Covalent bonding, immobilization

Abstract The immobilization of chiral catalysts through non-covalent methods, as opposed to covalent immobilization, allows an easier preparation of chiral heterogeneous catalysts with, in principle, less influence of the support on the conformational preferences of the catalytic complex. In this review the different possibilities for immobilization without forming a covalent bond between the chiral diazahgand and the support, which can be either solid or liquid, are presented. 

In the last 20 years a great deal of effort has been focused towards the immobilization of chiral catalysts [2] and disparate results have been obtained. In order to ensure the retention of the valuable chiral hgand, the most commonly used immobihzation method has been the creation of a covalent bond between the ligand and the support, which is usually a solid, hi many cases this strategy requires additional functionalization of the chiral hgand, and this change - together with the presence of the very bulky support - may produce unpredictable effects on the conformational preferences of the catalytic complex. This in turn affects the transition-state structures and thus the enantioselectivity of the process. 

Enzymes are immobilized by a variety of methods. Two general types of immobilization procedures are used. The first-type procedures are based on weak interactions between the support and the enzyme and are classified as physical methods. The second-type procedures rest upon the formation of covalent bonds between the enzyme and the support and are classified as chemical methods. 

Immobilization of Gold Nanoparticles onto Hydrogen-Terminated Silicon Surface by Si-C Covalent Bonds... 

Wet preparation of metal nanoparticles and their covalent immobilization onto silicon surface has been surveyed in this manuscript. Thiol-metal interaction can be widely used in order to functionalize the surface of metal nanoparticles by SAM formation. Various thiol molecules have been used for this purpose. The obtained functionalized particles can be purified to avoid the effect of unbounded molecules. On the other hand, hydrogen-terminated silicon surface is a good substrate to be covered by Si-C covalently bonded monolayer and can be functionalized readily by this link formation. Nanomaterials, such as biomolecules or nanoparticles, can be immobilized onto silicon surface by applying this monolayer formation system. 

Bovine serum albumin covalently bonded to silica and a-acid glycoprotein immobilized on silica have been used to resolve a wide range of acidic and basic drugs and amino acid derivatives [807-810]. Because of their complex structures, however, the... 

Hydrophilic polymers (Table 5) provide a matrix which is comparable to an aqueous environment. Ions can diffuse quite freely, but the possible water uptake (10-1000%) can cause significant swelling of the polymer. Swelling of the matrix affects the optical properties of the sensors and, consequently, the signal changes. Immobilization of the indicator chemistry usually is achieved via covalent bonding to the polymer. 

Since immunosensors usually measure the signals resulting from the specific immu-noreactions between the analytes and the antibodies or antigens immobilized, it is clear that the immobilization procedures of the antibodies (antigens) on the surfaces of base transducers should play an important role in the construction of immunosensors. Numerous immobilization procedures have been employed for diverse immunosensors, such as electrostatic adsorption, entrapment, cross-linking, and covalent bonding procedures. They may be appropriately divided into two kinds of non-covalent interaction-based and covalent interaction-based immobilization procedures. 

Protein is immobilized by combining with the surface of the electrode through a covalent bond, which is called covalent bonding of protein. The process requires low temperature (0°C), low ion intensity, and physiological pH conditions. Although covalent bonding onto the surface of an electrode is more difficult than adsorption, it can provide a more stable immobilized protein. 

The examples in Scheme 2-1 show that in intra-annular chirality transfer the resident asymmetric center connects to the enolate through annular covalent bonds. The geometric configuration of the enolate can be either immobile or irrelevant to the sense of asymmetric induction. 

---