Noncovalent immobilization approaches

3 NONCOVALENT IMMOBILIZATION APPROACHES Noncovalently Bound Metal Complexes 

Krijnen et al. reported on the noncovalent, nonionic immobilization of Ti-silsesquioxanes onto MCM-41, as an epoxidation catalyst.[132] An intriguing aspect 

To avoid the limitations of the small apertures and cavities provided by zeolites, mesopores have been created inside zeolites X, Y and DAY.[140] By dealumination of the zeolite structure, mesoporous regions that are completely surrounded by micropores were obtained, and these intrazeolitic cavities were then used as the space in which to assemble metal complexes. The preparation of a cobalt-salen-5 complex provided a catalytic material that shows improvements in the conversion, 

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Hyperbranched polymers can also be used for supramolecular immobilization (Scheme 15). Yet another approach for the noncovalent immobilization has been presented by Tzschucke and coworkers who used interactions between fluorous phase silica (FPS) and perfluoro-tagged palladium... 

An elegant alternative approach for noncovalent interaction relies on fluorous-lluorous interactions. A glycan array of monosaccharides and disaccharides bearing anomeric fluorous tags was noncovalently immobilized on fluorous-derivatized glass slides (19, 20). The attachment method is compatible with a wide range of functional groups and has been successfully used to probe carbohydrate-protein interactions. 

Noncovalent immobilization can also be used for I the site-selective coupling of antibodies to supports. One common approach for this involves absorbing the I antibody to a secondary ligand such as protein A or protein G, which both bind to the Fc region of many... 

Noncovalent adsorption of native carbohydrate probes on a substrate surface is the simplest way to prepare carbohydrate microarrays. This method relies on the formation of a variety of noncovalent interactions between the surface and the arrayed carbohydrates. In addition to its simplicity and high-throughput characteristics in array construction, these approaches may be favorable in supporting the preservation of the native structure of spotted carbohydrate antigens since there is no need to modify the carbohydrates before microarray application. However, given that the saccharides are noncovalently immobilized on an array substrate, the efficiency of immobilization must be verified for each spotted carbohydrate. 

Other Immobilization Techniques Along with noncovalent and covalent immobilization methods, other techniques have been developed for the preperation of affinity supports. Such methods include entrapment, molecular imprinting, and the use of the ligands as both the support and stationary phase. Although these methods are not as common as the approaches already examined, they have important advantages in some applications [8]. 

Recently, we extended the approach to dendrimers immobilized on silica. For this purpose, the first generation (and also larger generations) DAB den-drimer was functionalized with urea propyl-(SiOR)3 groups that via an easy process were grafted on silica. The catalyst was noncovalently anchored by simply adding a solution of a metal complex with ancillary ligands utilized with the typical the complementary motif (Fig. 7) [29]. 

Dai et al. [90] reported a very interesting work introducing a simple and general approach for noncovalent functionalization of the sidewalls of CNTs for further immobilization of ferritin, streptavidin and biotinyl-3,6-dioxaoetanediamine in a very efficient way. The first step was the noncovalent funetionalization of SWCNTs by irreversible adsorption of a bifimetional moleeule, 1-pyrenebutanoic acid, succinimidyl ester onto the hydrophobie surfaees of SWCNTs dispersed in DMF or methanol. 

One approach that can overcome immobilization-related problems involves the use of antigen (or ligand) covalently bound to biotin (Ag-B). The Ag-B is added to the solution at the desired concentration to allow solution-phase binding, so that the selection process is based mainly on the differential affinity of each antibody or other binding molecule. The mixture is then added to strepavidin-coated beads or wells, where the formation of the biotin-strepavidin noncovalent complex allows the selection of high affinity antigen-binding molecules. 

Similarly to Bianchini s approach, De Rege [26] also immobilized cationic [((R,R)-Me-duphos (26))Rh-(COD)]OTf complex noncovalently by the hydrogenbonding interaction of triflate counterion with surface silanols ofMCM-41 support. In contrast to the results obtained by Bianchini et al. [25c], the catalytic activity and selectivities of the immobilized 26-Rh complex on MCM-41 were equal to or greater than the homogeneous counterparts (Scheme 2.7). Moreover, the catalysts were recyclable (up to four times, with no loss of activity) and did not leach. Here again, the counteranion was very important for the successful immobilization of the catalyst onto MCM-41. Whereas, the DuPhos-Rh complex with triflate anion was effectively immobilized (6.7 wt% based on Rh), tlie analogous complex with the lipophilic BArp anion [BArp = R(3,l-((. i )2-C J I i was not loaded onto the support. 

Solid phase hybridization is in most cases achieved on membranes. Target nucleic acid is immobilized and subsequently detected by a probe. This approach forms the basis of slot/dot blot hybridization, Northern and Southern hybridization and colony or plaque hybridization. Dot/slot blot hybridization (Kafatos et al., 1979) demonstrates the presence of target sequences but not their size. Although solid phase hybridization is convenient for hybrid/free probe separation, it has the disadvantages that nucleic acid is most often bound noncovalently and that targets are immobilized at fre-... 

Due to the nature of carbon materials, the presentation of representative methods for surface derivatization will follow an approach different from that described in the preceding section, which is based on the spatial target site where physical-chemical modification can take place (1) immobilization performed at edges and/or ends and defects of graphitic sheets, (2) immobilization onto the graphene sheets, and (3) exclusively for CNTs we present some examples of endohedral encapsulation of metal complexes. For the first two cases, covalent bonding and noncovalent interactions can occur directly between the transition metal complex and carbon supports or via spacers grafted to the carbon surface. 

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