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

The complexation by beta-cyclodextrin (beta-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 beta-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 beta-CD. Therefore the conditions for a nucleophilic attack by the beta-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 beta-CD, but favoring instead the approach of an external OH- group, in agreement with the experiment.