A dual growth mode unique for organic crystals relies on mesoscopic liquid precursors

The Hematin-dihydroartemisinin Adduct Mobilizes a Potent Mechanism to Suppress β-hematin Crystallization

Malaria remains a significant public health challenge in equatorial regions of the world largely owing to the parasite's emerging resistance to the recently introduced drugs of the artemisinin (ART) family. In the human body most ART-derivative drugs are metabolized to dihydroartemisinin (DHA), which, in the parasite, after activation by heme, can form a hematin-dihydroartemisinin adduct (H-DHA). Here we test whether and how H-DHA inhibits hematin crystallization, the main constituent of the heme detoxification pathway of malaria parasites. We find that H-DHA is a poor inhibitor of classical crystal growth- it weakly blocks the growth sites on crystal surfaces-and, counterproductively, a promotor of β-hematin nucleation, driven by a boost in the formation of precursors. We establish that at elevated hematin concentrations H-DHA activates two non-classical pathways that transform it into a potent β-hematin growth inhibitor. First, β-hematin crystallites, whose nucleation is promoted by H-DHA, incorporate into large β-hematin crystals and suppress their growth, likely by straining the crystal lattice. A second consequence of H-DHA is the generation of macrosteps on β-hematin crystal surfaces that hinder growth. Importantly, the induced growth suppression is irreversible and persists even in the absence of H-DHA. Our findings suggest that a partial resistance mechanism to artemisinin-class drugs in trophozoite-stage parasites may be due to the reduced concentrations of hematin and H-DHA, which deactivate the dual non-classical mode of action of the adduct in the delayed-clearance parasite strains.

Direct observation of cholesterol monohydrate crystallization

Cholesterol crystallization is integral to the pathology of diseases such as atherosclerosis and gallstones, yet the relevant mechanisms of crystal growth have remained elusive. Here, we use a variety of in situ techniques to examine cholesterol monohydrate crystallization over multiple length scales. In this study, we first identified a biomimetic solvent to generate triclinic monohydrate crystals, while avoiding the formation of nonphysiological solvates and enabling crystallization at rates where the dynamics of surface growth could be captured in real time. Using a binary mixture of water and isopropanol, with the latter serving as a surrogate for lipids in physiological environments, we show that cholesterol monohydrate crystals grow classically by the nucleation and spreading of crystal layers. Time-resolved imaging confirms that layers are generated by dislocations and monomers incorporate into advancing steps after diffusion along the crystal surface and not directly from the solution. In situ atomic force microscopy (AFM) and microfluidics measurements concertedly reveal abundant macrosteps, which engender a self-inhibition mechanism that reduces the rate of crystal growth. This finding stands in contrast to numerous other systems, in which classical mechanisms lead to unhindered growth by spreading of single layers.

Vitamin B2 Operates by Dual Thermodynamic and Kinetic Mechanisms to Selectively Tailor Urate Crystallization

Here we demonstrate how a biologically relevant molecule, riboflavin (vitamin B2), operates by a dual mode of action to effectively control crystallization of ammonium urate (NH4HU), which is associated with cetacean kidney stones. In situ microfluidics and atomic force microscopy experiments confirm a strong interaction between riboflavin and NH4HU crystal surfaces that substantially inhibits layer nucleation and spreading by kinetic mechanisms of step pinning and kink blocking. Riboflavin does not alter the distribution of tautomeric urate isomers, but its adsorption on NH4HU crystal surfaces does interfere with the effects of minor urate tautomer by limiting its ability to induce NH4HU crystal defects while also suppressing NH4HU nucleation and inhibiting crystal growth by 80% at an uncharacteristically low modifier concentration. Time-resolved spectroscopy measurements, ab initio calculations, and molecular dynamics simulations confirm that each riboflavin molecule forms a complex with six or more urate molecules to lower supersaturation, thereby reducing the rate of NH4HU crystallization by a thermodynamically driven mechanism. The degree of complexation observed for riboflavin far exceeds that of common chelating agents, and results in crystal dissolution when the free urate concentration falls below NH4HU solubility. The synergism that is created by riboflavin's dual kinetic and thermodynamic mechanisms is rarely achieved by more conventional crystal growth inhibitors. These insights offer new approaches that could be influential for the design of molecular modifiers in crystal engineering applications, the development of therapeutics for pathological conditions, and establishing broader understanding of the roles played by foreign agents in natural and biological crystallization.