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Carbohydrates Importance of Carbohydrates • Distributed widely in nature • Key intermediates of metabolism (sugars) • Structural components of plants (cellulose) • Central to materials of industrial products: paper, lumber, fibers • Key component of food sources: sugars, flour, vegetable fiber • Contain OH groups on most carbons in linear chains or in rings Sources • Glucose is produced in plants through photosynthesis from CO2 and H2O • Glucose is converted in plants to other small sugars and polymers (cellulose, starch) • Dietary carbohydrates provide the major source of energy required by organisms Chemical Formula and Name • Carbohydrates have roughly as many O’s as C ’s (highly oxidized) • Since H’s are connected to each H and O the empirical formulas are roughly (C(H2O))n – Appears to be “carbon hydrate” from formula • Current terminology: natural materials that contain many hydroxyls and other oxygen-­‐containing groups Carbohydrates are Polyhydroxy Aldehydes or Polyhydroxy Ketones molecular formula = Cn(H2O)n The structures of glucose and fructose differ only at the top two carbons. Carbohydrates Simple carbohydrates are monosaccharides. Complex carbohydrates are disaccharides, oligosaccharides, and polysaccharides Polyhydroxy aldehydes are aldoses. Polyhydroxy ketones are ketoses. ClassificaXon of Carbohydrates • Simple sugars (monosaccharides) can't be converted into smaller sugars by hydrolysis. • Carbohydrates are made of two or more simple sugars connected as acetals (aldehyde and alcohol), oligosaccharides, and polysaccharides • Sucrose (table sugar): disaccharide from two monosaccharides (glucose linked to fructose), • Cellulose is a polysaccharide of several thousand glucose units connected by acetal linkages (aldehyde and alcohol) The Smallest Aldose EnanXomers of Glyceraldehyde D-­‐Sugars and L-­‐Sugars An L-­‐Sugar is the Mirror Image of a D-­‐Sugar D and L are Not Related to (+) and (–) D and L indicate configuraXon (+) and (–) indicate how the compound rotates the plane of polarized light. Aldoses and Ketoses • aldo-­‐ and keto-­‐ prefixes idenXfy the nature of the carbonyl group • -­‐ose suffix designates a carbohydrate • Number of C ’s in the monosaccharide indicated by root (tri-­‐, tet-­‐, pent-­‐, hex-­‐) Naturally Occurring D Sugars The Aldotetroses The aldotetroses have two asymmetric centers and four stereoisomers. ConfiguraXons of the Aldoses • Stereoisomeric aldoses are disXnguished by trivial names, rather than by systemaXc designaXons • EnanXomers have the same names but different D,L prefixes • R,S designaXons are difficult to work with when there are mulXple similar chirality centers • SystemaXc methods for drawing and recalling structures are based on the use of Fischer projecXons • Aldotetroses have two chirality centers • There are 4 stereoisomeric aldotetroses, two pairs of enanXomers: erythrose and threose • D-­‐erythrose is a diastereomer of D-­‐threose and L-­‐threose • Aldopentoses have three chirality centers and 23 = 8 stereoisomers, four pairs of enanXomers: ribose, arabinose, xylose, and lyxose Epimers Epimers differ in configuraXon at one asymmetric center. D-­‐mannose is a C-­‐2 epimer of d-­‐glucose. D-­‐galactose is a C-­‐4 epimer of d-­‐glucose. Base-­‐Catalyzed EpimerizaXon Base-­‐Catalyzed Enediol Rearrangement ReducXon An aldose forms one alditol. A ketose forms two alditols. OxidaXon to an Aldonic Acid Only aldehydes can be reduced by Br2. Ketones and alcohols cannot be oxidized by Br2. Aldoses and Ketoses are Oxidized by Tollens Reagent OxidaXon of Monosaccharides • Aldoses are easily oxidized to carboxylic acids by: Tollens' reagent (Ag+, NH3), Fehling's reagent (Cu2+, sodium tartrate), Benedict`s reagent (Cu2+ sodium citrate) – OxidaXons generate metal mirrors; serve as tests for “reducing” sugars (produce metallic mirrors) • Ketoses are reducing sugars if they can isomerize to aldoses OxidaXon to an Aldaric Acid The Kiliani–Fischer Synthesis The Kiliani–Fischer synthesis increases the carbon chain of an aldose by one carbon. The Kiliani–Fischer synthesis forms a pair of C-­‐2 epimers. The Wohl DegradaXon The Wohl degradaXon decreases the carbon chain of an aldose by one carbon. Determining the Structure of Glucose A Kiliani–Fischer synthesis on (–)-­‐arabinose forms (+)-­‐glucose and (+)-­‐mannose. Therefore, (+)-­‐glucose and (+)-­‐mannose are C-­‐2 epimers. OxidaXon of (+)-­‐glucose and (+)-­‐mannose forms opXcally acXve aldaric acids. Therefore, (+)-­‐glucose and (+)-­‐mannose are sugars 3 and 4, or 5 and 6. Determining the Structure of Glucose The aldaric acid of (–)-­‐arabinose is opXcally acXve. Therefore, (+)-­‐glucose and (+)-­‐mannose are sugars 3 and 4. Determining the Structure of Glucose Cyclic Structures of Monosaccharides: Anomers • Alcohols add reversibly to aldehydes and ketones, forming hemiacetals Monosaccharides Form Cyclic Hemiacetals D-­‐glucose exists in three different forms α-­‐D-­‐Glucose and β-­‐D-­‐Glucose are Anomers Monosaccharides Form Cyclic Hemiacetals Six-­‐Membered Ring Sugars are Called Pyranoses Five-­‐Membered Ring Sugars are Called Furanoses Furanoses and Pyranoses α-­‐D-­‐Glucose β-­‐D-­‐Glucose Glucose is the Most Stable Aldohexose Galactose is a C-­‐4 Epimer of Glucose, so the OH at C-­‐4 is Axial An L-­‐Sugar is the Mirror Image of a D-­‐Sugar Gulose differs in configuraXon from glucose at C-­‐3 and C-­‐4, so gulose has the OH groups at C-­‐3 and C-­‐4 in axial posiXons. Glycoside FormaXon • Treatment of a monosaccharide hemiacetal with an alcohol and an acid catalyst yields an acetal in which the anomeric –OH has been replaced by an –OR group – β-­‐D-­‐glucopyranose with methanol and acid gives a mixture of α and β methyl D-­‐glucopyranosides Mechanism for Glycoside FormaXon Glycosides • Carbohydrate acetals are named by first ciXng the alkyl group and then replacing the -­‐ose ending of the sugar with –oside • Stable in water, requiring acid for hydrolysis SelecXve FormaXon of C1-­‐Acetal • Synthesis requires disXnguishing the numerous –
OH groups • Treatment of glucose pentaacetate with HBr converts anomeric OH to Br • AddiXon of alcohol (with Ag2O) gives a β glycoside (Koenigs–Knorr reacXon) Koenigs-­‐Knorr ReacXon Mechanism • α and β anomers of tetraacetyl-­‐D-­‐glucopyranosyl bromide give β -­‐glycoside • Suggests either bromide leaves and caXon is stabilized by neighboring acetyl nucleophile from α side • Incoming alcohol displaces acetyl oxygen to give β glycoside FormaXon of N-­‐Glycosides The Eight EssenXal Monosaccharides • Cells need eight monosaccharides for proper funcXoning • More energeXcally efficient to obtain these from environment • Include L-­‐fucose, D-­‐galactose, D-­‐glucose, D-­‐mannose, N-­‐
acetyl-­‐D-­‐glucosamine, N-­‐acetyl-­‐D-­‐galactosamine, D-­‐
xylose, N-­‐acetyl-­‐D-­‐neuraminic acid The Eight EssenXal Monosaccharides (ConXnued) The Anomeric Effect The anomeric effect is the preference of certain subsXtuents bonded to the anomeric carbon for the axial posiXon. Disaccharides • A disaccharide combines a hydroxyl of one monosaccharide in an acetal linkage with another • A glycosidic bond between C1 of the first sugar (α or β) and the –OH at C4 of the second sugar is parXcularly common (a 1,4ʹ′ link) Reducing and Nonreducing Sugars A reducing sugar can reduce Ag+ or Br2. A nonreducing sugar cannot reduce Ag+ or Br2. A sugar with an aldehyde, a ketone, or a hemiacetal group is a reducing sugar. A sugar without one of these groups is a nonreducing sugar. Maltose Maltose is a disaccharide. Maltose is a reducing sugar. Maltose and Cellobiose • Maltose: two D-­‐
glucopyranose units with a 1,4ʹ′-­‐α-­‐glycoside bond (from starch hydrolysis) • Cellobiose: two D-­‐
glucopyranose units with a 1,4ʹ′-­‐β-­‐glycoside bond (from cellulose hydrolysis) Hemiacetals in Disaccharides • Maltose and cellobiose are both reducing sugars • The α and β anomers equilibrate, causing mutarotaXon Lactose Lactase is the enzyme that cleaves the glycosidic bond in lactose. Without lactase, lactose is turned into gases by flora in the lower bowel, producing flatulence and stomach cramps. Lactose • • • • A disaccharide that occurs naturally in milk Lactose is a reducing sugar. It exhibits mutarotaXon It is 1,4’-­‐β-­‐D-­‐galactopyranosyl-­‐D-­‐glucopyranoside The structure is cleaved in digesXon to glucose and galactose Sucrose Sucrose is not a reducing sugar. Sucrose • “Table Sugar” is pure sucrose, a disaccharide that hydrolyzes to glucose and fructose • Not a reducing sugar and does not undergo mutarotaXon (not a hemiacetal) • Connected as acetal from both anomeric carbons (aldehyde to ketone) Polysaccharides and Their Synthesis • Complex carbohydrates in which very many simple sugars are linked • Cellulose and starch are the two most widely occurring polysaccharides Cellulose • Consists of thousands of D-­‐glucopyranosyl 1,4ʹ′-­‐β-­‐
glucopyranosides as in cellobiose • Cellulose molecules form a large aggregate structures held together by hydrogen bonds • Cellulose is the main component of wood and plant fiber Starch and Glycogen • Starch is a 1,4ʹ′-­‐α-­‐glupyranosyl-­‐glucopyranoside polymer • It is digested into glucose • There are two components – amylose, insoluble in water – 20% of starch • 1,4’-­‐α-­‐glycoside polymer – amylopec:n, soluble in water – 80% of starch AmylopecXn • More complex in structure than amylose • Has 1,6ʹ′-­‐α-­‐glycoside branches approximately every 25 glucose units in addiXon to 1,4ʹ′-­‐α-­‐links Glycogen • A polysaccharide that serves the same energy storage funcXon in animals that starch serves in plants • Highly branched and larger than amylopecXn—up to 100,000 glucose units Synthesis of Polysaccharides – via Glycals • Difficult to do efficiently, due to many –OH groups • Glycal assembly is one approach to being selecXve • Protect C6 –OH as silyl ether, C3–OH and C4–OH as cyclic carbonate • Glycal C=C is converted to epoxide Glycal Coupling • React glycal epoxide with a second glycal having a free –OH (with ZnCl2 catayst) yields a disaccharide • The disaccharide is a glycal, so it can be epoxidized and coupled again to yield a trisaccharide, and then extended Some Other Important Carbohydrates • Deoxy sugars have an –OH group is replaced by an –H. – DerivaXves of 2-­‐deoxyribose are the fundamental units of DNA (deoxyribonucleic acid) Amino Sugars • –OH group that is replaced by an –NH2 • Amino sugars are found in anXbioXcs such as streptomycin and gentamicin Cell-­‐Surface Carbohydrates and Influenza Viruses • Polysaccharides are centrally involved in cell–
cell recogniXon -­‐ how one type of cell disXnguishes itself from another • Small polysaccharide chains, covalently bound by glycosidic links to hydroxyl groups on proteins (glycoproteins), act as biochemical markers on cell surfaces, determining such things as blood type Structures of the A, B, and O Blood-­‐Group AnXgenic Determinants The Synthesis of Vitamin C