3/10/14AcetalFormationAcetalFormation:MechanismIn acid solution, sugars can react with alcohols to formAcid-catalyzednucleophilicadditionacetalsknownasglycosides+H,OCH,OHCH,OHCH,OHCH,OHCH,OHROHR-OH1. Protonation of OH groupReaction is a nucleophilic addition of two alcohols to2.water.removal to formaldehydesCH,OHcarbocation3. alcohol addition andaldehydehemiacetalacetalrelease of protonglycosideopen-chain sugarcyclic sugarR'O-CH-OHR'O-CH-OR"O-CHAcetals:GlycosidesHydrolysis(AcetalBreakdown)Acid-catalyzednucleophilicaddition:·Forsugars,productcalledaglycoside+ROHH,OHCH,OH R group called aglycone.ReactionisreversibleinpresenceofwaterH,C1.Protonation of OR groupandacidCH,OH2.Formafion of carbocation3. Water addition and release-Acid hydrolysisof protonOH
3/10/14 1 Acetal Formation • In acid solution, sugars can react with alcohols to form acetals known as glycosides • Reaction is a nucleophilic addition of two alcohols to aldehydes aldehyde hemiacetal acetal open-chain sugar cyclic sugar glycoside O = CH RO-CH-OH RO-CH-OR CH2OH CH2OH + ROH OH + H2O H+ O OR O Acetal Formation: Mechanism Acid-catalyzed nucleophilic addition: H+ OH2 + R-OH + H2O 1. Protonation of OH group 2. water removal to form carbocation 3. alcohol addition and release of proton CH2OH OH O CH2OH O + CH2OH O + H+ CH2OH OR O Acetals: Glycosides • For sugars, product called a glycoside – R group called aglycone • Reaction is reversible in presence of water and acid – Acid hydrolysis Hydrolysis (Acetal Breakdown) Acid-catalyzed nucleophilic addition: H+ OH+ H2O + ROH 1. Protonation of OR group 2. Formation of carbocation 3. Water addition and release of proton CH2OH OR O CH2OH O + CH2OH O R + H+ CH2OH OH O
3/10/14OligosaccharidesreducingLink-Glycone-Aglycone面β-Lactoseβ-galactosyl glucoseShEβ-Maltose α-glucosyl glucoseSHONHOFβ-Cellobioseβ-glucosylglucosenonreducing endreducing endMaltose(Mall Sugar)OligosaccharidesCHOHHO-nonreducingHO.CH,OHCHOHOFCH,OHOHOHAn a-glucoside &SucroseHO.OHHOHHHOHa β-fructosideCHOHBeda(OH)OHoyFoconsznCH,OHCellobioseCHORTrehaloseCHOHHOHO-OHOHTHOAn a,α-diglucosideCH,O(H)OH!t(OH)Rettgsge,mitaiotatesMaltase hydrolyzes a but not 8!2
3/10/14 2 Oligosaccharides reducing β-Lactose Link-Glycone-Aglycone β-Maltose β-Cellobiose O H OH H H OH H OH C H2OH H O H H OH H OH OH C H2OH H O H O H H OH H OH OH C H2OH H O H H H OH H OH OH C H2OH O O H H O H H H H O H OH H OH H OH OH OH OH C H2OH C H2OH H β-galactosyl glucose α-glucosyl glucose β-glucosyl glucose nonreducing end reducing end Oligosaccharides nonreducing Sucrose Trehalose O O OH H OH H H O H H H OH H OH H OH C H2OH C H2OH C H2OH O H H H H OH H OH OH C H2OH C H2OH OH OH OH H H O H H O H An α-glucoside & a β-fructoside An α,α-diglucoside
3/10/14OHHCCOHa1-6α 1-6AHCOHOHHOHOHOOHHO.a1-4HOTaoβO-HOOHHOPolysaccharides.Classification,StructureOH·ConformationOHHO:PropertiesOHHOO.Individual PolysaccharidesHOOHβ1-4OH3
3/10/14 3 α 1-6 α α β α α 1-6 α 1-4 β 1-4 Polysaccharides • Classification, Structure • Conformation • Properties • Individual Polysaccharides
3/10/14Classification(Glycans)Conformation·Homoglycans-HeteroglycansMonosaccharide linkages willdetermine conformation.May·Linear-Branched (frequency&length)haveperiodicand nonperiodicregionsofsugars·Monsaccharideresiduesequencemaybeperiodicornonperiodicperiodicsequencesmaybelongorshort;theycanalsoconsistofperiodicoccurrenceofperiodicsequencesseparatedbynonperiodicsequencesPolysaccharidesExtendedorStretchedRibbonCellnloseiioType1,4 β-D-glucopranosyl residuesOHAmv(4.114)HOHOOHFigureX12The Jalridonamangromu aftecellulue chola Thdrow bondi abeneeeteringemngeeandthrCehdnfgnanafhencihiosIng ghwisr rerthw4
3/10/14 4 Classification (Glycans) • Homoglycans – Heteroglycans • Linear - Branched (frequency & length) • Monsaccharide residue sequence may be periodic or nonperiodic periodic sequences may be long or short; they can also consist of periodic occurrence of periodic sequences separated by nonperiodic sequences Conformation Monosaccharide linkages will determine conformation. May have periodic and nonperiodic regions of sugars Extended or Stretched RibbonType 1,4 β-D-glucopranosyl residues
3/10/14Chemical Modification of CelluloseCiOONa'h-O-CH-DaytO-CHHCHemiclluloseoe (CMC)CarheM1,4 α-D-glucopranosyl residuesamyloseH,OHM.Amyiose.anunbranchedstarch(4.119)5
3/10/14 5 Hemicellulose Chemical Modification of Cellulose 1,4 α-D-glucopranosyl residues amylose