Faraz Arastu
English 3301
5/30/13
Grand Challenges Explorations Grant
Word Count: 1257
Malaria is responsible for high morbidities and mortalities of children in tropical regions. In 2010, there were almost 216 million cases of malaria and nearly 655,000 deaths.1 This disease burden mainly falls on children less than five years old.1 Incidences of malaria result from two principle causes: parasites that infect human hosts and the mosquitoes that transmit them. Therefore, there are two main branches of combating the disease: treatment and prevention.
Current treatment efforts are based on early diagnosis and drug discovery targeting the parasites. Prevention efforts are focused mainly on vaccine development. Other prevention methods target mosquitos including nets, repellants, and pesticides.Drug and vaccine discovery are generally well-developed solutions, but mosquito control strategies have not improved since World War II. In the modern world, mosquitoes have evolved resistance to insecticides and repellants, and many mosquitoes breed around artificial water sources, making it difficult to destroy their habitats.
Research Idea
I propose research that focuses on disrupting the disease vectors, mosquitos, to curb parasite transmission. A recent study of Anopheles gambiae olfactory receptor neurons (AgORNs) shows that certain scents prompt females to bite and lay eggs.2 Table 1 shown below categorizes some human-emanated molecules and their effects on An. gambiae.2
Odorant | Class | Oviposition | Behavior |
3-methylindole | aromatic | + |
|
4-methylphenol | aromatic | + |
|
geraniol | terpene | + | |
indole | aromatic | + | + |
isovaleric acid | acid | + | + |
L(+) lactic acid | acid | + | |
phenol | aromatic | + |
|
Adapted from Supplementary Table 1.2 “+” indicates egg-laying behavior in “Oviposition” column and biting behavior in “Behavior” column.2
|
Based on this data, An. gambiae is very sensitive to aromatic compounds, which signal it to lay eggs and find blood meals, i.e. positive behavioral activity. Many of these human-produced compounds are volatile, suggesting a path of intervention.2 Strong non-covalent interactions of synthetic small molecules with the scents may disrupt mosquito olfaction. Indole is one of the aromatic odors that “contributes 30% of the volatile headspace in human sweat” and “signals blood meal recognition and oviposition,”making it a great starting point.3
Indole is one of the few compounds that exhibits high egg-laying and biting behaviors. Therefore, the synthetic small molecules (SSMs) I make will bind indole strongly to distort it from AgORN recognition and curb the biting and oviposition behaviors. Some mosquitos can sense their signals at concentrations as low as 10 parts per trillion.3To combat this, my research will focus on concentration-based competitive signal disruption. The SSMs will exploit intermolecular interactions of indole including π-π stacking between aromatic compounds as well as hydrogen bonding of the acidic proton.4 The π-hydrogen bonding interactions between aromatic p-orbitals and the acidic proton is also being considered as it is stronger than van der Waals forces.4
Figure 1: Generated via PerkinElmer Informatics |
Figure 1 illustrates an electron cloud of indole. Aromatic rings have been shown to bind strongly in π-hydrogen bonding interactions with indole.4 However, only one ring per molecule of indole can interact this way. The second ring can interact only through van der Waals forces because there is only one acidic proton.4 It may be possible to amplify both π-hydrogen bonding and π-π stacking using aromatic systems that interact on more than one face of indole.
Experimental Plan
Figure 2.5 |
In an organic synthesis lab, my task will be to synthesize these types of aromatic structures. My focus is on pyrogallolarenes (PGAs), specifically the compound shown in Figure 2.5 This carcerand molecule dimerizes with itself when partially deprotonated to form a cavity that hosts heterocycles like pyrazine and pyridine with high affinity.5 PGAs can be synthesized in ambient conditions, exhibit solubility in water and alcohols, are readily purified using crystallization, and have been confirmed to bind guest molecules in solution by NMR and in the solid-state by X-ray crystallography.6
Several classes of carcerands are biologically compatible including cyclodextrins and porphyrins. Although PGAs have not been previously used in a biological context, one study investigated them for use in drug delivery systems.5 Because PGA dimerization and binding to a guest are based on non-covalent interactions – mainly hydrogen bonding, these compounds are stable at room temperature but dissociate at high temperatures and variant salt conditions.7 This fits the criteria of a concentration based strategy for olfactory disruption.
My approach is to vary the backbone structure and the R-groups to increase affinity for indole, and decrease mosquito biting and breeding. I will specifically try replacing the cyclic ethers with cyclic amines and increase the length of the ether and amine carbon chains. In the dimer configuration, the R-groups face away from the cavity. Therefore, I will try substituting polarizable atoms like halides at these positions. The Figure 2 compound has a cavity suitable for hosting single ring molecules, but needs to be expanded using these modifications to bind indole. The cavity most likely cannot be expanded to the correct size by incorporating polycyclic aromatic systems. The planar faces of these molecules may disrupt the shape of the cavity.
Host-guest binding affinity between my SSM compounds and indole will primarily be assessed using fluorescence quenching spectroscopy (FQS), 1H NMR, and X-ray crystallography, which have been used to confirm π-π attractive and π-shielding interactions.7 The X-ray structural data will be corroborated using microwave spectroscopy, which has been shown to elucidate bond angles and lengths in indole systems.8
Future research will direct the synthesis of carcerands to mask the other six odorants in Table 1 responsible for oviposition and bloodmeal recognition. If this is successful, commonalities can be found between mosquito species to induce broad-spectrum olfactory disruption of other mosquito vector populations. Formulating these chemicals into a usable mosquito control product will be a challenge which is beyond the scope of this proposal.
As a rising chemist, my goal is to help prevent transmission of malarial parasites because it victimizes young children. Research in disrupting mosquito olfaction is both necessary and productive as it could lead to a powerful malaria prevention strategy to supplement drug discovery, vaccine development, and pesticide campaigns. Optimizing binding affinity, measuring the degree of olfactory disruption, and designing the delivery of PGAs are topics of new research questions I expect to confront during this research.
My research will impact the understanding of indole in molecular recognition via host-guest interactions. This can be applied to study indole receptor targets in small molecule drug discovery. Characterizing π-π attractions with indole can be applied in studying substrate interactions with tryptophan residues, resulting in better protein target inhibition. This research can be applied in understanding guest molecule intercalation in the 3.4Å gaps between DNA bases to improve transcriptional regulation or DNA staining methods.7
References
1 World Health Organization. Global Malaria Programme. 10 Facts on Malaria. N.p., Apr. 2012.
2 Carey, Allison F. et al. "Odorant Reception in the Malaria Mosquito Anopheles Gambiae." Nature 464 (2010): 66-71.
3 Beehler, J. W. et al. "Field Evaluation of Synthetic Compounds Mediating Oviposition In Culex Mosquitoes." J. Chem. Ecology 20.2 (1994): 281-91.
4 Braun, J. E. et al. "Van Der Waals versus Hydrogen-Bonding in Complexes of Indole with Argon, Water, and Benzene by Mass-Analyzed Pulsed Field Threshold Ionization." J. Phys. Chem. 102 (1998): 3273-278.
5 Warmuth, Ralf, and Juyoung Yoon. "Recent Highlights in Hemicarcerand Chemistry."Accounts of Chemical Research 34.2 (2001): 95-105.
6 Atwood, Jerry L. et al. "Organization of the Interior of Molecular Capsules by Hydrogen Bonding."PNAS 99.8 (2002): 4837-841.
7 Jones, G.B. Shielding in Organic Synthesis. Tetrahedron, 2001, 57, 7999.
8 Jones, G. B. ; Chapman, B. J. ; Pyrroles and their Benzo Derivatives:Structure, in Comprehensive Heterocyclic Chemistry II; Pergamon, Oxford, 1996, pp. 1-38.
9 Preventing Malaria Transmission Draft.docx