Inorganic supports, covalent immobilization

Probably the first non-covalent immobilization of a chiral complex with diazaligands was the adsorption of a rhodium-diphenylethylenediamine complex on different supports [71]. These solids were used for the hydride-transfer reduction of prochiral ketones (Scheme 2) in a continuous flow reactor. The inorganic support plays a crucial role. The chiral complex was easily... 

Weetall, H.H. (1993) Preparation of immobilized proteins covalently coupled through silane coupling agents to inorganic supports. Applied Biochemistry and Biotechnology, 41, 157-188. 

Other inorganic polymers, such as silicones, have excellent optical and mechanical properties for optical membranes. A great number of easily handled commercial silicone prepolymers are available but they have some disadvantages towards other materials. The surface is not easily modifiable to covalently immobilize indicators, they are not suited to combine with the customarily used support structures, owing to hard adhesion, and they are bad solvents for most of the indicators. 

Both organic and inorganic polymer materials have been used as solid supports of indicator dyes in the development of optical sensors for (bio)chemical species. It is known that the choice of solid support and immobilization procedure have significant effects on the performance of the optical sensors (optodes) in terms of selectivity, sensitivity, dynamic range, calibration, response time and (photo)stability. Immobilization of dyes is, therefore, an essential step in the fabrication of many optical chemical sensors and biosensors. Typically, the indicator molecules have been immobilized in polymer matrices (films or beads) via adsorption, entrapment, ion exchange or covalent binding procedures. 

Immobilization of triazacyclononane-type metal complexes on inorganic supports via covalent linking spectroscopy and catalytic activity in olefin oxidation... 

Of the inorganic supports, best results were reported for a mesoporous MCM-41 [337]. Support on ionic-liquid phases has been studied by different groups with variable results [338, 339], Of the non-conventional organic polymers, non-covalent immobilization on poly(diallyldimethylammonium) is notable [340], Catalysts 133 (15 mol.%) promoted the aldol reaction of acetone and benzaldehydes to afford the corresponding (i-hydroxyketones in 50-98% yields and 62-72% ee, which are clearly lower than those reported for other polymer-supported systems. Recycling of the catalysts was possible at least six times without loss of efficiency. More recently, proline has been attached to one DNA strand while an aldehyde was tethered to a complementary DNA sequence and made to react with a non-tethered ketone [341], To date, the work has focused more on conceptual development than on the analysis of its practical applications in organic synthesis. 

Immobilization of enzymes can be accomplished in several ways, including physical cnirapmeni in a polymer gel, physical adsorption on a porous inorganic support such as alumina, covalent bonding of the enzyme to a solid surface such as glass beads or a polv-rner. or copolynierizattt)nof the enzyme with a suitable monomer. 

This example illustratively shows that inorganic materials are well suited for continuous flow processes in column-like reactors. Thus, covalently immobilized NH-benzyl-(li, 2S)-(-)-norephedrine 10 on silica inside a column was doped with ruthenium. This setup was used to carry out continuous asymmetric transfer hydrogenation reactions (Scheme 10) [38]. Remarkably,no catalyst deactivation occurred over a period of one week, which the authors ascribed to the successful site isolation of the catalyst on the support. 

The properties of supported enzyme preparations are governed by the properties of both the enzyme and the carrier material. The interaction between the two provides an immobilized enzyme with specific chemical, biochemical, mechanical and kinetic properties. The support (carrier) can be a synthetic organic polymer, a biopolymer or an inorganic solid. Enzyme-immobilized polymer membranes are prepared by methods similar to those for the immobilized enzyme, which are summarized in Fig. 22.7 (a) molecular recognition and physical adsorption of biocatalyst on a support membrane, (b) cross-linking between enzymes on (a), (c) covalent binding between the biocatalyst and the membrane, (d) ion complex formation between the biocatalyst and the membrane, (e) entrapment of the biocatalyst in a polymer gel membrane, (f) entrapment and adsorption of biocatalyst in the membrane, (g) entrapment and covalent binding between the biocatalyst and the membrane, (h) entrapment and ion complex formation between the biocatalyst and the membrane, (i) entrapment of the biocatalyst in a pore of an UF membrane, (j) entrapment of the biocatalyst in a hollow-fiber membrane, (k) entrapment of biocatalyst in microcapsule, and (1) entrapment of the biocatalyst in a liposome. 

ILs can also be covalently immobilized on inorganic materials of silica, mesoporous silicates, aluminosilicate, and alumina. For that three ways are known (Scheme 10). The first method is the co-condensation of a trialkoxysdylalkyl onium salt and triethoxysilane (Scheme 10-A) [37-39]. The second one is the immobilization by the reaction between surface hydroxyl groups of the solid and the alkoxysilyl group (Scheme 10-B) [40,41]. For the last one (Scheme 10-C), the inorganic materials modified with alkyl halide (usually propylchloride) are used [42-46] on them, corresponding amine, phosphine, or pyridine can be immobilized and quaternized. For the preparation of imidazolium-based immobilized IL, imidazole is fixed on the modified support in the presence of alkali compounds, followed by quaternization with alkyl halides. 

Different attachment methods have been developped to immobilize enzymes on inorganic supports(Zaborsky 197 ) Two of them, which are particularly suited to the coupling on glass and porous ceramics have been tested in this study. The first one consists of the adsorption of the proteins to the carrier and the second is a covalent coupling through silanization of the support (See the contribution of Rao eh al. for more details). However, in contrast to the case of hydrogenase, catalase and BSA, the binding of the TSF I complexes... 

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