Molecular recognition and sensing of organophosphorus compounds and paraquat by endo-functionalized macrocyclic receptors

Abstract

Pesticides, including organophosphorus compounds and paraquat, are water-soluble toxic chemicals that can prevent, destroy or control pests. The abuse of pesticides can cause many problems such as threats to humans and endangered species, therefore their detection and detoxification are in urgent need. Traditional instrumental methods such as GC-MS and electrochemical methods for the detection of organophosphorus compounds and paraquat have been developed but cannot detect pesticides with enough efficiency and selectivity, and not convenient because the instruments are often not portable. The use of a complementary host is a good approach in the detection of pesticides. However, the recognition of pesticides in water by synthetic hosts is a great challenge since water provides high desolvation penalty for both the host and the guest upon binding. Therefore, endo-functionalized synthetic hosts, which contain inward-directed polar binding sites that are deeply embedded and protected from the polar environment inside their hydrophobic cavity, were developed to overcome the challenge. In this thesis, the recognition of organophosphorus compounds and paraquat by endo-functionalized naphthotube and naphthobox is described. In Chapter 1, an introduction on biomimetic molecular recognition, especially the recognition of polar molecules in water by endo-functionalized synthetic hosts, is described. On the other hand, the recognition methods for organophosphorus compounds and paraquat including instrumental methods, reaction-based, and recognition-based methods are also described. In Chapter 2, the recognition, detection, and toxic inhibition of organophosphorus compounds by naphthotubes 1a and 1b are described. Through 1H NMR, ITC titration experiments and molecular simulations, the binding of endo-functionalized naphthotubes to organophosphorus compounds were found to be contributed by hydrogen bonding and the hydrophobic effect. Moreover, 1b was found able to be used as “turn-on” fluorescence probes for benzyl organophosphorus with a sensitivity at the micromolar range. Finally, the in vitro enzyme-based experiments based on the monitoring of acetylcholinesterase (AChE) activity suggesting that naphthotube 1b was also able to inhibit the toxicity of paraoxon to AChE as a synthetic scavenger. In Chapter 3, the binding studies of two isomers of nitro-spiropyran 8-MC and 8-SP by naphthotube 1b were described. 1b was found to have high selectivity in binding to 8-SP over 8-MC, and able to shift the equilibrium of 8-SP and 8-MC. The presence of 1b was also found to lower the conversion of 8-SP into 8-MC. The detection of paraoxon 7 through an IDA of 1b and 8-MC was achieved with a sensitivity at the micromolar range. Naked-eye detection of paraoxon was also achieved by using a paper stripe that was absorbed with solid state 8-SP@1b complex. In Chapter 4, the recognition of paraquat 9 by naphthobox NPT is described. The X-ray crystal structure, UV-vis spectroscopy, and cyclic voltammetry curve of 9@(NPT)2 suggesting the binding between NPT and 9 is contributed by the formation of C-H···N hydrogen bonds and charge transfer interactions. The binding of NPT to other paraquat analogues was also studied, which indicating that steric hindrance and electronic effect largely influence their affinity to NPT, and therefore NPT can recognize 9 in high selectivity.