Chemical modification, stabilization

Conventional chemical modification Stability to weathering, compatibilization... 

We have emphasized that minor chemical modifications stabilize different structural minima in Fig. 21. The MBP-TCNQ complexes in Section 3.4 may illustrate, if supported by structure determinations, different structures for the same complex, presumably due to different preparative conditions. That one modification is metastable hardly matters in the solid state where there is no interconversion between different minima in Fig. 21. The precise control of crystallization conditions is obviously crucial. The impurity effects leading to MP-TCNQ or HMP-TCNQ, or to different MjP-TCNE adducts in Section 4.4, represent different structures for different complexes. We are not aware of definitive evidence for different structures of the scone complex, but suspect such behavior to be possible among phenazine complexes. 

By Chemical Modification. Stabilization by chemical pretreatment with organotins (22,23,24,25) has been discussed already. Pretreatment studies with other reagents have been reviewed briefly (22), and improved stability is now also said to result from the prior reaction of PVC with maleic anhydride (92). Kennedy s interesting work on stabilization reactions of organoaluminums (93) has been continued by a report of enhanced stability (compared with that of ordinary PVC) for poly (vinyl chloride-g-styrene) obtained from the Et2AlCl-induced reaction of PVC and styrene monomer (94). Selective destruction of defect sites seems a plausible explanation for this result (94), but improved stability has been observed also for the PVC parts of PVC-polystyrene blends (53,95,96) and for the PVC moieties of poly (vinyl chloride-g-styrene )s prepared by alternative methods (95,96,97). 

Forms of BR and polyisobutylene. The properties of butyl rubber and polyisobutylene depend on their moleeular weight, degree of unsaturation, nature of the stabilizer incorporated during manufacture and, in some cases, chemical modification. It is common to produce halogenated forms of butyl rubber to increase polarity and to provide a reactive site for alternate cure mechanisms [6],... 

The presence of allylic chlorines and tertiary chlorines and their influence on the thermal stability of PVC has now been established with some degree of confidence, and together they are considered to constitute the labile chlorine structures in the polymer. Numerous chemical modification methods involving the selective nucleophilic substitution of labile chlorines in PVC with other chemical moieties for identifying and quantifying labile structures have been reported in the literature. 

The interest in this type of copolymers is still very strong due to their large volume applications as emulsifiers and stabilizers in many different systems 43,260,261). However, little is known about the structure-property relationships of these systems 262) and the specific interactions of different segments in these copolymers with other components in a particular multicomponent system. Sometimes, minor chemical modifications in the PDMS-PEO copolymer backbone structures can lead to dramatic changes in its properties, e.g. from a foam stabilizer to an antifoam. Therefore, recent studies are usually directed towards the modification of polymer structures and block lengths in order to optimize the overall structure-property-performance characteristics of these systems 262). 

Applications of peroxide formation are underrepresented in chiral synthetic chemistry, most likely owing to the limited stability of such intermediates. Lipoxygenases, as prototype biocatalysts for such reactions, display rather limited substrate specificity. However, interesting functionalizations at allylic positions of unsaturated fatty acids can be realized in high regio- and stereoselectivity, when the enzymatic oxidation is coupled to a chemical or enzymatic reduction process. While early work focused on derivatives of arachidonic acid chemical modifications to the carboxylate moiety are possible, provided that a sufficiently hydrophilic functionality remained. By means of this strategy, chiral diendiols are accessible after hydroperoxide reduction (Scheme 9.12) [103,104]. 

To improve the dimensional stability of the oligoethyleneoxy phosphazene/ metal salt complexes, some attempts have been reported which concern the chemical modification of the parent polymers, their chemical or radiation cross-linking, or the use of blends. 

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. 

The expansion of our knowledge of the structure and function of Na,K-ATPase is reflected in a rapid succession of reviews on Na,K-ATPase genes and regulation of expression [17], subunit assembly and functional maturation [20], the isozymes of Na,K-ATPase [18], and the stability of a subunit isoforms during evolution [21], physiological aspects and regulation of Na,K-ATPase [22], reconstitution and cation exchange [23], chemical modification [24], and occlusion of cations [25]. Other valuable sources are the review articles [26] and recent developments [27] reported at the International Na,K-pump Conference in September 1990. 

Chemical surface modifications The first surface modification for the purpose of eliminating EOF and protein adsorption was recommended by Hjerten.28 The attachment of vinyl silanes allowed the polymerization of a variety of molecules to the surface. Most of the chemical modifications used for preparing capillaries for electrophoresis originated from the experience acquired over the years preparing GC and LC stationary phases. Chemical modification should conform to certain requirements, including the prevention of adsorption, the provision of stable and constant EOF over a wide pH range, chemical stability, ease of preparation, and reproduciblity of preparation. The effects of silanization of the inner surface of capillaries on electrophoretic separations have been extensively studied.26-29... 

Improvement of the properties of a drug may be achieved by the chemical modification of the parent drug. The preparation of an ester, salt, or other modification of the parent structure may be employed with parenteral drugs to increase stability, alter drug solubility, enhance depot action, avoid formulation... 

Formulation strategies for stabilization of proteins commonly include additives such as other proteins (e.g., serum albumin), amino acids, and surfactants to minimize adsorption to surfaces. Modification of protein structure to enhance stability by genetic engineering may also be feasible, as well as chemical modification such as formation of a conjugate with polyethylene glycol. 

A variety of approaches exist for stabilizing proteins, for example, chemical modification, immobilization, and site-directed mutagenesis [95,96], but these techniques are not within the scope of this chapter. The focus here will be on stabilization of proteins via formulation development. The principal formulation strategy is to stabilize the protein using clinically acceptable additives (excipients) or through the use of suitable pharmaceutical-processing technologies. 

The improvement of its activity and stability has been approach by the use of GE tools (see Refs. [398] and [399], respectively). A process drawback is the fact that the oxidation of hydrophobic compounds in an organic solvent becomes limited by substrate partition between the active site of the enzyme and the bulk solvent [398], To provide the biocatalyst soluble with a hydrophobic active site access, keeping its solubility in organic solvents, a double chemical modification on horse heart cytochrome c has been performed [400,401], First, to increase the active-site hydrophobicity, a methyl esterification on the heme propionates was performed. Then, polyethylene glycol (PEG) was used for a surface modification of the protein, yielding a protein-polymer conjugates that are soluble in organic solvents. 

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