Hydrolytic studies, micelles

Many hydrolytic studies have been reported utilizing both micellar and synthetic pol3nneric systems. Cordes (7 ) and co-workers, and Ocubo and Ise ( ) have reported on systems which incorporate both the novel features of micelles and polyelectrolytes, i.e., catalyst systems which have binding sites available for both strong hydrophobic and electrostatic interaction with suitable substrates. Such hydrophobic polyelectrolyte systems have been prepared and have been termed "polysoaps" (9,10). The catalytic properties of these polysoaps has remained largely unexplored. 

Inspired by the many hydrolytically-active metallo enzymes encountered in nature, extensive studies have been performed on so-called metallo micelles. These investigations usually focus on mixed micelles of a common surfactant together with a special chelating surfactant that exhibits a high affinity for transition-metal ions. These aggregates can have remarkable catalytic effects on the hydrolysis of activated carboxylic acid esters, phosphate esters and amides. In these reactions the exact role of the metal ion is not clear and may vary from one system to another. However, there are strong indications that the major function of the metal ion is the coordination of hydroxide anion in the Stem region of the micelle where it is in the proximity of the micelle-bound substrate. The first report of catalysis of a hydrolysis reaction by me tall omi cell es stems from 1978. In the years that... 

Block copolymers (228), consisting of a hydrophilic poly(ethylene glycol) and a hydrophobic polyphosphazene residue, have been investigated with respect to their micelle formation in aqueous solution Micelle formation in water has also been observed for polymers (229) with ethyl glycinato substituents. Hydrolytic degradation of these polymers has been studied in aqueous thf. ... 

Proteases, when solubilized in the aqueous core of reverse micelles, can catalyze the hydrolysis of various small model peptides. In most of the cases, the substrates are partitioned between the micelles and the external solvent while the hydrolytic reaction is taking place within the dispersed aqueous domains. Since the beginning of micellar enzymology, a-chymotrypsin and trypsin have been extensively studied in different reverse micellar systems, employing various model peptides as substrates. In almost all cases the reactions followed Michaelis-Menten kinetics. 

In 1988, Walde and coworkers studied the kinetic and structural properties of another serine protease, namely trypsin, in two reverse micellar systems, AOT/ isooctane and CTAB/chloroform/isooctane, employing three different model substrates, an amide and two esters [71], The main aim of this work was to compare the behavior of trypsin in reverse micelles with that of a-chymotrypsin. In the case of trypsin, superactivity was not observed and in general no obvious similarities between the two enzymes were recorded. Some years later, reverse micelles formulated with biocompatible surfactants such as lecithin of variable chain lengths in isooctane/alcohol were studied in relation to their capacity to solubilize a-chymotrypsin and trypsin [72]. The hydrolytic behavior of the same serine proteases, namely a-chymotrypsin and trypsin, in both AOT and CTAB microemulsions was studied and related to the polarity of the reaction medium as expressed by the logP value and measured by the hydrophilic probe 1-methyl-8-oxyquinolinium betaine [39], In this study a remarkable superactivity of trypsin in reverse micelles formed with the cationic surfactant CTAB was reported. 

In the following years some more studies appeared in the literature concerning trypsin activity in reverse micelles in relation to various characteristics of the reaction medium. Fadnavis et al. studied the pH dependence of hydrolytic activity of trypsin in CTAB reverse micelles toward a positively charged model ester substrate [74]. It was found that enzyme activity variations as a function of w are pH dependent. In 2005, Dasgupta and coworkers related the catalytic activity of trypsin in reverse micelles formulated with cationic surfactants with the concentration of the water-pool components and the aggregate size to delineate the independent role of both parameters [75]. Finally, in 2006, the influence of ethylene glycol on the thermostability of trypsin in AOT reverse micelles was examined and was found to exhibit a positive effect [76]. 

Ono et al. have synthesized series of open acetal surfactants anionics, nonionics, cationics, and amphoterics—and made a systematic study of the influence of the polar head group on hydrolytic reactivity (Fig. 16) [36,37]. The hydrophobic tail as well as the coimecting group was kept constant and the time for complete decomposition was recorded. The results, shown in Table 1, constitute an illustration of the effect of the micelle surface on the hydrolysis rate. With negatively charged micelles the reaction is very fast, with... 

Another experimental example, which could demonstrate the importance of the so-called empirical criteria described in the beginning of this section, is the kinetic study of alkaline hydrolysis of 4-nitrophthalimide (NPT) in the absence and presence of cationic micelles (cetyltrimethylammonium bromide, CTABr micelles) at 35°C. The pH-independent and pH-dependent rate of alkaline hydrolysis of NPT in the absence and, presumably, in the presence of micelles involve the reaction of HO with nonionized (SH) and ionized (S ) NPT, respectively Pseudo-first-order rate constants (kobs) for hydrolytic cleavage of NPT at 0.01-M NaOH, 35°C and varying total concentrations of CTABr ([CTABrlx), as listed in Table 7.14, have been treated with Equation 7.76 where [D ] = [CTABrlx - CMC, Ks is CTABr micellar binding constant of NPT, k and k, represent pseudo-first-order rate constants for hydrolysis of NPT in the aqueous pseudophase and micellar pseudophase, respectively. The nonlinear least-squares calculated values of k, . Kg, and least squares (Sd ) are shown in Table 7.14. The values of k ai are comparable with the corresponding kobs values within the domain of the acceptable residual errors dj (Table 7.14). 

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