Chemical modifications enzymatic

Within certain limits it is possible to change these properties using chemical modification, enzymatic hydrolysis, heat denaturation, etc. 

Nowadays, the development of TPEs concern many branches of macromol-ecular chemistry cationic and radical polymerizations, chemical modification, enzymatic catalysis or the use of microorganisms. Their respective contributions are analyzed in Section 2. The elastomers based on halogen-containing polyolefins [37] and those prepared by dynamic vulcanization [38] are also included in the TPE family. More information on these materials and techniques is given in Section 2. 

AH cephalosporins found in nature (Tables 1 and 2) have the D-a-aminoadipic acid 7-acyl side chain (21). AH of these compounds can be classified as having rather low specific activity. A substantial amount of the early work in the cephalosporin area was unsuccessfiiHy directed toward replacing the aminoadipic acid side chain or modifying it appropriately by fermentation or enzymatic processes (6,22). A milestone ia the development of cephalosporins occurred in 1960 with the discovery of a practical chemical process to remove the side chain to afford 7-ACA (1) (1). Several related processes were subsequendy developed (22,23). The ready avaHabHity of 7-ACA opened the way to thousands of new semisynthetic cephalosporins. The cephalosporin stmcture offers more opportunities for chemical modification than does that of penicillins There are two side chains that especiaHy lend themselves to chemical manipulation the 7-acylamino and 3-acetoxymethyl substituents. 

A variety of chemical and enzymatic reactions produce derivatives of the simple sugars. These modifications produce a diverse array of saccharide derivatives. Some of the most common derivations are discussed here. 

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]. 

The solubility of iridoids depends on their state (free, glycosylated, acetylated), but usually they are extracted with polar solvents methanol, ethanol, aqueous alcohols, and rarely acetone. Iridoid glycosides are more or less stable some of them are very sensitive to acids and alkalis. Some iridoid glycosides such as aucubin suffer color modification after chemical or enzymatic hydrolysis they give first a blue to green... 

Torres, E. Baeza, A., and Vazquez-Duhalt, R., Chemical modification of heme group improves hemoglobin affinity for hydrophobic substrates in organic media. Journal of Molecular Catalysis, B-Enzymatic, 2002. 19 pp. 437—441. 

The following sections describe the major enzymatic and chemical modification procedures used to label nucleic acids and oligonucleotides. 

Chemical modification also may be used to label directly an oligonucleotide, eliminating the enzymatic step altogether. The chemical modification of nucleic acids can encompass several... 

Van Lenten, L., and Ashwell, G. (1971) Studies on the chemical and enzymatic modification of glycoproteins. A general method for the tritiation of sialic acid-containing glycoproteins. J. Biol. Chem. 46, 1889-1894. 

The easiest way to detect a protein modification seems to be the mass measurement of all peptides generated by enzymatic digestion. The comparison with the predicted peptide masses from the sequence of the protein identifies unmodified peptides and unexplained masses would give indications to modified peptides. Unfortunately, this is not a suitable approach in practice. In many peptide mapping experiments done with the MALDI mass mapping technique, up to 30% of the measured masses remain unexplained. This is probably due to protein contaminations from human keratins, chemical modifications introduced by gel electrophoresis and the digestion procedure, and other proteins present at low levels in the piece excised from the sodium dodecyl sulfate gel. The detection of a protein modification requires a more specific analysis. 

Debono M, Abbott BJ, Malloy RM, Fukuda DS, Hunt AH, Daupert VM, Counter FT, Ott JL, CarreU CB, Howard LC, Boeck LD, Hamill RL. (1988) Enzymatic and chemical modifications of hpopeptide antibiotic A21978C The synthesis and evaluation of daptomycin (LY146032). JAntibiot 41 1093-1105. 

A promising approach to this topic is the development of biocompatible solid phase attachment systems for macrocycles that allow on-bead enzymatic and chemical modification [4]. While making use of recent developments in polymeric support for resins, we endeavored in constructing a new linker system, which allows easy attachment of macrocycles to the solid phase, simple organic or enzymatic reactions, and cleavage from solid support under mild conditions [98]. 

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