Ligands, support-bound

Porous glass (PG) modified with covalently adsorbed poly(p-nitrophenyl acrylate), as described in Sect. 4.1, turned out to be a highly suitable carrier for immobilization of various biospecific ligands and enzymes. When the residual active ester groups of the carrier were blocked by ethanolamine, the immobilized ligands when bound to the solid support via hydrophilic and flexible poly(2-hydroxyethyl acrylamide). The effective biospecific binding provided by the ligands... 

Support-bound transition metal complexes have mainly been prepared as insoluble catalysts. Table 4.1 lists representative examples of such polymer-bound complexes. Polystyrene-bound molybdenum carbonyl complexes have been prepared for the study of ligand substitution reactions and oxidative eliminations [51], Moreover, well-defined molybdenum, rhodium, and iridium phosphine complexes have been prepared on copolymers of PEG and silica [52]. Several reviews have covered the preparation and application of support-bound reagents, including transition metal complexes [53-59]. Examples of the preparation and uses of organomercury and organo-zinc compounds are discussed in Section 4.1. 

Metal Support Support-bound Ligand Application Ref. 

Immobilization is a popular means of simplifying separation of a catalyst from the reaction mixture. In contrast with immobilized metal complexes (via a solid-support-bound ligand) leaching problems are a less critical issue when using organocatalysts immobilized by covalent bonding to the solid support. 

Figure 6-4 Principle of affinity chromatography.The analyte (enzyme, antibody, antigen, tissue receptor, etc.) binds to the support-bound ligand. Subsequently, it is eluted with a general eluent (such as a chaotropic agent), pH change, or biospecific eluent (such as an inhibitor or substrate).

The complexation of neutral molecules by ligands covalently bound to polymer supports requires the study of a set of parameters different from those involving the complexation reactions of metal ions. While the latter emphasizes ion exchange mechanisms, the former relies on reactions such as acid/base interactions for selectivity. The role of the polymer support in molecular complexation reactions has been the focus of this part of our research studies are being undertaken to determine whether the polymer acts only as an inert matrix on which to bond appropriate ligands, or whether it can also influence the ligand-molecule interaction. 

Andersson [10 a, c] has used both acidic and basic polymeric supports based on phosphine-substituted poly(acrylic acid) (PAA) and poly(ethyleneimine) (PEI) respectively (polymer-bound ligands 4a, 4b and 5). In the latter case, presumably a branched dendritic PEI was employed, and the phosphine ligand was bound selectively to the multiple end groups. 

The support or matrix to which the ligand is bound must conform to stringent criteria for example, it must consist of spherical gel particles with good flow properties, a chemically inert macromolecular network with very large-sized pores must be present through which unbound protein molecules may freely pass, and suitable functional groups must be present on matrix to which the ligand can be bonded. 

When the affinity ligand is bound to the solid support, the equilibrium constant, AT, is affected to a certain extent. A modification of the affinant by the attachment to the support results in an increase in AT, as a consequence of the Umitation of the steric accessibility of the affinant. On the other hand, non-specific adsorption of the enzyme to the solid support and to the molecules of the already adsorbed enzyme causes a decrease in Af,. Assuming that a single enzyme of the crude proteins has an affinity for the specific adsorbent, the equilibrium between the attached affinant L and the isolated enzyme E is given by the equation... 

Based on X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), electron spin resonance (ESR), Mbssbauer, and extended X-ray absorption fine structure spectroscopy (EXAFS) , van Veen and collaborators concluded that the thermal treatment at temperatures where catalytic activity is maximum ( 500-600°C) does not lead to complete destruction of the macrocycles, but rather to a ligand modification which preserves the Me-N4 moiety intact. Furthermore, the stability of this catalytic site is improved because the reactive parts of the ligands are bound to the carbon support and thus are no longer susceptible to an oxidative attack. Thermal treatments at higher temperatures (up to 850°C) led to some decomposition of the Me-N4 moiety, and thus to a decrease of the catalytic activity, and to the reduction of some of the ions to their metallic state. 

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