《食品化学》课程教学资源（英文讲义）05 Carbohydrates part3

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 R
O-CH-OH R
O-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 Ribbon￾Type 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

3/10/14BetaGlucan1-3, β-linkedMixed linkages(4.118)B1-3 linikago"B-glucanLoosely-JointedHCOH(4.121)C

3/10/14 6 1-3, β-linked Beta Glucan Mixed linkages Loosely-Jointed

3/10/14CH,OHHHOHOHGlucoseOHHOCH,OHD-GalactoseHCHOHOHGalactoseStronglyPleatedorRibbonType1,4 linked α-D-galactopyranosyl-uroniate units(4.116)(4.115)

3/10/14 7 Strongly Pleated or Ribbon Type 1,4 linked α-D-galactopyranosyl-uronate units

3/10/14ConformationsofHeteroglycansFigure 3.7chioimohnsennMAyrarimwoPeriodicseguencesAperiodicFig,4.14. Schematic representation of a gel settingprocess (according to Rees, 1977)Properties.Fiber.Viscosity·Gelforming.Films and Coatingb.EnergyFig.4.15. Interchain aggregation between regular.Structuralconformations.a Double helix,b doublehelixbundle,c egg-box,d ribbon-ribbon, and e doublehelix,ribboninteraction8

3/10/14 8 Conformations of Heteroglycans Properties • Fiber • Viscosity • Gel forming • Films and Coating • Energy • Structural

3/10/14SeaweedPolysaccharides? Agar·AlginateSeaweed.Carrageenans·PlantExudates.FurcellaranProblem: Alginates are heteroglycans made from:Seedgumsβ-D-mannuronic aad a-L-guluronic acids join by 1> 4 linkages.The polysaccharide is linear and consists of periods af three·PlantstructuralmaterialDistinct tepeating regions as defined below·Microbialgums[→ 4)-β-α-ManpA(1→ 4)-β-α-ManpA(1→],.Starch[→ 4)-α-L-GulpA(1.→ 4)-a-L-GulpA(1→],[→ 4)-B-b-ManpA(1 → 4)-α-t-GulpA(1 →)Asolution of alginate will gel with calcium. DrawThestracturesof thethreeregions andpredict whichregionformsThe gel with calcium crossfinking-PlantExudates.Gum Arabic.GumGhatti:GumTragacanth.KarayaGumFountt9

3/10/14 9 Polysaccharides • Seaweed • Plant Exudates • Seed gums • Plant structural material • Microbial gums • Starch Seaweed • Agar • Alginate • Carrageenans • Furcellaran Problem: Alginates are heteroglycans made from β-D-mannuronic and α-L-guluronic acids join by 1-> 4 linkages. The polysaccharide is linear and consists of periods of three Distinct repeating regions as defined below A solution of alginate will gel with calcium. Draw The structures of the three regions and predict which region forms The gel with calcium cross linking. Plant Exudates • Gum Arabic • Gum Ghatti • Gum Tragacanth • Karaya Gum

3/10/14Plant Seed Gums.LocustBeanGum·GuaranGumTamarindFlour·ArabinogalactanCHCCH,OHReuRt 41A representative segment of a galactomannan molecule.PlantStructure.Cellulose·Hemicellulose:PectindOH-OHHOH,CHOH,OOHHOHC.HONClSHOHOFigure3,15The --1)linked fnuchofinamosechuinof iulin.sacose_resihneswlty Found widhing a mowdacing terminsfor uhar wmaldorherwtsehe thenuhicing end of the chain.10

3/10/14 10 Plant Seed Gums • Locust Bean Gum • Guaran Gum • Tamarind Flour • Arabinogalactan Plant Structure • Cellulose • Hemicellulose • Pectin