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Templates transition state binding

The most interesting developments involve catalysis of simple aldol reactions. The key to reactive immunisation is the use of a hapten that is chemically reactive, rather than a passive template. This means that (i) relevant chemistry is going on during the course of antibody induction, which thus happens in the presence of intermediates involved in the reaction, and so may be modified to favor the formation of antibodies which bind these intermediates (and perhaps transition states leading to them). Furthermore (ii) it becomes possible to select for antibodies that react with, rather than just bind, to the hapten. The system used for the development of aldolase antibodies is outlined in Scheme 2... [Pg.345]

The ability of a chemical to act as a template is frequently attributed to a combination of thermodynamic and kinetic factors. As has been defined by Busch [3] a thermodynamic template binds more strongly to one of the products present in an equilibrium (i.e. a mixture under thermodynamic control) shifting the reaction towards the formation of this specific product which is then obtained in higher yields. In contrast, kinetic templates operate under irreversible conditions by stabilising the transition state leading to the final product. [Pg.92]

Transition-state stabilization The active site often acts as a flexible molecular template that binds the substrate in a geometric structure resembling the activated transition state of the molecule (see T at the top of the curve in Figure 5.4). By stabilizing the substrate in its transition state, the enzyme greatly increases the concentration of the reactive intermediate that can be converted to product and, thus, accelerates the reaction. [Pg.56]

Optically active phosphine monoester 69 was prepared as a stable transition state analogue of the hydrolysis of 70, and used as a template (Scheme 13.17) [52], When heated in the presence of amidine 72, methyl methacrylate, ethylene glycol dimethacrylate and AIBN, polymerization took place and offered the polymer 73, in which the template 69 is attached to the binding sites by a combination of hydrogen bonds and electrostatic interactions. After washing in the presence of MeOH and aqueous NaOH, the template was removed from the cavity, thus leading to a catalytically active MIP. [Pg.443]

Mosbach prepared imprinted polymers with not only a more specific substrate binding but with a true catalytic turnover [148]. Certain N-protected amino acid derivatives formed pseudotetrahedral complexes in solution in the presence of Co and two molecules of 5-vinylimidazole. These were polymerised in the presence of divinylbenzene and the metal-ion and amino acids eluted together afterwards. Hydrolysis of the corresponding 4-nitrophenylester of the amino acid template showed an increase in catalytic activity (factor 2-4) compared with a blank prepared without the template. The polymer also exhibits a clear preference for the activated ester of the template, which suggests that hydrolysis indeed takes place inside the cavities. Two years later the scope was further broadened by the preparation of the first polymer imprinted with a transition state analogue. [Pg.105]

Scheme 4.V. Schematic representation of the polymerisation of template 19 in the presence of two equivalents of binding site monomer 13 (A), removal of 19 (B), and catalysis causing alkaline hydrolysis of 18 through a tetrahedral transition state [11]. Scheme 4.V. Schematic representation of the polymerisation of template 19 in the presence of two equivalents of binding site monomer 13 (A), removal of 19 (B), and catalysis causing alkaline hydrolysis of 18 through a tetrahedral transition state [11].
One of the most attractive applications would be molecularly imprinted catalysts. In principle, such catalysts could be prepared if substrate, product or transition-state analogs could be used as template molecules, since to natural catalytic antibodies are produced in a similar way. Since molecularly imprinted polymers are considered to be analogous to antibodies in that binding sites are tailor-made, catalytic antibody-like activity in imprinted polymers could also be conceived, enabling an artifi-cial catalytic antibody with the advantageous features of synthetic molecules to be produced. [Pg.108]

Metal ion cofactors have varied roles to enhance the catalytic efficiency of enzymes in hydrolytic reactions, including facilitate substrate binding (water and organic substrate), gathering/template effects, function as an electrostatic catalyst (carbonyl polarization and transition state stabilization), function as a Lewis acid to lower the pA a of metal-water and stabilize the formation of the leaving group. Although their properties make several... [Pg.575]

Busch classified templates as thermodynamic [13a, 27] or kinetic [ 13b, 28], A thermodynamic template shifts the position of equilibrium of a reversible reaction by preferentially binding one product. Kinetic templates operate on irreversible reactions by stabilizing the main transition states leading to the desired product. Kinetic templates almost invariably bind the product more strongly than the starting material, so they also favor the formation of the product thermodynamically. Conversely, thermodynamic templates are likely to accelerate formation of the product by transition state stabilization, so classification of the observed effect depends crucially on the reaction conditions and time scale. [Pg.5]


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See also in sourсe #XX -- [ Pg.280 ]




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