A multiscale framework for heme crystallization inhibition: From single-molecule step-capping to parasite suppression

Heme crystallization is central to malaria parasites' survival during the erythrocytic stage of the disease, yet how clinically relevant antimalarial drugs modulate this process at the molecular level remains unclear. Here we employ stage-resolved parasite assays and multiscale in vitro approaches to explore the interactions between lumefantrine (LF), one of the popular components of current combination therapies, and hematin crystallization. Pulse LF suppresses Plasmodium falciparum during both ring and trophozoite stages. Under biomimetic conditions, LF markedly reduces β-hematin crystal growth rates. High-resolution atomic force microscopy reveals that individual LF molecules bind directly to crystal steps, physically constraining step motion through a previously unreported step-capping mechanism. This localized inhibition propagates to the mesoscale, producing step broadening, reduced interstep spacing, and causing surface kinetic roughening. In contrast to its strong inhibition of crystal growth, LF enhances hematin nucleation by promoting the formation of mesoscopic hematin-rich clusters, as detected by a combination of scattering and microscopy techniques. Attachment of the nucleated crystallites to larger crystal surfaces initiates a nonclassical inhibition sequence by straining the crystal interface, which further impedes step growth.