Catalysis by Inverse Micelles in Non-Polar Solvents
The rates of numerous reactions are affected by the presence of micellar surfactants in aqueous solutions.1–4 Structural similarities between globular proteins and spherical micelles and the analogies between enzymatic and micellar catalysis have prompted recent investigations of micellar systems as possible models for the micro-environment of the active site of enzymes. Although the kinetics for micellar catalysis generally obeys the Michaelis-Menten equation and, in many cases, competitive inhibition has been observed, micelles in aqueous solutions rarely enhance the rates of reactions by factors greater than 102 and show relatively limited substrate specificity.1–3 Micelles, unlike enzymes, are in a dynamic equilibrium with the monomeric surfactant and have comparatively mobile structures in water,5 Additionally, micelles do not bind the substrate in a rigid configuration with a specific orientation. It appears, therefore, that the aqueous micellar systems investigated to-date provide somewhat poorer models for enzymatic interactions than originally anticipated. Since the active sites of many enzymes are in a relatively hydrophobic environment and since X-ray crystallographic studies have indicated ion pair and hydrogen bonding interactions in polar regions of some proteolytic enzymes,6,7 model studies in apolar solvents8 and at interfaces9 have provided a better understanding of the mechanisms involved. The hydrolysis of p-nitrophenyldodecanoate has recently been examined in hexanol systems containing water and hexadecyl-trimethylammonium bromide under conditions where formation of micelles, “reverse” micelles, and liquid crystalline phases have been demonstrated.10,11 Rate accelerations of ca. 20-fold have been found in the regions where water is solubilized in the polar interior of the reversed micelle. This rate enhancement was analogous to that observed previously for the reaction of p-nitrophenyldodecanoate in aqueous micellar hexadecyltrimethylammonium bromide solution12 indicating that solubilization of the hydrolyzing agent results in catalytic efficiency similar to that for substrate solubilization.
KeywordsCritical Micelle Concentration Surfactant Concentration Reversed Micelle Micellar System Rate Enhancement
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