Study of the key interactions in the self-recognition of the antimalarial drug chloroquine

Malaria is a parasitic disease that causes thousands of deaths every year, especially in undeveloped countries. The Plasmodium protozoa, responsible of the infection, kill human red blood cells by digesting hemoglobin. Many compounds have been employed in the last century against malaria, but nowadays the increasing resistance of Plasmodium is becoming a very serious problem. New drugs are required and to this end it is desirable to quantitatively understand the role of different functional groups in determining effective pharmacophores. This work focuses on chloroquine (CQ), a 4-aminoquinoline antiplasmodial whose effectiveness is now hampered by evolved parasite resistance. It is accepted that CQ interferes with a crucial detoxification process of the parasite [1], namely the inhibition of heme bio-crystallization, but several details of this process still remain rather obscure. In the acidic digestive vacuole of Plasmodium, CQ is supposed to interact in its diprotonated form directly with the monomeric heme in two possible ways: 1) π-π stacking interactions between quinoline ring and heme proto-porphyrin [2] or 2) a direct Fe-N quinoline coordinative bond, supported by strong charge-assisted hydrogen bonds (CAHBs) between the tertiary amine of CQ and the propionate groups of heme [3-4]. In this work, the self-recognition of chloroquine diphosphate dihydrate salt was studied both theoretically and experimentally. High-resolution single crystal X-ray data were collected at low temperature (103 K) and complemented by quantum simulations with CRYSTAL14 [5] at the B3LYP/6-31G(p,d) theory level. The salt crystalizes in a P21/c structure, with phosphate ions forming infinite chains parallel to the b axis. CQ molecules and phosphates are connected through strong N-H•••O CAHBs, while a π-π interaction is present between the quinoline rings (see figure). The topological analysis of the primary charge density, performed according with the Quantum Theory of Atoms in Molecules [6], along with the ab-initio energy decomposition, show that the coulombic interactions between the charged hydrocarbon chain of CQ and the phosphate ions seem to provide the dominant features in the molecular self-recognition, while the π-π stacking between the quinoline moieties has just an ancillary role. These evidences suggest that, in agreement with our previous DFT/EXAFS results [3], the protonated tertiary amine of CQ is an essential component of the drug pharmacophore. [1] A. F. G. Slater, W. J. Swiggard, B. R. Orton, W. D. Flitter, D. E. Goldberg. A. Cerami and G. B. Henderson, Proc. Natl. Acad. Sci. USA 1991, 88, 325 [2] M. S. Walckzak, K. Lawniczak-Jablonska, A. Wolska, A. Sienkiewicz, L. Suarez, A. J. Kosar and D. S. Bohle, J. Phys. Chem. B 2011, 115, 1145 [3] G. Macetti, S. Rizzato, F. Beghi, L. Silvestrini and L. Lo Presti, Physica Scripta 2016, 91, 023001 [4] A. C. De Dios, R. Tycko, L. M. B. Ursos and P. D. Roepe, J. Phys. Chem. A 2003, 107, 5821 [5] R. Dovesi, V. R. Saunders, C. Roetti, R. Orlando, C. M. Zicovich-Wilson, F. Pascale, B. Civalleri, K. Doll, N. M. Harrison, I. J. Bush, P. D’Arco, M. Llunell, M. Causà and Y. Noël, CRYSTAL14 2014 CRYSTAL14 User's Manual. University of Torino, Torino [6] R. F. W. Bader, Atoms in molecules. A quantum theory 1990, Oxford University Press. Oxford, U.K.