Morphology-sensitive Raman modes of the malaria pigment hemozoin

Abstract

Resonance Raman spectroscopy was applied for investigating the malaria pigment hemozoin, which is an important target structure of antimalarialdrugs. Morphology-sensitive low wavenumber modes of hemozoin were selectively enhanced with help of excitation wavelengths at λ = 633 nm and λ = 647 nm. The assignment of the most prominent bands in the Raman spectra at 343 cm⁻¹ and 368 cm⁻¹ was assisted by DFT calculations of the hemozoin dimer. The mode at 343 cm⁻¹ in the Raman spectrum of hemozoin is strongly enhanced with λ_exc. = 647 nm and is represented by a combined, symmetric doming mode of the two hematin units in the hemozoin dimer. The enhancement of this vibration is stronger in the resonance Raman spectrum of hemozoin compared with less crystalline β-hematin. The selective resonance enhancement of the morphology-sensitive Raman modes of hemozoin is caused by absorption bands in the UV-VIS-NIR spectrum. This absorption spectrum of the crystalline malaria pigment hemozoin shows a strong band at 655 nm. Another broad absorption band at 870 nm is the reason for the strong relative resonance enhancement of the mode at 1372 cm⁻¹ in the Raman spectrum of crystalline hemozoin with λ_exc. = 830 nm. In conclusion, resonance Raman micro-spectroscopy with λ_exc. = 647 nm was shown to have great potential as an analytical tool to probe the morphology of hematin samples.