demonstrate the formation of β-hematin in both media. The β-hematin crystals prepared in aqueous-acetate are composed by needle-like particles with average lengths around 760-770 nm; their lattice parameters and unit cell volumes are larger than those reported in the literature. They are paramagnetic at 300 K and antiferromagnetic at very low temperatures. Their Mössbauer spectra at 298 K, 77 K, and 10 K are consistent with the presence of high-spin Fe (III) and are less asymmetric as a result of the occurrence of fast spin-spin relaxation time, and their surface composition is complex, showing the presence of a multiplicity of iron oxidation and spin states (although with a majority of high-spin Fe³⁺ ions). In comparison, the β-hematin crystals prepared in aqueous-oily medium have significantly smaller lengths (ca. 560 nm) and slightly larger unit cell volume in comparison to the previous sample. The magnetic measurements show that they are affected by superparamagnetism and paramagnetism at 298 K and the coexistence of weakly ferromagnetic or possibly ferrimagnetic and paramagnetic phases at 80 K. Their Mössbauer spectra at 298 K, 77 K, and 10 K, also consistent with the presence of high-spin Fe (III), show longer spin-spin relaxation times, and their surface composition is also complex containing less surface OH^- groups and higher amounts of Fe (II) ions in low- and high-spin states. The observed differences are discussed in relation to the specific formation conditions present in the synthesis medium. The results reported here are of outmost importance for understanding how the physicochemical properties of β-hematins depend on the synthesis conditions.