(720f) Mechanisms of Beta-Hematin Crystallization and Inhibition By Antimalarial Growth Modifiers

Katy N. Olafson, Peter G. Vekilov, Jeffrey D. Rimer

Department of Chemical and Biomolecular Engineering, University of Houston, 4800 Calhoun Rd., Houston, TX 77204

Crystallization is a dynamic series of events driven by a system’s thermodynamics and kinetic properties. These fundamental processes mediate natural and synthetic crystallization pathways. We study the crystallization of hematin [1], which is a byproduct of the heme detoxification pathway in malaria that ensures the proliferation of the Plasmodium falciparum parasite. Antimalarial compounds are believed to inhibit the growth of hematin crystals, referred to as hemozoin (in vivo) or beta-hematin (in vitro). Identifying the modes of antimalarial action would lay the foundation for a rational method for novel drug design to combat parasite drug resistance and in this way minimize the one million deaths that currently result from this disease. Due to current antimalarial drug resistance, the design of novel treatments is imperative.

We employ in situ atomic force microscopy (AFM) as a platform to study the mechanisms of crystal growth and inhibitor-crystal interactions in a physiologically-relevant crystallization environment [2]. Our findings on the crystallization mechanism revealed that growth occurs by a classical pathway whereby layers are generated on crystal surfaces by 2D nucleation, forming islands with heights equal to the lattice parameter perpendicular to the studied surface. We measured the rate of 2D nucleation and island growth as a function of hematin supersaturation in the absence and presence of antimalarials. Our in situAFM measurements revealed that beta-hematin crystals grow by the association of single molecules. Furthermore, the quantification of nucleation and step velocity provide a foundation for determining antimalarial drug actions (i.e. inhibition of layer generation and/or step propagation) for hemozoin crystals. We identify the modes by which antimalarial compounds bind to beta-hematin faces and the roles they play as crystal growth inhibitors. We have shown that chloroquine – a common antimalarial medication with high resistance – specifically interacts with beta-hematin crystal surfaces and eliminates the generation of new layers and step growth at small dosages (e.g., ~1 mM). This allows us to discuss the physicochemical factors regulating crystal modifier efficacy towards the rational design of antimalarial drugs.

References:

[1] Olafson, K.N., Rimer, J.D., Vekilov, P.G., Growth of Large Hematin Crystals in Biomimetic Solutions, Cryst. Growth Des.14 (2014) 2123-2127

[2] Ketchum, M.A., Olafson, K.N., Rimer, J.D., Vekilov, P.G., Hematin Crystallization from Aqueous and Organic Solvents, J. Chem. Phys. 139 (2013) 121911(1-9)