The Mechanism Study And Theoretical Design Of Pyridine Compounds For The Detoxification Of Nerve Agents

Organophosphorus(OP) compounds disordered the nervous system mainly by inhibiting the activity of acetylcholinesterase(AChE), causing the toxicosis. The main treatment for organophosphorus(OP) compounds poisoning in clinicis to restore the activity of acetylcholinesterase(AChE) through oxime-induced reactivation of the phosphorylated OP-AChE adduct. However, it suffers from a competitive and irreversible aging reaction of the phosphorylated OP-AChE adduct, resulting in permanent inactivity of AChE.It was recently reported that N-methyl-2-methoxypyridinium species can act as methylating agent to methylate the methyl methane-phosphonate monoanion, in which the reaction mimics the reverse of the aging reaction of the phosphorylated OP-AChE adduct. However, such reaction with N-methyl-2-methoxypyridinium species in the enzyme is still not reported so far. It is of great interest to know whether or not this reaction is observable in the enzyme, and more importantly, if it turns out not observable in enzyme, why such reaction proceed fast in aqueous solution but not in enzyme. In the present study, we explored the mechanism of the N-methyl-2-methoxypyridinium methylating the methyl methane-phosphonate(a simple model of aged OP-AChE adduct), and predicted the erergy barrier of N-methyl-2-methoxypyridinium methylating the real OP-AChE adducts. Compared the difference between structures and mechanism of the two reactions, we get the key points which effect the methylate reaction, giving the guidance for the design of methylating reagents which have higher activity.We performed DFT calculations and Quantum Mechanical/Molecular Mechanical(QM/MM) calculations to reveal the fundamental mechanism for the methylations of both the methyl methane-phosphonate monoanion and the aged Sarin-AChE adduct by N-methyl-2-methoxypyridinium species, respectively. The obtained results support the SN2 reaction mechanism, not the stepwise mechanism, for the methylation of the methyl methane-phosphonate monoanion by 9 reported N-methyl-2-methoxypyridinium compounds. The calculated free energy barriers are in good agreement with the experimental data. The methylation of the aged Sarin-AChE adduct by one N-methyl-2-methoxypyridinium compound(labeled as compound 2, N-methyl-2-methoxy-3-fluorine-pyridinium) also employ the SN2 reaction mechanism with an extremely high free energy barrier of 30.4±3.5 kcal/mol(arithmetically averaged free energy) or 26.6 kcal/mol(Boltzmann averaged free energy), which is consistent with the experimental observation that this reaction in enzyme is hardly to occur. Our results clearly show that compound 2 forms a strong π-π stacking interaction with W86 residues of AChE, making itself unable to approach sarin for the reverse of aging process. On the basis of the structure and mechanism, several possible strategies have been suggested for designing methylating agents with higher activity against the aged Sarin-AChE adduct, giving the guidance of reversing the aged Sarin-AChE adducts.