Immobilization by Ionic Interaction

Incidentally, this is the first reported heterogeneous osmium-catalyst-mediated AD reaction of alkenes using molecular oxygen as the cooxidant. Under identical conditions, the turnover numbers of the heterogeneous catalyst are similar to those of the homogeneous system. 

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Heck Reactions with Catalysts Immobilized by Ionic Interactions. 65... 

Mode of immobilization. Immobilization can be effected either chemically, by covalent bonding of the biocatalyst on a surface (Figure 5.6, option 1), by adsorption, or by ionic interactions between catalyst and surface (option 2), as well as by cross-linking of biocatalyst molecules for the purpose of enlargement (option 3), or physically by encapsulation in matrices or by embedding in a membrane (option 4). 

Another approach towards immobilization via ionic interactions is the use of tethered metal ions, such as Cu, Co, or Ni, in order to bind the enzyme [53, 54]. This is particularly so when the enzyme contains an easily accessible imidazole residue from histidine [55] or a His tag (Figure 2.6) [56-59], that is a short tag with six histidines. This tag can be readily introduced by genetically modifying the enzyme. Little influence of the tag on the catalytic performance has been noticed. When benzaldehyde lyase [60] was immobilized on an Ni -containing polyvinylpyrrolidinone-based matrix, it could be reused several times for the formation of benzoin (12) (Scheme 2.4) [58]. 

It can be concluded that CH6 as well as other aptamers bind cytochrome c rather unspeciflcally by ionic interactions of the polyanionic DNA and a protein containing a cationic cluster, at least when the aptamers are immobilized, as in SPR experiments. 

Ionic species can be immobilized by electrostatic interactions, specifically as counterions however, these systems can be sensitive to the redox transformations of the polymer, i.e., counterion desorption is expected when the film becomes neutral or oppositely charged. Other interactions (e.g., complex formation) can also be exploited. 

P-D-Fructofuranosidase attached to a cellulose derivative by ionic interactions was more stable than the free enzyme, and its activation energy was less than that of the free enzyme.The use of controlled diffusion to reduce the concentration of substrate in the vicinity of the enzyme was studied. p-o-Fructofuranosidase adsorbed onto immobilized, succinylated concanavalin A has been activated (with glutaraldehyde) and coupled to urease. An active, immobilized, water-soluble conjugate containing both enzymes was obtained following desorption from the matrix. 

Electrostatic immobilization employs similar complementarity. The fiber surface can be coated with a thin polyelectrolyte membrane, which interacts electrostatically with oppositely charged sensing materials. For example, a negative surface, such as sulfonated polystyrene, can be prepared to efficiently immobilize a positively charged dye. Similarly, cationic polymers can bind various anions. Ionic indicators or biological molecules (i.e., either cations or anions) can be immobilized by electrostatic interaction. 

Due to their better biomimetic properties, phospholipids have been proposed as an alternative to 1-octanol for lipophiiicity studies. The use of immobilized artificial membranes (lAM) in lipophiiicity determination was recently reviewed and we thus only briefly summarize the main conclusions [108]. lAM phases are silica-based columns with phospholipids bounded covalently. lAM are based on phosphatidylcholine (PC) linked to a silica propylamine surface. Most lipophiiicity studies with lAM were carried out using an aqueous mobile phase with pH values from 7.0 to 7.4 (log D measurements). Therefore, tested compounds were neutral, totally or partially ionized in these conditions. It was shown that the lipophiiicity parameters obtained on I AM stationary phases and the partition coefficients in 1-octanol/water system were governed by different balance of intermolecular interactions [109]. Therefore the relationships between log kiAM and log Poet varied with the class of compounds studied [110]. However, it was shown that, for neutral compounds with log Poet > 1, a correspondence existed between the two parameters when double-chain lAM phases (i.e., lAM.PC.MG and IAM.PC.DD2) were used [111]. In contrast, in the case of ionized compounds, retention on lAM columns and partitioning in 1 -octanol / water system were significantly different due to ionic interactions expressed in lAM retention but not in 1-octanol/water system and due to acidic and basic compounds behaving differently in these two systems. 

Kelly and Leyden10 studied the interaction of APTS with silica gel by thermometric enthalpy titration. This technique provides information regarding kinetic and thermodynamic parameters, which govern the reactivity of immobilized functional groups. They found that 26% of the APTS molecules were irreversibly bonded to the silica surface and attributed this stability to ionic interactions. Both values are equal within experimental error. 

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