Reversible covalent templates

The most convenient way of categorizing the classes of cathepsin inhibitors is based on the nature of the electrophilic warhead that interacts with the sulfhydryl group of the active site cysteine residue. Since a large portion of the binding energy of a cysteine protease inhibitor comes from the covalent interaction with this thiol, the properties of the resulting molecules are largely derived from the electrophile. In broad terms, these inhibitors can be broken down into ketone and nitrile-based reversible covalent inhibitors, or the more recent non-covalent inhibitors based on an aminoaniline template. 

Two different techniques have been developed for MIP production, namely the covalent and the non-covalent approaches. The covalent way is based on the chemical derivatization of the template with molecules containing polymerizable groups using reversible covalent bonds. Different chemical reactions can be ap-... 

The two most used reversible covalent reactions are disulfide exchange and palladium-catalyzed olefin metathesis. We first probed the incorporation of olefin units into the H bonded duplexes by subjecting the modified duplexes to a Pd (Gmbb s) catalyst. Based on a duplex template with the same unsymmetrical H bonding sequence used for directing the formation of the /3-sheet structures, we prepared two groups (strands 17 and 18) of five olefins covalently linked to the two template strands (Fig. 9.13). Mixing each one of components 17 with each one of components 18 in a 1 1 fashion results in a small library of 25 (5 x 5) members. 

There are two general classes of imprinted polymers covalent and noncovalent MlPs. These two categories refer to the types of interactions between the functional monomer and the template in the prepolymerization complex. There are also hybrid MlPs that utilize a combination of covalent and noncovalent interactions in the preparation and rebinding events (Klein et al. 1999). Covalent MlPs utilize reversible covalent interactions to bind the template to the functional monomers. In contrast, noncovalent MlPs rely on weaker noncovalent functional monomer-template interactions. Each type has specific advantages and disadvantages with respect to sensing applications that will be addressed in subsequent sections. 

Attachment of the alkene monomers to the template (Fig. 6.5), which must be reversible - readily formed and readily broken - to permit removal of the template after polymerisation, can generally be accomplished in two ways covalently or non-covalently. While the latter interactions (ionic, hydrophobic, n-n, hydrogen bonding) can easily be reversed, there is less scope for reversible covalent linkages. One of these is the formation of boronic esters - from boronic acid units of the monomers and OH groups of sugar templates. 

Ni(II) salts give rise to trimeric 31 and tetrameric 32 macrocycles (Fig. 16B).45 111 116 There was some speculation about the mechanism of the condensation, although when the reaction was run with cyclic intermediate 33114 there was a predominant formation of trimeric 31. Apparently, a rapid interconversion of oligomeric Schiff bases would in the presence of Ni(II) cations become directed to form thermodynamically stable and metallated 31 and 32. Since these pioneering discoveries, there have been many reports about using templating molecules for directing the outcome of reversible covalent processes.42,44,66... 

Fig. 3.1. A highly schematic representation of the molecular imprinting process. A monomer mixture with ehemical functionality complementary to that of the template is allowed to form solution adducts through the complementary interacting functionalities (reversible covalent or non-covalent interactions). Polymerisation in the presence of a cross-linking agent, followed by removal of the template, leads to the defining of recognition sites of complementary steric and functional topography to the template molecule.

The synthesis of MIPs is performed by copolymerization of appropriate monomers in the presence of a template ligand. The monomers interact with the template by non-covalent interactions, reversible covalent interactions, or metal ion-mediated interactions. The types of interactions that are usually exploited in molecular imprinting are as follows ... 

The preparation of the materials starts by positioning the functional monomers around a template molecule. The monomers interact with sites on the template via interactions that can be reversible covalent or non-covalent (hydrogen, ionic, Van der Waals, n-n, etc.). They are then polymerised and cross-... 

DCL) of equilibrating compounds.In this approach, building blocks that form reversible covalent bonds are used to build a DCL (Figure 21). Stabilization of a library member upon addition of a template results in a new equilibrium. The end result in accord with Le Chatelier s principle, is amplification of stabilized products in the mixture. 

The requirement for ultimate removal of the covalent template precludes examples in which the reaction has been changed from an intermolecular to an intramolecular one by connecting the reaction partners by some kind of permanent tether which remains in the molecule. Even with this restriction, organic synthesis provides an extremely rich variety of reversible templating and tethering, as illustrated in Chapter 10 here, we would only like to mention the silicon connection chemistry introduced by G. Stork. Claimed template effects sometimes deserve critical mechanistic examination, and this is nicely illustrated in Chapter 9. 

Abstract This review presents an overview of the area of anion-templated synthesis of molecules and supramolecular assemblies. The review is divided into two main sections the first part deals with anion-templated systems where the final products are linked by bonds that are not reversible under the conditions of the experiment Several recent examples of macrocycles, cages and interlocked species are presented in this section. The second part of the chapter, presents a discussion of anion-templation in systems containing reversible bonds that give rise to dynamic combinatorial libraries (either by formation of coordination metal-ligand bonds or by reversible covalent bonds). 

Two main approaches are used to produce MIPs the noncovalent [48] and the covalent [49] approach. In the covalent approach (Figure 5.12a), the functional monomer is covalently bonded to the template molecule before polymerization. When polymerization is complete, the covalent bonds between the template molecule and the polymer are cleaved and the template molecule is extracted. The resulting imprint is then able to recognize and rebind the imprinted analyte via reversible covalent bonds. However, this technique suffers from lack of generality owing to the difficulties of finding suitable monomers. 

The covalent approach or the pre-organized approach implies the formation of a template-functional monomer complex through reversible covalent bonds prior to polymerization. After synthesis and removal of the template, in the subsequent rebinding step, the initial covalent linkage is reconstituted between the polymer and template. Therefore, only a low number of non-selective binding sites are expected to be formed because of the well-defined stoichiometry taking place between the functional monomer and template. Unfortunately, this approach is only applicable to a limited number of template molecules. 

The semi-covalent approach combines the advantages of the previous two methods, employing reversible covalent bonds in the imprinting step and noncovalent interactions in the recognition process, after the cleavage of the template from the polymer. 

In the covalent approach (Figure 2), a polymerizable derivative of the template is obtained by linking the template with a vinyl functional monomer via a strong reversible covalent bond (e.g., boronate esters, Schiff bases, or ketal). The derivatized print molecule is then radical copolymerized with an excess amount of a cross-linking agent, using either thermal or photochemical radical initiation. Reaction conditions are... 

The principal idea behind MIP synthesis is the generation of solution state complexes between the template ligand in hand and appropriate functional monomers, followed by subsequent freezing of these complexes by copolymerization of the above with an excess of a cross-linking monomer. These monomer-template complexes are stabilized by non-covalent interactions, reversible covalent interactions, or metal ion-mediated interactions. The types of interactions that are usually exploited in molecular imprinting are 1) cleavable covalent bonds 2) tt-tt interactions 3) hydrogen bonds 4) hydrophobic van der Waals interactions 5) crown-ether/cyclodextrin type interactions 6) metal-Ugand... 

Fig. 2.12 Schematic representation of the molecularly imprinting process the formation of reversible interactions between the template and polymerizable functionality may involve one or more of the following interactions (A) reversible covalent bond, (B) covalently attached polymerizable binding groups that are activated for nrai-covalent interaction by template cleavage, (C) electrostatic interactions, (D) hydrophobic or Van der Waals interactirais, (E) co-ordination with a metal center each kind of interaction occurs with complementary functional groups or structural elements of the template, (a-e), respectively. A subsequent polymerization in the presence of crosslinker(s) results in the formation of an insoluble matrix in which the template sites reside. Template is then removed from the polymCT through disruptirai of polymer-template interactions, and extraction from the matrix. (Reproduced fiom Ref. [97] with the permission of Wiley)...

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