Stabilization by chemical modification

Stabilization by chemical modification has been reviewed recently (100), and details have been published on a new modification technique involving pretreatment with aqueous ethanol (100). This method improves the static and dynamic stabilities of PVC by factors ranging up to 3-3.5 its effectiveness is believed to derive from the deactivation of labile halogen sites by solvolytic displacements (100). 

Transition metal aUcoxides are versatile precursors for sol-gel synthesis because they are known for almost aU transition metal elements [12], With the notable exception of silicon, most transition metals are extremely reactive to water, which can present problems such as the formation of precipitates. Therefore, most transition metal alkoxides must be handled with great care, in a dry environment, and are often stabilized by chemical modification to prevent precipitation upon hydrolysis. 

For technical application as adhesives and for food thickening purposes, w. are commonly stabilized by chemical modification (- modified starches, - cross-linked starches). 

In the same way, trypsin from bovine pancreas was also stabilized by chemical modification with activated PM with a molecular weight of about 100,000 [75]. PM-trypsin was much more stable than unmodified enzyme not only toward autolytic digestion but toward heat or urea denaturation as well. Table 8 represents the stability of the PM-modified and unmodified enzymes together with succinylated trypsin. In the case of trypsin preparations incubated at 37 C and pH 7.6, unmodified trypsin markedly reduced its enzymic activity its residual activities were only 28% and 15% after 60 min and 120 min... 

Commercially produced metal-substituted chlorophylls such as copper chlorophylls and copper chlorophyllins that can be obtained by chemical modification of natural chlorophylls have better stability, solubility, and tinctorial strength, but they cannot be considered natural food colorants and will be discussed later. 

Hill etal. (2001) modified wood surfaces with methacrylic anhydride and grafted the activated surface with styrene in order to see if this would improve the UV stability of the modified substrate. There was no evidence to suggest that UV stability was improved either by chemical modification or by modification plus grafting. 

Rowell, R.M., Tilhnan, A.-M. and Zhengtian, L. (1986e). Dimensional stabilization of flakeboard by chemical modification. Wood Science and Technology, 20(1), 83-95. 

Since the protein scaffold is commonly not very stable, many methods have been used for stabilization presence of additives, immobilization by multiple-point attachment, stabilization by chemical or biochemical modification and by protein engineering, and several others. 

An outstanding property of these polymers is their shear stability. The sonic shear stability testsfci indicate that these polymers are superior to some of the currently used polymers of ethylene-propylene or methacrylate type. The excellent stability of the hydrogenated diene-styrene polymers is attributed to their relatively low molecular weight and narrow distribution consistent with the established theory of shear degradation of polymers. The most recent developments in this field are block polymer VI improvers with dispersancy properties, built into the molecule by chemical modification of the rubber block. 2... 

Plettner E., DeSantis G., Stabile M. and Jones J. B. (1999) Modulation of esterase and amidase activity of subtilisin Bacillus lentus by chemical modification of cysteine mutants. J. Am. Chem. Soc. 121, 4977-4981. 

Properties of rice starches are changed by chemical modification in the same way as the properties of other starches (see Chapter 18). Starches prepared via the alkali method have been modified to provide additional pH and shear stability. In general, hypochlorite-oxidized rice starch has a lower gelatinization temperature and lower maximum paste viscosity producing a softer, clearer gel. Hydroxypropylated rice starches have lower gelatinization temperatures, whereas crosslinked rice starch has an increased gelatinization temperature, increased shear resistance and acid stability. 

Liu JZ, Wang M (2007) Improvement of activity and stability of chloroperoxidase by chemical modification. BMC Biotechnol 7 23-30... 

Most of the research in the area of chemical modification of wood was conducted for improving either its dimensional stability or its biological resistance. Wood is made up primarily of cellulose, hemicellulose, and lignin. Originally, chemical modification of wood was a chemical reaction between some reactive part of a wood component and a simple chemical reagent to form a covalent bond between the two [1]. The most abundant reactive sites in wood are the hydroxyl groups. The major important types of covalent bonds formed by chemical modification of wood are ethers and esters. The treated wood must still possess the desirable properties of wood. Past research work on the chemical modification of wood was reviewed extensively in 1975 [1] and 1984 [2] by Rowell. 

For enzymes another point can be important Not all pure enzymes are stable in solution. In particular in organic solvents enzymes can be very unstable (4). The stability of enzymes can be improved by genetic modifications, by chemical modification, and most easily and without specialist equipment by immobilization. Furthermore, immobibzation opens up the possibility of improving other properties of the enzyme, such as its activity, substrate specificity, and enantioselectivity. Although the underlying principles are not always understood enzyme immobilization is a powerful tool for the improvement of activity and enzyme stability, specificity, and selectivity. 

Petcavich et al. (1978) employed IR subtraction techniques to elucidate the mechanism of the oxidative degradation of polychloroprenes at 60 °C. The spectra were taken at 60 2°C. The results lead to the conclusion that 1,2- and 3,4-structural irregularities are involved in the initial stage of the thermal oxidation of these compounds at 60 °C. In addition, a simple free radical mechanism seems to be consistent with the experimental results. The observed results suggest that polychloroprenes may be stabilized towards oxidative degradation by eliminating the 1,2- and 3,4-structures by chemical modification of the polymer after synthesis. 

These enzymes must have a large activity spectrum, and ideally enantioselectivity for toxic stereoisomers. Their mass production under GMP conditions must be realizable at a reasonable cost. Long-term storage without activity loss (in solution, lyophilized, or adsorbed/bound on a matrix) must be possible under field conditions. Conformational stability can be optimized by chemical modification or addition of stabihzers such as polyols. Thermostable enzymes from thermophilic bacteria (Merone et al., 2005) or mutated/ evolved highly stable enzymes from mesophilic bacteria (Elias et al., 2008) are promising alternatives. 

Polymer-supported TADDOL-Ti catalyst 79 prepared by chemical modification was poorly active in the Diels-Alder reaction of 3-crotonoyloxazolidinone with cyclo-pentadiene (Eq. 24) whereas polymeric TADDOL-Ti 81 prepared by copolymerization of TADDOL monomer 80 with styrene and divinylbenzene had high activity similar to that of the soluble catalyst. In the presence of 0.2 equiv. 81 (R = H, Aryl = 2-naphthyl) the Diels-Alder adduct was obtained in 92 % yield with an endolexo ratio of 87 13. The enantioseleetivity of the endo product was 56 % ee. The stability and recyclability of the catalyst were tested in a batch system. The degree of conversion, the endolexo selectivity, and the enantioseleetivity hardly changed even after nine runs. Similar polymer-supported Ti-TADDOLate 82 was prepared by the chemical modification method [99]. Although this polymer efficiently catalyzed the same reaction to give the (2R,2S) adduct as a main product, asymmetric induction was less than that obtained by use of a with similar homogeneous species. 

Current research efforts are directed toward improving the thermal and low-temperature stability of vegetable oils by chemical modification, blending with functional fluids and additive response studies. 

Several 2-aryl- and 2-phenylmethyl-3,5-isoxazolidine-dlones were synthesized and found to be bleaching herbicides with good tolerance by soybeans. The most active member, 2-(2-chlorophenyl)methyl-4,4-dimethyl-3,5-isoxazolidinedione, failed to perform in the field due to its instability in soil. To improve the chemical stability by molecular modifications, a series of 3-isoxazolidinones were prepared and found to be highly active bleaching herbicides with excellent soybean tolerance. Synthesis and structure-activity relationships are discussed. One of the most active compounds,... 

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