《食品化学》课程教学课件（国外讲稿）05 Carbohydrates part3

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 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

3/10/14 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 Ribbon￾Type 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

3/10/14 1-3,B-linked Beta Glucan Mixed linkages H.OH 0 (4.118) 学当%的】 Loosely-Jointed CH.O 人交 (4.121) 4

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

3/10/14 CH,OH HO HO H OH Glucose OH HO CH,OH -0 HO D-Galactose H OH Galactose OH Strongly Pleated or Ribbon Type 1.4linked a-D-galactopyranosyl-uronate unit (4.116) (4.115 7

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

3/10/14 Conformations of Heteroglycans 的ences prof a sin Properties ·Fiber ·Viscosity ·Gel forming ·Films and Coating ·Energy Fig.4.15.Interchain aggregation between regular conformations.a Double helix,b double helix ·Structural bundle.e egg-box,d ribbon-ribbon,and e double helix.ribbon interaction

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

3/10/14 Polysaccharides Seaweed ·Agar ·Seaweed ·Alginate Carrageenans ·Plant Exudates Furcellaran ·Seed gums Plant structural material ·Microbial gums [4)-0-8-ManpA(1-4)-8-p-ManpA(1-1 ·Starch [→4-at-GulpA(1 →4-a-L-GulpA(1→] →4)-go-ManpA(1→4a-t-GulpA0→]b Plant Exudates ·Gum Arabic 人2 ·Gum Ghatti ·Gum Tragacanth ·Karaya Gum

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

3/10/14 Plant Seed Gums ·Locust Bean Gum ·Guaran Gum ·Tamarind Flour ·Arabinogalactan 0 u41 Plant Structure ·Cellulose ·Hemicellulose ·Pectin 9-Ch 三004 10

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