Micelle-enzyme complexes

The above enantioselectivities are obviously complex functions of many factors, perhaps even more complex than in natural enzymes. Complexity is partly due to the present co-micellar system in which it is difficult to analyze separately the interaction of the substrate with the achiral micelle, and that of the substrate with the catalyst complex. 

Biomimetic oxidation of cyclohexane was performed by Fish et al. in aqueous micelle system with t-butyl hydroperoxide (TBHP) and in the presence of biomimetic methane monooxygenase (MMO) enzyme complex [Fe20( 7-H20) ( )-OAc)L2] +. While in biphasic systems or in microemulsions, obtained by vigorous stirring, the reaction did not occur, the use of cetyltrimethylammonium hydrogensulfate (CTAHS) accelerated the reaction up to the rate observed in acetonitrile. Monti and collaborators developed another biomimetic system related to cytochrome P450, based on a porphyrin derivative, and successfully used it for the... 

In contrast to the situation in the absence of catalytically active Lewis acids, micelles of Cu(DS)2 induce rate enhancements up to a factor 1.8710 compared to the uncatalysed reaction in acetonitrile. These enzyme-like accelerations result from a very efficient complexation of the dienophile to the catalytically active copper ions, both species being concentrated at the micellar surface. Moreover, the higher affinity of 5.2 for Cu(DS)2 compared to SDS and CTAB (Psj = 96 versus 61 and 68, respectively) will diminish the inhibitory effect due to spatial separation of 5.1 and 5.2 as observed for SDS and CTAB. 

In contrast to SDS, CTAB and C12E7, CufDSjz micelles catalyse the Diels-Alder reaction between 1 and 2 with enzyme-like efficiency, leading to rate enhancements up to 1.8-10 compared to the reaction in acetonitrile. This results primarily from the essentially complete complexation off to the copper ions at the micellar surface. Comparison of the partition coefficients of 2 over the water phase and the micellar pseudophase, as derived from kinetic analysis using the pseudophase model, reveals a higher affinity of 2 for Cu(DS)2 than for SDS and CTAB. The inhibitory effect resulting from spatial separation of la-g and 2 is likely to be at least less pronoimced for Cu(DS)2 than for the other surfactants. 

The catalytic principle of micelles as depicted in Fig. 6.2, is based on the ability to solubilize hydrophobic compounds in the miceUar interior so the micelles can act as reaction vessels on a nanometer scale, as so-called nanoreactors [14, 15]. The catalytic complex is also solubihzed in the hydrophobic part of the micellar core or even bound to it Thus, the substrate (S) and the catalyst (C) are enclosed in an appropriate environment In contrast to biphasic catalysis no transport of the organic starting material to the active catalyst species is necessary and therefore no transport limitation of the reaction wiU be observed. As a consequence, the conversion of very hydrophobic substrates in pure water is feasible and aU the advantages mentioned above, which are associated with the use of water as medium, are given. Often there is an even higher reaction rate observed in miceUar catalysis than in conventional monophasic catalytic systems because of the smaller reaction volume of the miceUar reactor and the higher reactant concentration, respectively. This enhanced reactivity of encapsulated substrates is generally described as micellar catalysis [16, 17]. Due to the similarity to enzyme catalysis, micelle and enzyme catalysis have sometimes been correlated in literature [18]. 

MICELLAR SUBSTRATES. Phospholipids in micelles are frequently found to be more active substrates in lipolysis than those phospholipids residing in a lipid bilayer". Dennis first described the use of Triton X-100 to manipulate the amount of phospholipid per unit surface area of a micelle in a systematic analysis of the interfacial interactions of lipases with lipid micelles. Verger and Jain et al have presented cogent accounts of the kinetics of interfacial catalysis by phospholipases. The complexity of the problem is illustrated in the diagram shown in Fig. 2 showing how the enzyme in the aqueous phase can bind to the interface (designated by the asterisk) and then become activated. Once this is achieved, E catalyzes conversion of S to release P. ... 

As previously stated, the use of templates such as DBSA, HDTMAB, and PEOPE allows the formation of well-defined micellar structures in aqueous solution when the template concentration is above its CMC. In a recent publication [38], the polymerization with a bifunctional sodium dodecyl diphenyloxide disulfonate (DODD) as template was proposed to proceed by a micellar mechanism in the same way (Scheme 2). In an aqueous acid solution of DODD and aniline, anilium ions locate at the micellar interface, with benzene parts penetrating into the hydrophobic core of the DODD micelle to form the complex (as illustrated in Scheme 3b). At the concentration of 0.055 mol a slight turbidity was observed in solution, indicating micellar aggregation. Once the micellar structure is formed and the enzyme is added to the aqueous medium, addition of H2O2 triggers the polymerization of anilinium ions around micelles (Scheme 3). 

Enzymes and micelles resemble each other with respect to both structure (e.g., globular proteins and spherical aggregates) and catalytic activity. Probably the most common form of enzyme catalysis follows the mechanism known in biochemistry as Michaelis-Menton kinetics. In this the rate of the reaction increases with increasing substrate concentration, eventually leveling off. According to this mechanism, enzyme E and substrate A first react reversibly to form a complex EA, which then dissociates to form product P and regenerate the enzyme ... 

K-casein is physically inaccessible to enzyme. This theory is also supported by Creamer et al (1978), who, with an electron microscope, observed protein complexes formed by heating skim milk at 100°C for 30 min. These complexes are large, containing hundreds of individual protein molecules attached to casein micelles. At higher pH levels the complexes change, becoming more filamentous and associating less with micelles. 

Milk-clotting is a complex process, involving a primary enzymic phase in which K-casein is altered and loses its ability to stabilize the remainder of the caseinate complex, a secondary non-enzymic phase in which aggregation of the altered caseinate takes place, a third step where the aggregate of casein micelles forms a firm gel structure and a possibly separate fourth step where the curd structure tightens and syneresis occurs (McMahon and Brown 1984B). 

Hatada, A. and K. Kataoka. 1998. Novel polyion complex micelles entrapping enzyme molecules in the core preparation of narrowly-distributed micelles from lysozyme and poly(ethylene glycol)-poly(aspartic acid) block copolymer in aqueous mediuWiacromoleculeSI 288-294. 

Adapting techniques based on in vitro protein synthesis to the isolation of enzymes requires establishing a link between a nucleic acid-protein complex and product formation. Methods based on binding, analogous to those developed for phage displayed libraries, may be used to enrich catalysts from noncatalysts. In addition, Tawfik and Griffiths (1998) exploited the aqueous core of reverse micelles as artificial compartments... 

Artificial enzymes with metal ions can also hydrolyze phosphate esters (alkaline phosphatase is such a natural zinc enzyme). We examined the hydrolysis of p-nitro-phenyfdiphenylphosphate (29) by zinc complex 30, and also saw that in a micelle the related complex 31 was an even more effective catalyst [118]. Again the most likely mechanism is the bifunctional Zn-OH acting as both a Lewis acid and a hydroxide nucleophile, as in many zinc enzymes. By attaching the zinc complex 30 to one or two cyclodextrins, we saw even better catalysis with these full enzyme mimics [119]. A catalyst based on 25 - in which a bound La3+ cooperates with H202, not water - accelerates the cleavage of bis-p-nitrophenyl phosphate by over 108-fold relative to uncatalyzed hydrolysis [120]. This is an enormous acceleration. 

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