Comparative analysis of complexation of pesticides (fenitrothion, methylparathion, parathion) and their carboxylic ester analogues by β-cyclodextrin. Theoretical semiempirical calculations

Abstract

The complexation by -cyclodextrin ( -CD) of three organophosphorus pesticides, fenitrothion, parathion and methylparathion, and of their carboxylic ester analogues was analyzed using PM3 and molecular mechanics methods. The objective was to elucidate structural features and energy changes that accompany the complexation and could possibly affect the hydrolysis process, which is catalyzed by -CD in the case of carboxylic esters but inhibited for the pesticides, in alkaline medium. The complexation of fenitrothion was further explored, since experiments proved its hydrolysis is relatively less inhibited and progresses mainly through a different pathway than that usually accepted. The results of this study show that complex structures involving esters enable effective interactions between the guest carbonyl and the rim of the host; methylparathion and parathion, however, appear to be deeply included in the cavity of -CD. Therefore the conditions for a nucleophilic attack by the -CD are favorable for the carboxylic esters but not for the two pesticides. Different complex geometries resulted for fenitrothion depending on its mode of inclusion in the cavity, none apparently being prone to an attack by the -CD, but favoring instead the approach of an external OHgroup, in agreement with the experiment.

Interactions of cyclodextrins and their derivatives with toxic organophosphorus compounds

“…Indeed the two latter can form deeply included complexes (high α in parameter) compared to fenitrothion which is therefore less shielded from hydroxide ions, leading to the highest hydrolysis rate observed in this series ( Table 3 ) [ 42 ]. The origin of this trend was disclosed by theoretical semi-empirical calculations of parathion-, methyl parathion- and fenitrothion-β-CD complexes [ 55 ]. It was shown that the methyl group, adjacent to the nitro function slightly impairs both deep and axial inclusion of fenitrothion into the cavity.…”

Effect of cyclodextrins on the reactivity of fenitrothion

“…9,15,17 Theoretical calculation of the energies of the two types of complexes indicates that in the gas phase there is no significant difference in their values. 18 Therefore it is likely that the permethylation of the rim of the b-CD (TRIMEB) changes the combination of interactions between the host and the guest which results in a change of guest orientation in the cavity.…”

Inclusion of the insecticide fenitrothion in dimethylated-β-cyclodextrin: unusual guest disorder in the solid state and efficient retardation of the hydrolysis rate of the complexed guest in alkaline solution

“…This was suggested for parathion, methyl parathion, and paraoxon [17,21,23]. Theoretical calculation of the energies of the two types of complexes in the gas phase indicates that there is no significant difference in their values [24]. Single-crystal X-ray analysis shows that for β-CD and TRIMEB, complexes of type A are formed while for DIMEB (see above) in the unit cell a small percentage of the inclusion complexes exhibit the type B orientation.…”