Inhibition of Water-Catalyzed Ester Hydrilysis in Hydrophobic Microdomains of Poly(methacrylic acid) Hypercoils

Article abstract of DOI:10.1021/ja00323a045

The water-catalyzed hydrolysis of p-methoxyphenyl dichloroacetate (1) and 2,2-dichloropropionate (2) in aqueous solution at 25 deg C is strongly retarded by neutral atactic (at) and syndiotactic (st) poly(methacrylic acid) (PMAA), but not by poly(acrylic acid) and poly(N-vinylpyrrolidone).The rates and thermodynamic activation parameters are consistent with binding of the substrates to hydrophobic microdomains within the PMAA hypercoil.A conformational transition of PMAA to an extended coil leads to disappearance of the rate inhibition.This transition is induced either by ionization of PMAA or, at constant pH (ca. 3), by addition of urea and can be monitored by potentiometric titrations.Solubility measurements employing the water-insoluble dye Orange OT further established hydrophobic bonding to neutral at-PMAA.Whereas inhibition of the hydrolysis of 1 and 2 in water in the presence of hydrophobic cosolvents or micelles is characterized by initial-state stabilization, it appears that the inhibition by at- and st-PMAA primarily involves destabilization of the transition state.The effect of PMAA may be explained in terms of a lack of water penetration into the hydrophobic microdomains.