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Carbohydrates. Carbohydrate Broad class of polyhydroxylated aldehydes and ketones commonly called sugars Synthesized by green plants during photosynthesis Name derived from glucose Glucose was the first simple carbohydrate obtained in pure form

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carbohydrates
Carbohydrates

Carbohydrate

  • Broad class of polyhydroxylated aldehydes and ketones commonly called sugars
  • Synthesized by green plants during photosynthesis
  • Name derived from glucose
    • Glucose was the first simple carbohydrate obtained in pure form
    • Molecular formula of glucose, C6H12O6, was thought to be a “hydrate of carbon, C6(H2O)6”
    • ~ 50% of the dry weight of earth’s biomass consists of glucose polymers
carbohydrates1
Carbohydrates

Carbohydrates act as chemical intermediates by which solar energy is stored and used to support life on earth

21 1 classification of carbohydrates
21.1 Classification of Carbohydrates

Carbohydrates are classed as simple or complex

  • Simple sugars , or monosaccharides
    • Carbohydrates like glucose and fructose that cannot be converted into more simple sugars by hydrolysis
  • Complex carbohydrates
    • Made up of two or more simple sugars
    • Sucrose is a disaccharide comprised of one glucose and one fructose
    • Cellulose is a polysaccharide comprised of several thousand linked glucose units
classification of carbohydrates
Classification of Carbohydrates

Monosaccharides are classified as aldoses or ketoses

  • -ose suffix designates a carbohydrate
  • aldo- prefix identifies an aldehyde carbonyl group in the sugar
  • keto- prefix identifies a ketone carbonyl group in the sugar
  • Number of carbons indicated by the numerical prefix tri-, tetra-, pent-, hex-
21 2 depicting carbohydrate stereochemistry fischer projections
21.2 Depicting Carbohydrate Stereochemistry: Fischer Projections

Fischer projections

  • Suggested by Emil Fischer (1891)
  • Method to project a tetrahedral carbon onto a flat surface
  • Tetrahedral carbon represented by two crossed lines
    • Horizontal lines come out of the page
    • Vertical lines go back into page
depicting carbohydrate stereochemistry fischer projections
Depicting Carbohydrate Stereochemistry: Fischer Projections

A Fischer projection of (R)-glyderaldehyde

depicting carbohydrate stereochemistry fischer projections1
Depicting Carbohydrate Stereochemistry: Fischer Projections

Rules for manipulating Fischer projections:

  • A Fischer projection can be rotated on the page by 180°, but not by 90° or 270°
    • Only a 180° rotation maintains the Fischer convention by keeping the same substituent groups going into and coming out of the plane
depicting carbohydrate stereochemistry fischer projections2
Depicting Carbohydrate Stereochemistry: Fischer Projections
  • A 90° rotation breaks the Fischer convention by exchanging the groups that go into and come out of the plane
    • A 90° or a 270° rotation changes the representation to the enantiomer
depicting carbohydrate stereochemistry fischer projections3
Depicting Carbohydrate Stereochemistry: Fischer Projections
  • A Fischer projection can have one group held steady while the other three rotate in either a clockwise or a counterclockwise direction
    • Effect is to simply rotate around a single bond
depicting carbohydrate stereochemistry fischer projections4
Depicting Carbohydrate Stereochemistry: Fischer Projections

Three steps for assigning R,S stereochemical designations in Fischer projections

  • Assign priorities to the four substituents in the usual way
  • Place the group of lowest priority, usually H, at the top of the Fischer projection by using one of the allowed motions
    • The lowest-priority group is thus oriented back away from viewer
  • Determine the direction of rotation 1→2→3 of the remaining three groups and assign R or S configuration
depicting carbohydrate stereochemistry fischer projections5
Depicting Carbohydrate Stereochemistry: Fischer Projections

Carbohydrates with more than one chirality center are shown in Fischer projection by stacking the centers on top of one another

  • By convention the carbonyl carbon is always placed at or near the top
worked example 21 1 assigning r or s configuration to a fischer projection
Worked Example 21.1Assigning R or S Configuration to a Fischer Projection

Assign R or S configuration to the following Fischer projection of alanine:

worked example 21 1 assigning r or s configuration to a fischer projection1
Worked Example 21.1Assigning R or S Configuration to a Fischer Projection

Strategy

  • Follow the steps listed in the text
    • Assign priorities to the four substituents on the chiral carbon
    • Manipulate the Fischer projection to place the group of lowest priority at the top by carrying out one of the allowed motions
    • Determine the direction 1→2→3 of the remaining three groups
worked example 21 1 assigning r or s configuration to a fischer projection2
Worked Example 21.1Assigning R or S Configuration to a Fischer Projection

Solution

  • The priorities of the groups are (1) –NH2, (2) –CO2H, (3) –CH3, and (4) –H
  • To bring the lowest priority (–H ) to the top we might want to hold the –CH3 group steady while rotating the other three groups counterclockwise
worked example 21 1 assigning r or s configuration to a fischer projection3
Worked Example 21.1Assigning R or S Configuration to a Fischer Projection
  • Going from first- to second- to third-highest priority requires a counterclockwise turn, corresponding to S stereochemistry
21 3 d l sugars
21.3 D,L Sugars

Glyceraldehyde

  • Simplest aldose
  • One chirality center
  • Two enantiomeric (mirror-image) forms
    • Only dextrorotatory enantiomer (–)-glyceraldehyde occurs naturally
    • (+)-Glyceraldehyde has the R configuration
    • (R)-(+)-glyceraldehyde is also referred to as D-glyderaldehyde (D for dextrorotatory)
    • (S)-(–)-glyceraldehyde in also known as L-glyceraldehyde (L for levorotatory)
d l sugars
D,L Sugars

Virtually all naturally occurring monosaccharides have the same R stereochemical configuration as D-glyceraldehyde at the chirality center farthest from the carbonyl group

  • In Fischer projections most naturally occurring sugars have the hydroxyl group at the bottom chirality center pointing to the right
    • Such compounds known as D sugars
d l sugars1
D,L Sugars

L sugars have an S stereochemical configuration at the chirality center farthest from the carbonyl group

  • –OH group pointing to the left in Fischer projections
  • An L sugar is the mirror image (enantiomer) of the corresponding D sugar

D and L sugars can be either dextrorotatory or levorotatory

    • D and L designations only specify the stereochemical configuration at the one chirality center farthest away from the carbonyl group
21 4 configurations of the aldoses
21.4 Configurations of the Aldoses

Aldotetroses are four-carbon sugars with two chirality centers and an aldehyde carbonyl group

  • 22 = 4 possible stereoisomeric aldotetroses
    • Two D,L pairs or enantiomers named erythrose and threose

Aldopentoses are five-carbon sugars with three chirality centers and an aldehyde carbonyl group

  • 23 = 8 possible stereoisomeric aldopentoses
    • Four D,L pairs of enantiomers named ribose, arabinose, xylose, and lyxose
    • All but lyxose occur widely
      • D-Ribose is an important constituent in RNA
      • L-Arabinose is found in plants
      • D-Xylose is found in both plants and animals
configurations of the aldoses
Configurations of the Aldoses

Aldohexoses are six-carbon sugars with four chirality centers and an aldehyde carbonyl group

  • 24 = 16 possible stereoisomeric aldohexoses
    • Eight D,L pairs of enantiomers named allose, altrose, glucose, mannose, gulose, idose, galactose, and talose
    • D-Glucose from starch and cellulose and D-galactose from gums and fruit pectins occur widely in nature
configurations of the aldoses1
Configurations of the Aldoses

Configurations of D-aldoses

  • -OH groups on right side (R) or left side (L) of the chain
configurations of the aldoses2
Configurations of the Aldoses

Remembering the names and structures

of the eight D aldohexoses:

  • Set up eight Fischer projections with the –CHO group on top and the –CH2OH group at the bottom
  • At C5, place all eight –OH groups to the right (D series)
  • At C4, alternate four –OH groups to the right, four to the left
  • At C3, alternate two –OH groups to the right, two to the left
  • At C2, alternate –OH groups right, left, right, left
  • Name the eight isomers using the mnemonic “All altruists gladly make gum in gallon tanks.”

(Structures of the four D aldopentoses: “Ribs are extra lean.”

worked example 21 2 drawing a fischer projection
Worked Example 21.2Drawing a Fischer Projection

Draw a Fischer projection of L-fructose.

worked example 21 2 drawing a fischer projection1
Worked Example 21.2Drawing a Fischer Projection

Strategy

  • Since L-fructose is the enantiomer of D-fructose, look at the structure of D-fructose and reverse the configuration at each chirality center.
21 5 cyclic structures of monosaccharides anomers
21.5 Cyclic Structures of Monosaccharides: Anomers

Aldehydes and ketones undergo a rapid and reversible nucleophilic addition reaction with alcohols to form hemiacetals

Monosaccharides undergo intramolecular nucleophilic additions

  • The carbonyl and hydroxyl groups of the same molecule react to form cyclic hemiacetals
cyclic structures of monosaccharides anomers
Cyclic Structures of Monosaccharides: Anomers

Glucose exists in aqueous solution primarily in the six-membered, pyranose ring form

  • Results from intramolecular nucleophilic addition of the –OH group at C5 to the C1 carbonyl group
  • The name pyranose is derived from pyran
    • Pyran is the name of the unsaturated six-membered cyclic ether
  • Pyranose rings have chairlike geometry with axial and equatorial substituents
cyclic structures of monosaccharides anomers1
Cyclic Structures of Monosaccharides: Anomers
  • Pyranose rings are drawn placing the hemiacetal oxygen at the right rear
    • –OH group of hemiacetal can either be on the top or bottom face of the ring
    • Terminal –CH2OH group is on the top face of the ring in D sugars and on the bottom face of the ring in L sugars
  • When an open-chain monosaccharide cyclizes to a pyranose ring form a new chirality center is generated at the former carbonyl carbon
    • The two diastereomers are called anomers and the hemiacetal carbon atom is referred to as the anomeric center
cyclic structures of monosaccharides anomers2
Cyclic Structures of Monosaccharides: Anomers

Two anomers formed by cyclization of glucose

  • The molecule whose

newly formed –OH

group at C1 is cis

to the oxygen atom

on the lowest chirality

center (C5) in a Fischer

projection is the

a anomer

  • The molecule whose

newly formed –OH

group at C1 is trans

to the oxygen atom

on the lowest chirality

center (C5) in a Fischer

projection is the

b anomer

cyclic structures of monosaccharides anomers3
Cyclic Structures of Monosaccharides: Anomers

Some monosaccharides also exist in a five-membered cyclic hemiacetal form called a furanose

  • D-Fructose exists in both the pyranose and the furanose forms
    • The two pyranose anomers result from addition of C6 –OH group to the C2 carbonyl
    • The two furanose anomers result from addition of C5 –OH group to the C2 carbonyl
cyclic structures of monosaccharides anomers4
Cyclic Structures of Monosaccharides: Anomers

Both anomers of D-glucopyranose can be crystallized and purified

  • Pure a-D-glucopyranose
    • Melting point = 146 °C
    • [a]D specific rotation = +112.2
  • Pureb-D-glucopyranose
    • Melting point = 148-155 °C
    • [b]D specific rotation = +18.7
cyclic structures of monosaccharides anomers5
Cyclic Structures of Monosaccharides: Anomers
  • When a sample of either pure anomer of D-glucopyranose is dissolved in water its optical rotation slowly changes and reaches a constant value of +52.6
    • The specific rotation of a-D-glucopyranose decreases from +112.2 to +52.6 when dissolved in aqueous solution
    • The specific rotation of b-D-glucopyranose increases from +18.7 to +52.6 when dissolved in aqueous solution
    • This change in optical rotation is due to the slow conversion of the pure anomers into a 37 : 63 equilibrium mixture and is known as mutarotation
cyclic structures of monosaccharides anomers6
Cyclic Structures of Monosaccharides: Anomers

Mutarotation of D-glucopyranose

  • Mutarotation occurs by a reversible ring opening of each anomer to the open-chain aldehyde followed by reclosure
  • Mutarotation is catalyzed by both acid and base
worked example 21 3 drawing the chair conformation of an aldohexose
Worked Example 21.3Drawing the Chair Conformation of an Aldohexose

D-Mannose differs from D-glucose in it stereochemistry at C2. Draw D-mannose in its chairlike pyranose form.

worked example 21 3 drawing the chair conformation of an aldohexose1
Worked Example 21.3Drawing the Chair Conformation of an Aldohexose

Strategy

  • First draw a Fischer projection of D-mannose
  • Lay it on its side and curl it around so that the –CHO group (C1) is toward the right front and the –CH2OH group (C6) is toward the left rear
  • Connect the –OH at C5 to the C1 carbonyl group to form the pyranose ring
  • In drawing the chair form raise the leftmost carbon (C4) up and drop the rightmost carbon (C1) down
worked example 21 4 drawing the chair conformation of an aldohexose
Worked Example 21.4Drawing the Chair Conformation of an Aldohexose

Draw b-L-glucopyranose in its more stable chair conformation

worked example 21 4 drawing the chair conformation of an aldohexose1
Worked Example 21.4Drawing the Chair Conformation of an Aldohexose

Strategy

  • It’s probably easiest to begin by drawing the chair conformation of b-D-glucopyranose
  • Then draw its mirror-image L enantiomer by changing the stereochemistry at every position on the ring
  • Carry out a ring-flip to give the more stable chair conformation
  • Note that the –CH2OH group is on the bottom face of the ring in the L enantiomer
21 6 reactions of monosaccharides
21.6 Reactions of Monosaccharides

Ester and Ether Formation

  • Monosaccharides exhibit chemistry similar to simple alcohols
    • Usually soluble in water but insoluble in organic solvents
    • Do not easily form crystals upon removal of water
    • Can be converted into esters and ethers
      • Ester and ether derivatives are soluble in organic solvents and are easily purified and crystallized
reactions of monosaccharides
Reactions of Monosaccharides
  • Esterification is normally carried out by treating the carbohydrate with an acid chloride or acid anhydride in presence of base
    • All –OH groups react including the anomeric –OH group
reactions of monosaccharides1
Reactions of Monosaccharides

Carbohydrates are converted into ethers by treatment with an alkyl halide in the presence of base – the Williamson ether synthesis

  • Silver oxide (Ag2O) gives high yields of ethers without degrading the sensitive carbohydrate molecules
reactions of monosaccharides2
Reactions of Monosaccharides

Glycoside Formation

  • Hemiacetals yield acetals upon treatment with an alcohol and an acid catalyst
  • Treatment of monosaccharide hemiacetals with an alcohol and acid catalyst yields an acetal, called a glycoside
reactions of monosaccharides3
Reactions of Monosaccharides
  • Glycosides are named by first citing the alkyl group and then replacing the –ose ending of the sugar with –oside
  • Glycosides are stable in neutral water and do not mutarotate
  • Glycosides hydrolyze back to free monosaccharide plus alcohol upon treatment with aqueous acid
  • Glycosides are abundant in nature
    • Digitoxigenin – used for treatment of heart disease
reactions of monosaccharides4
Reactions of Monosaccharides

Biological Ester Formation: Phosphorylation

Glycoconjugates

  • Carbohydrates linked through their anomeric center to other biological molecules such as lipids (glycolipids) or proteins (glycoproteins)
  • Constitute components of cell walls and participate in cell-type recognition and identification
reactions of monosaccharides5
Reactions of Monosaccharides
  • Glucoconjugate formation occurs by reaction of the lipid or protein with a glycosyl nucleoside diphosphate
    • Glycosyl nucleoside diphosphate is initially formed by phosphorylation of monosaccharide with ATP to give glycosyl phosphate
reactions of monosaccharides6
Reactions of Monosaccharides
  • Reaction with UTP forms a glycosyl uridine 5′-diphosphate
  • Nucleophilic substitution by an –OH (or –NH2) group on a protein then gives the glycoprotein
reactions of monosaccharides7
Reactions of Monosaccharides

Reduction of Monosaccharides

  • Treatment of an aldose or ketose with NaBH4 reduces it to a polyalcohol called an alditol
    • Reduction occurs by reaction of the open-chain form present in aldehyde/ketone hemiacetal equilibrium
    • D-Glucitol, also known as D-sorbitol, is present in many fruits and berries and is used as a sweetener and sugar substitute
reactions of monosaccharides8
Reactions of Monosaccharides

Oxidation of Monosaccharides

  • Aldoses are easily oxidized to yield corresponding carboxylic acids called aldonic acids
    • Oxidizing agents include:
      • Tollen’s reagent (Ag+ in aqueous NH3)
        • Gives shiny metallic silver mirror on walls of reaction tube or flask
      • Fehling’s reagent (Cu2+ in aqueous sodium tartrate)
        • Gives reddish precipitate of Cu2O
      • Benedict’s reagent (Cu2+ in aqueous sodium citrate)
        • Gives reddish precipitate of Cu2O

(All three reactions serve as simple chemical tests for reducing sugars)

reactions of monosaccharides9
Reactions of Monosaccharides
  • Fructose is a ketose that is a reducing sugar
    • Undergoes two base-catalyzed keto-enol tautomerizations that result in conversion to a mixture of aldoses (glucose and mannose)
reactions of monosaccharides10
Reactions of Monosaccharides
  • Br2 is a mild oxidant that gives good yields of aldonic acid products
    • Preferred over Tollen’s reagent because alkaline conditions in Tollen’s oxidation cause decomposition of the carbohydrate
reactions of monosaccharides11
Reactions of Monosaccharides
  • Aldoses are oxidized in warm, dilute HNO3 to dicarboxylic acids called aldaric acids
    • Both the –CHO group at C1 and the terminal –CH2OH group are oxidized
reactions of monosaccharides12
Reactions of Monosaccharides
  • Enzymatic oxidation at the –CH2OH end of aldoses yields monocarboxylic acids called uronic acids
    • No affect on the –CHO group
21 7 the eight essential monosaccharides
21.7 The Eight Essential Monosaccharides

Humans need to obtain eight monosaccharides for proper functioning

  • All are used for synthesis of glycoconjugate components of cell walls
the eight essential monosaccharides
The Eight Essential Monosaccharides
  • Fucose is a deoxy sugar
    • The –OH group at C6 is replaced by –H
  • N-Acetylglycosamine and N-acetylgalactosamine are amide derivatives of amino sugars
    • The –OH group at C2 is replaced by an –NH2 group
  • N-Acetylneuraminic acid is the parent compound of sialic acids
the eight essential monosaccharides1
The Eight Essential Monosaccharides
  • All eight essential monosaccharides all synthesized from D-glucose
    • Galactose, glucose, and mannose are simple aldohexoses
    • Xylose is an aldopentose
21 8 disaccharides
21.8 Disaccharides

Cellobiose and Maltose

  • Disaccharides contain a glycosidic acetal bond between the anomeric carbon of one sugar and an –OH group at any position on another sugar
    • A glycosidic bond between C1 of the first sugar and the –OH at C4 of the second sugar is a common glycosidic link called a 1→4 link
disaccharides
Disaccharides

Maltose consists of two a-D-glucopyranose units joined by a 1→4-a-glycoside bond

  • Maltose is the disaccharide obtained by enzyme-catalyzed hydrolysis of starch

Cellobiose consists of two b-D-glucopyranose units joined by a 1→4-b-glycoside bond

  • Cellobiose is the disaccharide obtained by partial hydrolysis of cellulose
disaccharides1
Disaccharides
  • Maltose and cellobiose are both reducing sugars because the anomeric carbons on the right-hand glucopyranose units have hemiacetal groups and are in equilibrium with the aldehyde forms
    • Maltose and cellobiose also exhibit mutarotation of a and b anomers
    • Maltose is digested by humans and is fermented readily by yeast
    • Cellobiose cannot be digested by humans and is not fermented by yeast
disaccharides2
Disaccharides

Lactose

  • Lactose is a disaccharide that occurs naturally in human and cow’s milk
  • Lactose is a reducing sugar and exhibits mutarotation
  • Lactose contains a 1→4-b-link between C1 of galactose and C4 of glucose
disaccharides3
Disaccharides

Sucrose

  • Sucrose is ordinary table sugar and is among the most abundant pure organic chemicals in the world
    • Sucrose is obtained from sugar cane (20% sucrose by weight) or from sugar beets (15% sucrose by weight)
    • Sucrose is a disaccharide that consists of 1 equivalent of glucose and 1 equivalent of fructose
      • 1:1 mixture often referred to as invert sugar because the sign of optical rotation inverts (changes) during hydrolysis from sucrose ([a]D = +66.5) to a glucose/fructose mixture ([a]D = -22.0)
      • Honeybees have enzymes called invertases that catalyze the hydrolysis of sucrose
      • Honey is primarily a mixture of sucrose, glucose, and fructose
disaccharides4
Disaccharides
  • Sucrose is not a reducing sugar and does not undergo mutarotation
  • Glucose and fructose are joined by a glycoside link at the anomeric carbons of both sugars, C1 of glucose and C2 of fructose
21 9 polysaccharides and their synthesis
21.9 Polysaccharides and Their Synthesis

Polysaccharides are complex carbohydrates in which tens or even thousands of simple sugars are linked together through glycoside bonds

  • Only one free anomeric –OH on end of long polymeric chain
    • Not reducing sugars
    • Do not exhibit noticeable mutarotation
  • Cellulose and starch are the two most widely occurring polysaccharides
polysaccharides and their synthesis
Polysaccharides and Their Synthesis

Cellulose

  • Cellulose consists of several thousand D-glucose units linked by 1→4-b-glycoside bonds like those in cellobiose
    • Used by nature to impart strength and rigidity to plants
    • Used commercially as raw material for cellulose acetate (acetate rayon) and cellulose nitrate (guncotton) the major ingredient of smokeless gun powder
polysaccharides and their synthesis1
Polysaccharides and Their Synthesis

Starch and Glycogen

  • Starch is a polymer of glucose found in potatoes, corn, and cereal grains
    • Monosaccharide units are linked by 1→4-a-glycoside bonds like those in maltose
    • Starch is separated into two fractions:
      • Amylose accounts for about 20% by weight of starch
      • Amylopectin accounts for about 80% by weight of starch
        • Amylopectin is nonlinear and contains 1→6-a-glycoside branches approximately every 25 glucose units
polysaccharides and their synthesis4
Polysaccharides and Their Synthesis
  • Starch is digested in the mouth and stomach by a-glycosidase enzymes which catalyze the hydrolysis of a-glycoside links but leave the b-glycoside links in cellulose untouched
    • Humans can digest potatoes and grains but cannot digest grasses and leaves
  • Glycogen is a polysaccharide that serves as long-term storage of energy for the human body
    • Glycogen contains both 1→4 and 1→6 links
polysaccharides and their synthesis5
Polysaccharides and Their Synthesis

Polysaccharide Synthesis

  • Glycal assemble method
    • A glycal is an unsaturated sugar with a C1-C2 double bond
    • The C6 –OH group is protected as a silyl ether (R3Si-O-R)
    • The C4 and C3 –OH groups are protected as a cyclic carbonate ester
    • Carbons C1 and C2 are epoxidized
polysaccharides and their synthesis6
Polysaccharides and Their Synthesis
  • Treatment of the protected glycal with another glycal containing a free C6 –OH group in the presence of ZnCl2 yields a dissacharide
  • The dissacharide can be epoxidized and treated with a third glycal to yield a trisaccharide
  • Process is continued to prepare a polysaccharide
polysaccharides and their synthesis7
Polysaccharides and Their Synthesis

Lewis Y hexasaccharide

  • Synthesized complex polysaccharide
  • Tumor marker that is currently being explored as a potential cancer vaccine

Gal

Gal

GlcNAc

Glc

21 10 cell surface carbohydrates and carbohydrate vaccines
21.10 Cell-Surface Carbohydrates and Carbohydrate Vaccines

Small polysaccharide chains covalently bound by glycosidic links to –OH or –NH2 groups on proteins act as biochemical markers on cell surfaces

  • If human blood from one donor type (A, B, AB, or O) is transfused into a recipient with another blood type the red blood cells clump together, or agglutinate
  • Agglutination results from the presence of polysaccharide markers on the surface of the cells
cell surface carbohydrates and carbohydrate vaccines
Cell-Surface Carbohydrates and Carbohydrate Vaccines
  • Types A, B, and O red blood cells each have their own unique markers, or antigenic determinants, and type AB red blood cells have both A and B markers
summary of reactions
Summary of Reactions

Summary of Carbohydrate Reactions

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