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1. (Article ID: 11117)
 
Puls, J
Chemistry and biochemistry of hemicelluloses: Relationship between hemicellulose structure and enzymes required for hydrolysis
Macromolecular Symposia 120 (1997) 183-196
 

The lignified plant cell wall is a composite material in which cellulose, hemicelluloses and lignin are in tight association. Xylans constitute the predominant hemicellulose of hardwoods and straw, wherease galactomannans represent the largest hemicellulose fraction in softwoods. Naturally occuring hemicelluloses, commercially available hemicelluloses and xylans and mannans from technical processes have distinct differences with respect to degree of polymerization and extent of branching. In principle, naturally occuring hemicelluloses carry more side chains compared to those which have been extracted with alkali or under pressure and heat. The presence or absence of substituents is also reflected by the enzymes required for complete hydrolysis. The action of the depolymerizing xylanases and mannanases may be partly dependent on debranching enzymes. Special empasis is therefore given on the substrate requirements of the so-called accessory enzymes (i.e. α-glucuronidase, α-arabinosidase, α-galactosidase, acetyl-and phenolic acid esterase) as well as on synergy effects with the depolymerizing enzymes.

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2. (Article ID: 11993)
 
Alluis B, Pérol N, El hajji H, Dangles O
Water-soluble flavonol (= 3-hydroxy-2-phenyl-4H-1-benzopyran-4-one) derivatives: Chemical synthesis, colouring, and antioxidant properties
Helvetica Chimica Acta 83(2) (2000) 428-443
 

Water-soluble derivatives of rutin, a very common glycoside of quercetin (=3,3′,4′,5,7-pentahydroxyflavone=2-(3,4-dihydroxyphenyl)-3,5,7-trihydroxy-4H-1-benzopyran-4-one) and a potent plant antioxidant from the flavonol family, were synthesized by simple chemical procedures aimed at introducing carboxy or sulfo groups at the sugar moiety (Scheme 1). Such derivatives form stable molecular complexes with malvin, a polyphenolic pigment from the anthocyanin family, and thereby prove to be very effective in the enhancement (hyperchromism) and variation (bathochromism) of natural colours. The H2O-solubilizing carboxylate and sulfate groups are shown to deeply modify the enthalpy-entropy balance of the pigment-flavonol complexation (copigmentation). A molecular interpretation of the complexation-induced bathochromic shift in the pigment VIS band is proposed. Finally, the H2O-soluble rutin derivatives are shown to retain the high antioxidant ability of rutin as evidenced by their efficient trapping of the coloured radical DPPH (=2,2-diphenyl-1-(2,4,6-trinitrophenyl)hydrazyl).

chemical synthesis, derivatives, rutin, malvin, natural pigments

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3. (Article ID: 12005)
 
Ashida H, Fukuda I, Yamashita T, Kanazawa K
Flavones and flavonols at dietary levels inhibit a transformation of aryl hydrocarbon receptor induced by dioxin
FEBS Letters 476(3) (2000) 213-217
 

Dioxins invade the body mainly through the diet, and produce toxicity through the transformation of aryl hydrocarbon receptor (AhR). An inhibitor of the transformation should therefore protect against the toxicity and ideally be part of the diet. We examined flavonoids ubiquitously expressed in plant foods as one of the best candidates, and found that the subclasses flavones and flavonols suppressed antagonistically the transformation of AhR induced by 1 nM of 2,3,7,8-tetrachlorodibenzo-p-dioxin, without exhibiting agonistic effects that transform AhR. The antagonistic IC(50) values ranged from 0.14 to 10 μM, close to the physiological levels in human.

flavonoid, cancer prevention, dioxin antagonist, aryl hydrocarbon receptor, transformation inhibitor, dietary phytochemical

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4. (Article ID: 12594)
 
Perez S, Mazeau K, Herve du Penhoat C
The three-dimensional structures of the pectic polysaccharides
Plant Physiology and Biochemistry 38(1-2) (2000) 37-55
 

The application of molecular modelling methods to the realistic description of cell wall polysaccharides is described. In a first section, the basic principles and tools are presented; they provide a consistent description of the levels of structural organization of polysaccharides, from the basic monomeric components to the more complex assemblies of macromolecular chains and their interactions. The second section provides a comprehensive illustration which includes the application of these methods to the elucidation of the structural features of the plant cell wall polysaccharides, pectins. The ordered and disordered states of both the `smooth' and the `hairy' regions of the pectin components are described, along with a discussion of the role of some key structural moieties such as the degree of esterification or acetylation. The stereochemical and electrostatic requirements for ionic binding to the polysaccharide chain are also presented. In an effort to provide a more global view, a `canonical' structure of pectin has been constructed and described; it corresponds to a hypothetical model having a molecular mass of 50 kDa. The main structural features of rhamnogalacturonan II, which have been determined to date, are also reported.

molecular modelling, cell wall, nuclear magnetic resonance, arabinogalactan, pectins, rhamnogalacturonan, 3D-structure

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