Propionic acid side chain hydrogen bonding in the malaria pigment β-hematin

Abstract

Malaria pigment, or β-hematin, the insoluble heme detoxification product resulting from the intraerythrocitic digestion of hemoglobin by young malaria trophozoites has been structurally characterized by X-ray powder diffraction and shown to contain chains of propionic acid linked dimers. Although there is considerable spectroscopic evidence for a monodentate propionate–iron interaction in this crystalline material, the spectroscopic characterization of the propionic acid dimer is limited. Herein we demonstrate the presence of the propionic acid dimer unit by H/D isotope substitution in carboxylic acid dimer. In the Raman spectrum of the deuterium substituted compound there is a circa 12 cm −1 shift, H: 1629 cm −1 vs. D: 1617 cm −1 in the symmetric ring breathing mode for the propionic acid dimer. On the other hand, the IR active asymmetric stretch has a very small shift, <3 cm −1, upon deuteration. These, and other vibrational data, are consistent with the presence of a planar carboxylic acid dimer in the structure of β-hematin.