Deciphering the Molecular Interactions Between Antimalarials and Hematin Crystal Surfaces

Date

2017-05

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Abstract

The unique physicochemical properties of crystals are essential for a variety of commercial applications, consumer products, and functional roles in diverse natural, synthetic, and biological systems. Facile methods of tailoring crystal properties, such as altering growth conditions, are characteristically inadequate and nontrivial. One versatile approach to control crystallization involves the use of modifiers, which are additives that interact with crystal surfaces to alter their growth rates. Elucidating their binding specificity is a ubiquitous challenge that is critical to their design as well as understanding their role in natural processes. In this dissertation project, we selected hematin, a pathological crystal that is relevant to malaria, as a model system to examine the complementarity of antimalarial drugs to crystal surfaces. 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 parasite lifecycle in human erythrocytes, heme released during hemoglobin catabolism is detoxified by sequestration into crystals. Many antimalarials are believed to suppress the parasite by inhibiting hematin crystallization. Despite significant advancement, fundamental questions regarding hematin growth and inhibition remain. We employ time-resolved in situ atomic force microscopy paralleled with bulk crystallization to show that the lipid sub-phase in the parasite may be a preferred growth environment. We present the first evidence of a molecular mechanisms of hematin crystallization and antimalarial drug action as crystal growth inhibitors. Our observations demonstrate that hematin crystals grow via a classical mechanism wherein layers are generated by two-dimensional nucleation and spread. Four classes of surface sites are identified for binding of potential drugs and could advance future design and/or optimization of new antimalarials. We provide evidence that antimalarials suppress crystal growth by binding directly to surfaces. We elucidate how subtle rearrangement and removal of functional moieties on quinoline isomers and analogues can create effective or ineffective modifiers, along with tuning their inhibitory modes of action. These findings enable the identification and optimization of chemical moieties that bind to surface sites, thus providing guidelines for the discovery of antimalarials to combat parasite resistance. Our findings highlight the importance of specific functional moieties in quinoline-class compounds, leading to an improved understanding of modifier – crystal interactions that could prove to be broadly applicable to the design of new generation antimalarial drugs to combat the increased resistance of malaria parasites to existing therapies.

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Keywords

Crystal growth, Crystallization, Malaria, Hematin, Morphology, Modifiers, Impurity, Inhibition, Antimalarials

Citation

Portions of this document appear in: Olafson, Katy N., Megan A. Ketchum, Jeffrey D. Rimer, and Peter G. Vekilov. "Molecular mechanisms of hematin crystallization from organic solvent." Crystal Growth & Design 15, no. 11 (2015): 5535-5542. And in: Olafson, Katy N., Megan A. Ketchum, Jeffrey D. Rimer, and Peter G. Vekilov. "Mechanisms of hematin crystallization and inhibition by the antimalarial drug chloroquine." Proceedings of the National Academy of Sciences 112, no. 16 (2015): 4946-4951. And in: Ketchum, Megan A., Katy N. Olafson, Elena V. Petrova, Jeffrey D. Rimer, and Peter G. Vekilov. "Hematin crystallization from aqueous and organic solvents." The Journal of chemical physics 139, no. 12 (2013): 09B611_1. And in: Vekilov, Peter G., Jeffrey D. Rimer, Katy N. Olafson, and Megan A. Ketchum. "Lipid or aqueous medium for hematin crystallization?." CrystEngComm 17, no. 41 (2015): 7790-7800. And in: Olafson, Katy N., Jeffrey D. Rimer, and Peter G. Vekilov. "Growth of large hematin crystals in biomimetic solutions." Crystal growth & design 14, no. 5 (2014): 2123-2127. And in: Olafson, Katy N., Rui Li, Bryan G. Alamani, and Jeffrey D. Rimer. "Engineering crystal modifiers: Bridging classical and nonclassical crystallization." Chemistry of Materials 28, no. 23 (2016): 8453-8465. And in: Olafson, Katy N., Tam Q. Nguyen, Peter G. Vekilov, and Jeffrey D. Rimer. "Deconstructing Quinoline‐Class Antimalarials to Identify Fundamental Physicochemical Properties of Beta‐Hematin Crystal Growth Inhibitors." Chemistry–A European Journal 23, no. 55 (2017): 13638-13647.