1. Structure and Mode of Action of Organophosphate Pesticides:
A Computational Study
Lasantha Rathnayake
Scott H. Northrup
Department of Chemistry
Tennessee Technological University

2. Organophosphate (OP) compounds
An important class of man-made chemicals actively released into the environment is the organophosphates
Dichlorvos

3. Applications of OP compounds
Pesticides
Parathion, paraoxon, and diazinon
Chemical warfare agents
Sarin, soman, tabun, and VX
Therapeutic agents
Echothiophate iodide to treat glaucoma

4. Applications of OP Compounds
Presence of OP-containing pesticides in the environment is abundant.
Pesticide exposure arises from living next to treated areas or in agricultural regions, as well as from house and yard pesticide treatment.

5. Pesticide transportation paths to organism
Pesticide transportation paths to untargeted organism
World Health Organization (WHO), 2001

6. Main action of OPs as pesticides
Inhibit the activity of the enzyme acetylcholinesterase (AcHE) which is hydrolase of the neurotransmitter acetylcholine (ACh) in the nervous system (central and peripheral nervous system)
Mouse AcHE
Human AcHE
mAcHE: Carletti et. al., Journal of the American Chemical Society, 2008
hAcHE: Carletti et. al., Journal of medicinal chemistry, 2010

7. Action of OPs with serine
Phosphorylation dealkylation and hydroxylation mechanisms of OPs with serine at active site of AcHE
Elersek and Filipic, InTech, 2011

8. Tabun (an OP) with mouse AcHE
Crystal structures of mouse AcHE with covalently bonded tabun:
nonaged (left) and aged (right)
Carletti et. al., Journal of the American Chemical Society, 2008

9. Remarks
Although a basic understanding exists as to how these compounds function and their toxicological effects, much remains to be understood about how details of their molecular structure relate to toxicity and species specificity.
What makes one OP a good candidate for a household insecticide while another OP is a chemical warfare agent?
What structural features give rise to specificity for toxicity in different organisms?
What are the fates of these compounds in the environment?
Are there ways to more effectively remediate or reverse their detrimental effects on organisms in the environment?

10. Remarks
Better understanding of the thermodynamics/kinetics of the reactions of OPs could lead to following
Design superior pesticides with greater margins of safety.
Development of preventative strategies for bodily exposure to chemical warfare agents.
Reversal of detrimental effects of exposure of OPs to humans and the environment.

11. Project Goals
Explore the equilibrium structures and reactivities of ten OPs widely used as pesticides.
Perform conformational analyses of the compounds to determine most stable conformations.
Explore pathways of reaction of OPs with nucleophiles mimicking their target enzyme AcHE.
Use advanced knowledge-based ligand docking algorithms available in the software Rosetta to predict the docking of OPs to AChE and subsequent pathways for binding.
Rosetta : Kaufmann et. al., Biochemistry, 2010

12. Computational Methodologies
Gaussian 03
Hybrid density functional theory (HDFT) method of mPW1B95-44 in conjunction with G(d,p) basis set.
Reaction paths
Conformations
Rosetta
Obtain and analyze docking poses
QSAR

13. Some of the conformations of OPs in consideration
Atom colors
C - dark green
O - red
Cl - light green
N - blue
H - gray
S - yellow
P - orange
Toxicity – Category I and II
Low molecular weight
Currently in use
Variety of functional groups

14. Reaction of OP with methanol:
Direct H transfer
Methanol mimics the activity of serine residue in active site of AcHE
ΔE≠ = (kcal/mol)
Pre complex
Transition state
H transfer from methanol to leaving group of dichlorovos

15. Reaction of OP with methanol: Multi step Wright mechanism
Wright and White, J. Molecular Structure (Theochem), 1998

16. Reaction of OP with methanol: Water assisted Wright mechanism

17. Direct vs water assisted Wright mechanisms
Purple – direct Wright mechanism
Blue –water assisted mechanism

18. Work in progress
Histidine assisted H – transfer

19. Work in progress Docking using Rosetta and analyzing the outcomes
Dichlorovos in hAcHE

20. Work in progress Docking using Rosetta and analyzing the outcomes
Dichlorovos in hAcHE

21. Current conclusions
Reactivity of OPs with methanol, which mimics AChE, was studied in different perspectives.
Wright mechanism shows substantially lower activation energies than the direct mechanism. However, values are still high.
Water-assisted Wright mechanism dramatically lowers the activation energies suggesting a biologically feasible reaction path for the reaction of OPs and AChE.

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