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16.5 Glycosides

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16.5Clycosides 505 . PRACTICEEXERCISE I5.7 ' Draw the Haworth projection ', of B-o-grucopyranose.Identiff the hemi- ocetSlcarbon. : .. PRACTICEEXERCISE I5.S Draw the.Haworth projection of a-o-ribofuranose. Identiff the hemi- ,, a.c€td carbon. .:: PRACTICEEXERCISE I6.9 , the Haworth projection Prury of a-o-fructofuranose. Identify the hemiketal carbon. Interconversion of stralght-chain aa4 ring for.trs of sugars The straight-chain sugar.forms are in equilibrium with the ring forms. ring forms The are usually quite predominant. For example, ifstlieZcfremically pure a-o-glucopyranose is dissolved in an acidic iol'tion, the ring will open and close repe.atedry. In reclosing, some B-o-glucopyranose is formed. The final equitibrium *i"t.tt" .orrists of about 63voB-o-grucopy- ranose, about 37To a-o-gtrucopyranose, and only u ti"y uriolrr, of the straight-chain aldehyde. From ihe percentagesof products formed, we say that can B-o-glycgn{anose is_only srightry morapreferred than a-o-glu- copyranose and that both o-glucopyranos-e*o-"r, u." -.r.h -ore ferred -aa"frya" pre_ than the straight-chai" form of t_glucose. CHO I H-C-OH cH2OH I HO-C-H I oroH H-C_OH (9) OH (cr) H-C_OH I cH2OH OH o.-o-Glucopytanose o-Glucose B-o-Glucopyranose (about 37%) (lessthan l%) (about 63%) 16.5Glycosides AIM: To describethe formotion of grycosidic- ' hoindsond the products of their hydrotysis. The closed-chain hemiacetal Focus or hemiketal forms of sugars may react with tgrgr1nSCerdso{rcfals (sees".. rs.ol.The acitats or ietals Alcohols react with closed- arsl-""Ittare calledglycosideC. - - ofsug- chain forms of sugars to form glycosidic bonds. Glycosidic bonds Thecoualent ether link betweenthe sugar hydroxyr and the arcohoris asy- cosidicbond. A simple arcoholsuch ai ''"ihurroi u. o-o_ ""J;;;;;lr"n ll l,l r'f 506 CHAPTER16 Carbohydrates ,"1 _o J\ ucHroH Figure15.7 _o Clycosidicbond Someof the common 5\ O, ,.-90-+6) I J glycosidicbonds found in 4 polysaccharides.The acetal andhemiacetal portions of the moleculesare shbwn in color.The wavy line connectingthe hydroryl groupto carbonI indicates that the carbon-orygen linkagemay be eitheralpha or beta. glucopyranoseproduce methyl a-o- glucopyranoside: cHzoH i.",o".ts4 Ho\|--{ocH3E>Z*.:::,ji*. OH ct-o-Glucopyranose Methanol Methyl a-o-glucopyranoside The alcohol used to make a glycosidic bond is often more complex than methanol-in fact, sugarsthemselves are alcohols.As shovvnin Figure 16'7' the individual saccharide units are attached through glycosi<lic bonds. Gly- cosidic bonds between sugars are designated according to the position numbers of the carbons of the sug:us that are linkdd and also according to the stereochemistry of the linkage. For example, suppose the beta hydroxyl group at carbon I in a hexose is linked by a glycosidic bohd to carbon 4 of another hexose. This linkage is called a BQ-4) glycosidic bond. Other common linkagesare c(l-4), a(l-6), and B(l-6). Once the anomeric -OH group of a sugar is tied up as an acetal, it is no longer freeto go from the ring form to the straight-chain form. PRACTTCEEXERCISE r6.t0 = .. The hydroxyl group of carbon I in a-o-glucopyr:rnoseis lin\ed bya gly- t cosidic bond to carbon 4 of another o-glucopyranose molecule. Draw ?,= the structure of the glycoside that is formed. Identifii the acetal car-bon. 16.6 Polysaccharides 507 Hydrolysis of glycosidic bonds Glycosidic bonds may be cleaved by hydrolysis reactions. We can take the hydrolysis of an a(1-4) glycosidic bond between two hexosesas an exam- ple. For simplicity, only the carbon skeleton and the glycosidic bond are shown: Fo. L-o. L-o. L-o. L)"(_)+Ho-H=(_),*n_L) The chemical hydrolysis of most complexsugars can be done byheating an aqueous solution of the carbohydrate.A trace of acid is added as a catalyst. En4rmes act as the catalyst in biological systems.Hydrolysis reactions will be important as we proceed into biochemistry since they are the means by which sugars, fats, and proteins are broken dornmto simple materials by digestion. ] PRACTICEEXERCISE I6.II ,, The glycosidic bondin the following compound is hydrolyzed.What are -: the structures of the products? - cH2oH F-O t/' \ \oH / "oHlrol o cH2oH 16.6Polysaccharides AIM: To list the structures,sources, ond usesof the following polysoccharides:storch, omylose, amylopectin, glycogen, ond cellulose. Individual sugar units may be connected Focus to one another to forrir linear, branched, or circular polymers, as shor,rmin Figure 16.8. Polysaccharides Polysaccharidesare composed haue many monosaccharidesbonded togetherto form a long polymer chain. of many monosaccharide units The bonds connecting the sugar units are glycosidic. The 1-4 and L,*6 connected by glycosidic linkages are the ones most commonly found in natural polysaccharides linkages. consistingofhexoses. .
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