3/10/14 Acetal Formation Acetal Formation:Mechanism hhls tom Acid-catalyzed nucleophilic addition: CH.OH CH,OH CH,OH CH,OH CH,OH ROH OH OR Reaction is a nucleophilic addition of two alcohols to aldehydes CH.OH aldehyde hemiacetal acetal open-chain suga cyclic sugar glycoside O-CH R'O-CH-OH R'O-CH-OR" Acetals:Glycosides Hydrolysis (Acetal Breakdown) Acid-catalyzed nucleophilic addition: For sugars,product called a glycoside CH,OH CH OH -R group called aglycone Reaction is reversible in presence of water 1.Prolonation of OR group and acid 2.Formafion of carbocation OH -Acid hydrolysis
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
3/10/14 Oligosaccharides reducing Link-Glycone-Aglycone B-Lactose 学卧 B-galactosyl glucose B-Maltose 学到 a-glucosyl glucose B-Cellobiose 行 B-glucosyl glucose nonreducing end reducing end Oligosaccharides nonreducing Sucrose An a-glucoside OH H OH Trehalose An a,a-diglucoside Maltase hydrolyet not
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
3/10/14 1-6 H90 HO O B OH HO Polysaccharides OH Classification,Structure OH ·Conformation HO HO wOH ·Properties HO Individual Polysaccharides 1-4
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
3/10/14 Classification(Glycans) Conformation Homoglycans-Heteroglycans Monosaccharide linkages will Linear-Branched (frequency length) determine conformation.May have periodic and nonperiodic Monsaccharide residue sequence may be regions of sugars periodic or nonperiodic periodic sequences may be long or short; they can also consist of periodic occurrence of periodic sequences separated by nonperiodic sequences Polysaccharides Extended or Stretched Ribbon- Type 1.4 B-D-glucopranosyl residues CH,OH O 0 HO (4.114)
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
3/10/14 27鸭气兴 Chemical Modification of Cellulose 毛- 营 7%227 1,4 a-D-glucopranosyl residues amylose (4.119) 5
3/10/14 5 Hemicellulose Chemical Modification of Cellulose 1,4 α-D-glucopranosyl residues amylose