We show that in solutions of human hemoglobin (Hb)—oxy- and deoxy-Hb A or S—of near-physiological pH, ionic strength, and Hb concentration, liquid–liquid phase separation occurs reversibly and reproducibly at temperatures between 35 and 40°C. In solutions of deoxy-HbS, we demonstrate that the dense liquid droplets facilitate the nucleation of HbS polymers, whose formation is the primary pathogenic event for sickle cell anemia. In view of recent results that shifts of the liquid–liquid separation phase boundary can be achieved by nontoxic additives at molar concentrations up to 30 times lower than the protein concentrations, these findings open new avenues for the inhibition of the HbS polymerization.

Significance: Approximately 40% of the global population is at risk for malaria infection and 300–660 million clinical episodes of Plasmodium falciparum malaria occur annually. During the malaria parasite lifecycle in human erythrocytes, heme released during hemoglobin catabolism is detoxified by sequestration into crystals. Many of the common antimalarials are believed to suppress the parasite by inhibiting hematin crystallization. We present, to our knowledge, the first evidence of the molecular mechanisms of hematin crystallization and antimalarial drug action as crystal growth inhibitors. These findings enable the identification and optimization of functional moieties that bind to crystal surface sites, thus providing unique guidelines for the discovery of novel antimalarials to combat increased parasite resistance to current drugs.

Significance: Approximately 3.2 billion people are at risk for malaria. The resistance of malaria parasites to current advanced multidrug treatments, recently recorded to spread out of Southeast Asia, has raised concerns of dire public health consequences. We demonstrate that antimalarial drugs suppress heme detoxification in the malaria parasites in a manner that is counter to the prevailing hypothesis in the field. We find that quinoline-class drugs work by specific interactions with β-hematin crystals, which are the by-product of heme detoxification within the digestive vacuole of the parasites. We also identify specific drug adsorption sites on crystal surfaces. These insights may potentially spur development of antimalarial drugs that overcome parasite resistance through a rational approach, far superior to currently used combinatorial methods.