Comparative Analysis of Complexation of Pesticides (fenitrothion, methylparathion, parathion) and their Carboxylic Ester-Analogues by beta-Cyclodextrin. Theoretical Semiempirical Calculations.

ETHEL N. COSCARELLO; DORIS A. BARBIRIC; EDUARDO A. CASTRO; RAQUEL V. VICO; ELBA I. BUJÁN; RITA H. DE ROSSI

Journal of Structural Chemistry

Springer

Lugar: New York; Año: 2008

TRABAJO EN REVISION CON CORRECCIONES MENORES. SE ADJUNTA CARTA DE EDITOR EN VERSION ELECTRONICA Y EJEMPLAR IMPRESO______The complexation by b-Cyclodextrin (b-CD) of three organophosphorus pesticides, fenitrothion, parathion and methylparathion, and of their carboxylic ester analogues was analyzed via PM3 and molecular mechanics methods.  The objective was to shed light on structural features and energy changes that accompany complexation and could possibly affect the hydrolysis process, which is catalyzed by b-CD in the case of carboxylic esters but inhibited for the pesticides, in alkaline medium.  Complexation of fenitrothion was further explored, since experiments proved its hydrolysis to be relatively less inhibited and to progress mainly through a different pathway than usually accepted. Results show that complex structures involving esters enable effective interactions between the guest carbonyl and the rim of the host; methylparathion and parathion, however, appear deeply included in the cavity of the b-CD. So conditions for a nucleophilic attack by the b-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 prone to an attack by the b-CD, but favorable instead to the approach of an external HO– group, in agreement with experiment.