Noncovalent Approach

An approach based on coordinative bonds was followed by Bielawski et al., who developed electrically conductive coordination polymers that can be healed after damage by thermal treatment [62]. Incidentally, one further successful approach does not rely on dynamic features within the material. It simply carries microencapsulated monomer within its matrix, a polymer, which contains a polymerization catalyst When a crack propagates through the polymer matrix, monomer is released and polymerized within the crack, restoring the initial material properties of the sample [63, 64]. 

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The noncovalent approach is based on the use of complexes containing suitable moieties which are able to recognize specific proteins, primarily human serum albumin (HSA). When the targeting protein is confined in the blood, the adduct between the serum albumin and the functionalized complex may function as a blood pool agent. Due to the reversible nature of binding between the protein and the paramagnetic chelate, these adducts maintain excretory pathways typical of small complexes which, from the pharmacological point of view, favors them over covalently bound macromolecules. 

Functionalization of carbon nanotubes with metals can be achieved by different techniques exploiting either the covalent or the noncovalent approach. This topic, which is important for many applications, will be briefly discussed in a separate section after the description of the two methods. 

Alternatively, benzyl and pyrene derivatives offer a noncovalent approach to alter nanocarbon surface chemistry and have been studied extensively [62]. For example,... 

Zeng HQ, Yang XW, Flowers RA, Gong B. A noncovalent approach to antiparallel j8-sheet formation. J Am Chem Soc 2002 124 2903-2910. 

A recent example of the noncovalent approach is that of Roy and Imperiali.M Earlier work on noncovalent and disulfide-bridge semisynthesis is dted in a review 2 and we limit ourselves in this chapter to the covalent preparation of semisynthetic proteins, using peptide bonds, oximes, hydrazones, and thiazolidines. 

In the noncovalent approach, the monomer is self-assembled around the tern-plating molecule and then again copolymerized with the additional monomer. The template is then removed by using a porogenic solvent. 

Noncovalent approaches can usually preserve the structures and properties of carbon nanotubes after functionalization17 (though not necessarily the near-infrared absorption characteristics due to well-established doping effects), thus are equally important to the biocompatibilization and bioapplications of nanotubes.15 Among commonly employed noncovalent schemes are surfactant dispersion,18 tt-tt stacking with aromatic compounds,19 and polymer wrapping.20... 

While technically simpler than the covalent approach, the self-assembly approach creates more heterogeneous sites and also requires templates with specific functional groups.8 Since sol-gel chemistry is aqueous based, H-bonding interactions are significantly weaker compared to the conventional organic polymerization methods. Often, hydrophobic effects and ionic interactions are utilized. A number of other examples of the noncovalent approach to imprinting in sol-gel-derived materials are provided in recent reviews.5 17 In the sections below, the focus will be on some of the newer aspects of small molecule imprinting in silica that involve the use of chiral templates... 

The groups of Reek and Meijer have also applied the noncovalent approach for catalyst anchoring to a dendrimer support [62], Phosphine functionalized ligands were attached to the periphery of polypropylene imine) dendrimers via combined ionic interactions and H-bonding using a specific binding motif that is complementary to that of the support (Fig. 6). 

To address this sort of reaction we needed to develop a new approach to catalytic antibodies or enzymes in general that differed significantly from the noncovalent approach of Pauling and Jencks. We sought a method that would allow for the programming of detailed aspects of chemical reaction mechanisms, down to the level of the chemical identity... 

Heme proteins are one of the largest classes of metalloproteins studied to date see Iron Heme Proteins Dioxygen Transport Storage Iron Heme Proteins, Peroxidases, Catalases <6 Catalase-peroxidases). More than 5% of protein structures in the Protein Data Bank contain at least one heme moiety. It is no wonder that designing heme proteins has been one of the most active areas of research. Since the dominant secondary structure in heme proteins is a-hehces (accounting for 77% of all secondary structures in known heme proteins),a number of Q -helix-containing metalloporphyrin-peptides have been synthesized using either covalent or noncovalent approaches. 

An alternative to noncovalent approaches is covalent attachment of ligands to the surface. Methods for covalent attachment can be broadly categorized as follows 1) condensation with the aldehyde of a reducing sugar, 2) nucleophilic addition to or... 

Fig. 12 Concept of DNA-based catalysis in the noncovalent approach. A Class 1 ligands. B-D Class 2 ligands...

A one-dimensional porphyrin array is the most basic product to come from the connection of porphyrin units. The preparation of one-dimensional porphyrin arrays has been approached in various ways, using not only covalent but also noncovalent approaches. There are various ways to connect 7t-conjugated molecular systems 1 nm in size. Planar, conjugated, fused, orthogonal, and helical array connections are all available. Depending on the mode of connection, these arrays may be used as materials for molecular wires, photocurrent generation, optoelectronics, and so on. Therefore, many studies have been reported on the preparation of one-dimensional porphyrin arrays. 

Figure 17.1 Scheme of molecular imprinting in chiral analysis using the noncovalent approach. 

Before the molecularly imprinted polymer may be used for rebinding studies, one step remains—extraction of the original template molecules from the polymer matrix. A number of extraction protocols have been developed for different imprinting procedures. In the covalent and metal coordination approaches, an appropriate reagent is needed to break the bonds formed between the template and the functional elements. For the noncovalent approach, a polar solvent, often... 

The noncovalent approach to imprinting remains the area of the technique for which Mosbach is best known, although he has also worked with a variety of other... 

This in turn leads to a much broader range of applicability compared to covalent and semi-covalent techniques. The diversity of target compounds that have been imprinted using noncovalent techniques is vast and a brief general overview of substance classes is listed in Table 2. Finally, compared to covalent techniques, the noncovalent approach utilizes interactions that are more similar to those seen in natural, biochemical processes. Thus, the noncovalent approach is regarded as the most promising technique for mimicking nature s selectivity. 

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