Gas-Phase Mechanisms of Degradation of Hazardous Organophosphorus Compounds: Do They Follow a Common Pattern of Alkaline Hydrolysis Reaction As in Phosphotriesterase?
journal contributionposted on 14.08.2008, 00:00 by Edyta Dyguda-Kazimierowicz, W. Andrzej Sokalski, Jerzy Leszczynski
A comprehensive ab initio analysis of the gas-phase mechanisms of alkaline hydrolysis for a number of phosphotriesterase substrates―O,O-diisopropyl phosphorofluoridate (DFP), O-isopropyl methyl phosphonofluoridate, O,O-diethyl p-nitrophenyl phosphate (paraoxon), O,O-diethyl p-nitrophenyl thiophosphate (parathion), N-acetyl phosphoramidothioate (acephate), O,O-diethyl S-2-ethylthioethyl phosphorothioate (demeton-S) and O-ethyl N,N-dimethyl phosphoramidocyanidate―has been presented herein. The results indicate that, although an associative mechanism of alkaline hydrolysis is followed by all these compounds, P−F and P−CN bonds are cleaved according to the multistep addition−elimination scheme, whereas the breakage of P−O and P−S bonds appears to be consistent with the one-step direct-displacement mechanism. Of the two alternative reaction pathways present in all those cases (except of acephate), the most probable one involves the proton from a nucleophilic hydroxide experiencing an additional stabilization by the phosphoryl oxygen atom.