The induction of aggregate formation of cationic polysoaps ((CL)-Copol Cl-12), cetyltrimethylammonium bromide (CTAB), n-dodecyltrimethylammonium bromide (DTAB), and n-dodecylmethyldiallylammonium bromide (DMDAAB) by low concentrations of Methyl Orange (10-5-10-4 M) and anionic surfactants (concentrations below their cmc's) in aqueous solutions was studied using UV-vis absorption and fluorescence spectroscopy. Reduced viscosities were also investigated as a function of polysoap concentration in the presence of low concentrations of the same additives. It was found that the cationic poly soaps, CTAB, DTAB, and DMDAAB aggregate far below their normal cmc in the presence of Methyl Orange in aqueous solution. The cationic polysoaps exhibited an about 5-fold decrease of the reduced viscosity in the presence of the hydrophobic anionic additives but no decrease of the reduced viscosity was found in the presence of hydrophobic nonionic additives and cationic additives (CTAB, DTAB). Depending on the length of the alkyl chains of the organic anionic additives, a conformational transition of the cationic polysoaps was indicated by changes of the reduced viscosity. Pyrene was used as a fluorescence probe to investigate the conformational state of the cationic polysoap CL-Copol Cl-12 in the presence of low concentrations of additives. Pirene fluorescence spectra revealed the formation of hydrophobic microdomains in the presence of the hydrophobic anionic additives depending on the length of alkyl chains. These domains were not observed in the case of hydrophobic cationic additives and nonionic additives.
Electrostatic interactions are considered to be major contributors to protein structure and specificity of enzyme catalysis. ['] However, few investigations have explored the control of substrate specificity of enzymes with ions of different sizes and charge densities. The control of substrate specificity in enzyme catalysis by use of organic solvents is well established.[' -31 Aggregation of amphiphilic polymers in water and water/organic solvent mixtures is well-known to lead to complex supramolecular structures with multiple morph~logies.[~~ Upon decreasing the number of generations, the aggregate morphology of polystyrene with poly(propy1enimine) dendrimers in aqueous solution has been shown to change from spherical micelles through micellar rods to vesicles.[61 The same morphological changes have also been found for polystyrene-b-poly(acry1ic acid) (PS-b-PAA) as poly(acry1ic acid) content de~reases.1~1 All of these studiesc4-' I suggest that an increase in hydrophobic effects of amphiphilic macromolecules upon an increase in ratio of hydrophobic to hydrophilic monomers leads to such changes of aggregate morphology. Consistent with the notion that ioninduced hydrophobic effects control aggregate morphology of amphiphiles, the morphology of aggregates of PS-b-PAA, polystyrene-b-poly(ethy1ene oxide) (PS-b-PEO), and PS-bpoly(4-vinylpyridinium methyl iodide) in water/organic solvent mixtures can also be changed from spheres to rods, and to vesicles by addition of salting-out agents such as NaCl and CaCI, .r81Polymers functionalized with the 4-(dialky1amino)pyridine group have been regarded as useful model systems for investigating the origins of enzymic efficiency and sele~tivity.[~ -"1 We have reported that macromolecule 1 containing the 4-(dialkylamino)pyridine functionality and a bis(trimethy1ene)disiloxane backbone as a nucleophilic catalyst exhibits enzyme-like substrate selectivity for the solvolysis of 2 in aqueous and methanol/ water solutions.[". 13, 14] To our knowledge, ion-induced substrate specificity changes have not been reported previously for catalytic ester solvolysis.' 1 2 ( n = 2 , 4 , 6 , 8 , 10, 12,14, 16.18) We present here ion-induced substrate specificity changes in the 1-catalyzed solvolysis of 2 ( n = 2, 4, 6,8, 10,12,14,16,18) in aqueous and methanol/water solutions. These results encourage more precise modeling studies of the molecular origins of catalytic efficiency and specificity in biological and chemical
COMMUNICATIONS
The substrate specificity in solvolysis reactions of
p-nitrophenyl alkanoates 2 (n = 2−18)
catalyzed
by 4-(dialkylamino)pyridine-functionalized polymer 1
can be controlled by the concentration of 1 in
50:50
(v/v) methanol−aqueous phosphate buffer solution at pH 8.0 and 30
°C. Below 1.0 × 10-5 unit mol
L-1,
macromolecule 1 exhibits substrate specificity for
2 (n =14). As the concentration of
1 increases to 2.5 ×
10-5 unit mol L-1,
the substrate preference changes from 2 (n =
14) to 2 (n = 12). The substrate
specificity
changes again from 2 (n = 12) to 2
(n = 10) when the concentration of 1 increases
further to 7.5 × 10-5 unit
mol L-1. The control of substrate
specificity by polymer catalyst concentration is believed to be
unprecedented
for catalysis of ester solvolysis.
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