Microenvironmental effects

Microenvironmental effects of micelles and related organic media 474... 

The source of the enormous rate enhancements in enzymatic catalysis has been discussed from physical organic points of view (Jencks, 1969 Bruice, 1970). The kinetic behavior is attributed to factors such as an orientation effect, a microenvironmental effect and multifunctional catalysis. The active sites of enzymes are generally located in a hydrophobic hole or cleft. Therefore, the microenvironmental effect is mainly concerned with the behavior of enzyme catalytic groups in this hydrophobic microenvironment and the specific... 

Many of the basic elements of enzyme catalysis have been illustrated here, including binding of substrate, multifunctional catalysis, microenvironmental effects, covalent catalysis, and strain effects. The most remarkable rate enhancements reported to date are those brought about by apolar derivatives of PEI, a polycation. These rate enhancements are very en-... 

Microenvironmental effects such as electrostatic or hydrophobic ones provided by molecular assemblies or polymers on the photoinduced charge separation and electron relay are described in this chapter together with the solid phase and macroheterogene-ous photoinduced charge separation system constructed with polymer solids. 

The microenvironmental effect of various sulfonated polystyrene beads was studied using the Prins reaction as a probe.372 Reaction rates were found to be lower when carbomethoxy or carbobutoxy neighboring groups were present compared to phenyl. A less ionic microenvironment appears to allow for a higher concentration of styrene within the polymer and leads to an immediate reaction with protonated formaldehyde. 

Murakami et al. utilized catalytic bilayer membranes to catalyze the (1-replacement reaction of serine with indoles [44], The bilayer vesicle formed with 32 and 36 drastically accelerated the (1-replacement reaction by 51-fold (krel) relative to pyridoxal in homogeneous aqueous solution. They attributed this to the hydrophobic microenvironmental effect provided by the bilayer vesicle, which affords effective incorporation of indole molecules and elimination of water molecules in the reaction site. The imida-zolyl group of 33 enhanced the reaction further, krd being 130, possibly due to general acid-base catalysis by the imidazolyl group. Copper(n) ions also improved the reaction. 

The foUowing condusions may be obtained from these results (1) The catalytic efficiency increases with pH, because of the increaang involvement of the benzimidazole anion. Lower pK j value of the benzimidazole group is effective in this reject However, the benzimidazole anion is not a good nudeophile toward the anionic substrate NABS because of electrostatic repulsion. (2) Polyvinylbenzimidazole 2 is a better catalyst than benzimidazole. Although this was daimed to be due to the bifunctional catalysis of the pdyn r, substrate binding and microenvironmental effects cannot be neglected. 

Studies have been made on the microenvironmental effects of cyclodextrins these include excimer formation in polymethylene-... 

Ionic, hydrophobic, or other interactions between the enzyme and the matrix (microenvironmental effects) may also result in changed KM and Vmax values. These essentially reversible effects are caused by variations in the dissociation equilibria of charged groups of the active center. 

Accessibility of functional groups for an immobilization reaction is further influenced by microenvironmental effects. Steric hindrance by neighbouring groups, interaction with neighbouring groups and solvent molecules, or alterations of pKa-values of dissociable groups can lead to a shifted reactivity. 

Nucleophilic substitution of the chloromethylated cross-linked polystyrene resin has been used to attach a wide variety of groups. Among these are those shown (5.31) Potassium diphenylphosphide was used to make the first phosphine.131 The microenvironmental effects of this catalyst in the formation of benzyl benzoate by the Mit-... 

More generally, it may be concluded that hydrous transition metal oxides are effective matrices for enzyme, etc, immobilisation. Their advantages include low cost, convenience of preparation (which may be conducted in any location without specialised facilities), the absence of any need for prepreparation, ability to couple enzyme at neutral pH, high retention of enzyme specific activity of the enzyme on immobilisation, and the ability of modification to exert microenvironmental effects on and thereby alter the characteristics of the immobilised enzyme. 

Microenvironmental effects on the intrinsic catalytic parameters of the enzyme such effects due to the perturbation of the catalytic pathway of the enzymic reaction would reflect events arising from the fact that enzyme-substrate interactions occur in a different microenvironment when an enzyme is immobilized on a solid support. 

Flavin mononucleotide (FMN) is an anion at pH 7 therefore it can be doped into poly(pyrrole) by growing films in the presence of the ion. Absorption spectra confirm the presence of FMN within films, and cyclic voltammery shows that entrapped flavin can be oxidized and reduced. It appears that entrapped reduced flavin is oxidized before the polymer backbone while the reduction of FMN and the backbone coincide. Electrochemistry of the entrapped material is shifted from that for the solution species, as we expect, due to the difference in the environment. The FMN has also been incorporated into poly(pyrrole-alkylammonium) films by ion exchange of the anion into the polymerized film. Reversible voltammetry for incorporated FMN was observed with a slight shift in potential with respect to the solution species, and it was attributed to microenvironmental effects. These films were shown to catalyze oxygen reduction. 

Enzyme Catalysis. Biocatalysis is extremely sensitive to microenvironmental effects, and subtle changes in the structural, conformational, and electronic makeup of the enzyme are well known to result in extreme variations in catalysis rates. The catalysis experiments not only reveal structural integrity but more so persuasively the conformational integrity as well. 

Microenvironmental effects on equilibrium and photoredox chemistry of bromo complexes in reverse micelles... 

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