Chair Conformations

2020

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Chair ConformationsChair Conformations• For pyranoses, the six-membered ring is more accurately represented as a chair chair conformationconformation.

OCH2OH

HOHO

OHOH()

CHOH

HO

CH2OHOHHO

OHO

OH()HO

HO

CH2OHO

(-D-Glucose)

(-D-Glucose)

-D-Glucopyranose

-D-Glucopyranose

D-Glucose

anomericcarbon

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Chair ConformationsChair Conformations• In both Haworth projections and chair conformations, the orientations of groups on carbons 1- 5 of -D-glucopyranose are up, down, up, down, and up.

OCH2OH

HOHO

OHOH()H

H OH

HHO

HOH()

OH

H

CH2OHO

-D-Glucopyranose(chair conformation)

-D-Glucopyranose(Haworth projection)

123

4

5

6

1

23

4

5

6

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MutarotationMutarotation• Mutarotation: Mutarotation: the change in specific rotation that accompanies the equilibration of - and -anomers in aqueous solution.• Example: when either -D-glucose or -D-glucose is dissolved in water, the specific rotation of the solution gradually changes to an equilibrium value of +52.7°, which corresponds to 64% beta and 36% alpha forms.

[]D25 = + 18.7°

-D-Glucopyranose-D-Glucopyranose[]D

25 = +112°

OHOH

HOHO

CH2OHO HO OH

OC

CH2OH

HO

HOH

OCH2OH

HO

HOOH

HO

Open-chain form

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Physical PropertiesPhysical Properties• Monosaccharides are colorless crystalline solids, very soluble in water, but only slightly soluble in ethanol• Sweetness relative to sucrose:

Carbohydrate

fructose

glucose

galactose

sucrose (table sugar)

lactose (milk sugar)

honey

SweetnessRelative to Sucrose

1.741.000.970.74

0.320.16

Artificial Sweetener

SweetnessRelative to Sucrose

maltose 0.33

saccharin 450acesulfame-K 200aspartame 180

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Formation of GlycosidesFormation of Glycosides• Treatment of a monosaccharide, all of which exist almost exclusively in cyclic hemiacetal forms, with an alcohol gives an acetal.

Acetals α + βMutarotation = No

HH OH

HHO

HOH

OH

H

CH2OHO

CH3OHH

+

-H2O

OCH2OH

H

OH

OCH3H

HOH

OHH

H

OCH2OH

H

OH

HH

HOH

OHH

OCH3

(-D-Glucose)-D-Glucopyranose

Methyl -D-glucopyranoside(Methyl -D-glucoside)

anomeric carbon

+

+


glycosidicbond

HH OH

HHO

HOH

OH

H

CH2OHO

CH3OHH+

-H2O

OCH2OH

H

OH

OCH3H

HOH

OHH

H

OCH2OH

H

OH

HH

HOH

OHH

OCH3



anomeric carbon

+

+


glycosidicbond

Hemiacetal mutarotation =Yes

HH OH

HHO

HOH

OH

H

CH2OHO

CH3OHH

+

-H2O

OCH2OH

H

OH

OCH3H

HOH

OHH

H

OCH2OH

H

OH

HH

HOH

OHH

OCH3



anomeric carbon

+

+


glycosidicbond

yes no

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Formation of GlycosidesFormation of Glycosides• A cyclic acetal derived from a monosaccharide is called a

glycosideglycoside.•The bond from the anomeric carbon to the -OR group is called a glycosidic bondglycosidic bond.

•Mutarotation is not possible in a glycoside because an acetal, unlike a hemiacetal, •is not in equilibrium with the open-chain carbonyl-containing compound.

Glycosides are stable in water and aqueous base, but like other acetals, are hydrolyzed in aqueous acid to an alcohol

and a monosaccharide.

•Glycosides are named by listing the alkyl or aryl group bonded to oxygen followed by the name

of the carbohydrate in which the ending -ee is replaced by -ide-ide.

HH OH

HHO

HOH

OH

H

CH2OHO

CH3OHH

+

-H2O

OCH2OH

H

OH

OCH3H

HOH

OHH

H

OCH2OH

H

OH

HH

HOH

OHH

OCH3



anomeric carbon

+

+


glycosidicbond

HH OH

HHO

HOH

OH

H

CH2OHO

CH3OHH+

-H2O

OCH2OH

H

OH

OCH3H

HOH

OHH

H

OCH2OH

H

OH

HH

HOH

OHH

OCH3



anomeric carbon

+

+


glycosidicbond

HH OH

HHO

HOH

OH

H

CH2OHO

CH3OHH+

-H2O

OCH2OH

H

OH

OCH3H

HOH

OHH

H

OCH2OH

H

OH

HH

HOH

OHH

OCH3



anomeric carbon

+

+


glycosidicbond

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Reduction to AlditolsReduction to Alditols• The carbonyl group of a monosaccharide can be reduced to an hydroxyl group by a variety of reducing agents, including NaBH4 and H2 in the presence of a transition metal catalyst.• The reduction product is called an alditolalditol.

OHOH

HOHO

CH2OHO

CHOOHHHHOOHH

CH2OHOHH

NaBH4

CH2OHOHHHHOOHH

CH2OHOHH

D-Glucitol(D-Sorbitol)

D-Glucose-D-Glucopyranose

alditol

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Reduction to AlditolsReduction to Alditols• Sorbitol is found in the plant world in many berries and in cherries, plums, pears, apples, seaweed, and algae.

• It is about 60 percent as sweet as sucrose (table sugar) and is used in the manufacture of gums, candies and as a sugar substitute for diabetics.

CH2OH

CH2OH

OHHOHH

CH2OH

CH2OH

OHHHHOOHH

CH2OHHHOHHOOHH

CH2OHOHH

D-Mannitol XylitolErythritol

OHOH

HOHO

CH2OHO

CHOOHHHHOOHH

CH2OHOHH

NaBH4

CH2OHOHHHHOOHH

CH2OHOHH

D-Glucitol(D-Sorbitol)

D-Glucose-D-Glucopyranose

•These four alditols are also common in the biological world.

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Oxidation to Aldonic Oxidation to Aldonic AcidsAcids

• The aldehyde group of an aldose is oxidized under basic conditions to a carboxylate anion.

• The oxidation product is called an aldonic acidaldonic acid.• A carbohydrate that reacts with an oxidizing agent to form an aldonic acid is classified as a reducing reducing sugarsugar (it reduces the oxidizing agent).

OCH2OH

HOHO

OHOH

COHHHHOOHH

CH2OHOHH

O HC

OHHHHOOHH

CH2OHOHH

O O-

oxidizingagent

D-GluconateD-Glucose-D-Glucopyranose(-D-Glucose)

basicsolution

enediol intermed.

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Oxidation to Uronic AcidsOxidation to Uronic Acids• Enzyme-catalyzed oxidation of the primary alcohol at C-6 of a hexose yields a uronic aciduronic acid.

Fisher proj.

Chair conformation

CHO

CH2OH

OHHHHOOHHOHH

D-Glucose

enzyme-catalyzedoxidation

CHO

COOH

OHHHHOOHHOHH

D-Glucuronic acid(a uronic acid)

OHO

HOOH

OH

COOH

e.g. Enzyme-catalyzed oxidation of D-glucose,

yields D-glucuronic acid.

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D-Glucuronic AcidD-Glucuronic Acid• D-Glucuronic acid is widely distributed in the plant and animal world.

OHOHO

OHO

COO-

HO

Propofol A urine-soluble glucuronide

OHOHO

OHO

COO-

HO

Propofol A urine-soluble glucuronide

•In humans, it is an important component of the acidic polysaccharides of connective tissues, e.g. collagen.

Used by the body to detoxify foreign phenols and alcohols; in the liver, compounds converted to glycosides of

glucuronic acid and excreted in the urine.

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Two Volunteers Two Volunteers needed. . .needed. . .

1 male1 female

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Testing for GlucoseTesting for Glucose

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Phosphate EstersPhosphate Esters• Mono- and diphosphoric esters are intermediates in the metabolism of monosaccharides.•For example, the first step in glycolysis is conversion of

•D-glucose to a-D-glucose 6-phosphate.

•Note that at the pH of cellular and intercellular fluids, both acidic protons of a diphosphoric ester are ionized, giving it a charge of -2.

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DisaccharidesDisaccharides

O

HOOH

OH

CH2OH

O

OH

HOO

CH2OH

HOCH2

OHO

HO

O

OH

CH2OH

OH

HOO

CH2OH

HOCH2

1

1

2

1

2

1

a unit of -D-glucopyranose

a unit of -D-fructofuranose

-1,2-glycosidic bond

e.g. Sucrose (table sugar)Sucrose is the most abundant disaccharide in the biological world; it is obtained principally from

the juice of sugar cane and sugar beets.

•Sucrose is a nonreducing sugar.

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DisaccharidesDisaccharides• Lactose

Lactose sugar present in milk; 5-8% human milk, 4-6% cow's milk.

O

OH

HOOH

O

CH2OH

O

HOOH

OH

CH2OHOOH O

OH

OH

CH2OH

O OH

OH

OH

CH2OH

1

1

4

4

-1,4-glycosidic bond

•Little sweetness: added to cheap foods ~ fillers . . .

•D-galactopyranose bonded by a b-1,4-glycosidic bond to carbon 4 of D-glucopyranose.

•Lactose is a reducing sugar. Why? In equilb. w/ Open chain form~ can be oxidized to carboxylate

OCH2OH

HOHO

OHOH

COHHHHOOHH

CH2OHOHH

O HC

OHHHHOOHH

CH2OHOHH

O O-

oxidizingagent

D-GluconateD-Glucose-D-Glucopyranose(-D-Glucose)

basicsolution

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A, B, AB and O Blood A, B, AB and O Blood typestypes

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AntigenAnti-bodies

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DisaccharidesDisaccharides• Maltose

• Present in malt, the juice from sprouted barley and other cereal grains.

• Maltose consists of two units of D-glucopyranose joined by an -1,4-glycosidic bond.

• Maltose is a reducing sugar.

OHO

HOOH

OOHO OH

OH

CH2OH

CH2OHO

OH

O

OHHO

O OH

HO

OH

CH2OH

HOCH2 1

4

-1,4-glycosidicbond1 4

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PolysaccharidesPolysaccharides• Polysaccharide:Polysaccharide: a carbohydrate of many monosaccharides joined by glycosidic bonds.

• Starch:Starch: a polymer of D-glucose, E stores in plants

• Two forms: amylose and amylopectin.

• 25% Amylose, unbranched chains w/ 4000 D-glucose units joined by -1,4-glycosidic bonds.

• 75% Amylopectin, has branched chains with up to 10,000 D-glucoses, joined by -1,4-glycosidic bonds;

at branch points, new chains of 24 to 30 units are started by -1,6-glycosidic bonds.

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PolysaccharidesPolysaccharides• Figure 20.3 Amylopectin.

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PolysaccharidesPolysaccharides• GlycogenGlycogen is the energy-reserve carbohydrate for animals.• Glycogen is a branched polysaccharide of approximately 106 glucose units joined by -1,4- and -1,6-glycosidic bonds.

• The total amount of glycogen in the body of a well-nourished adult human is about 350 g, divided almost equally between liver and muscle.

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PolysaccharidesPolysaccharides• CelluloseCellulose linear polysaccharide - D-glucose units, -

1,4-glycosidic bonds.• ~ molecular weight of 400,000 g/mol, corresponding to

approximately 2200 glucose units per molecule.

• Cellulose align themselves side by side into fibers . . OH groups form numerous intermolecular H-bonds.

• This arrangement of parallel chains in bundles gives cellulose fibers their high mechanical strength.

• reason why cellulose is insoluble in water.

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PolysaccharidesPolysaccharides

• Cellulose (cont’d)• Humans can’t use cellulose, lack digestive enzymes -

glucosidases, catalyze hydrolysis of -glucosidic bonds.

• Humans have only -glucosidases; hence, the polysaccharides we use as sources of glucose are starch and glycogen.

• Many bacteria and microorganisms have -glucosidases and can digest cellulose.

• Termites have such bacteria in their intestines and can use wood as their principal food.

• Ruminants (cud-chewing animals) and horses can also digest grasses and hay.

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Acidic Acidic PolysaccharidesPolysaccharides• Acidic polysaccharides:Acidic polysaccharides: a group of

polysaccharides that contain carboxyl groups and/or sulfuric ester groups, and play important roles in the structure and function of connective tissues.

• There is no single general type of connective tissue.

• Large number of highly specialized forms, such as cartilage, bone, synovial fluid, skin, tendons, blood vessels, intervertebral disks, and cornea.

• Most connective tissues are made up of collagen, a structural protein, in combination with a variety of acidic polysaccharides.

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Acidic Acidic PolysaccharidesPolysaccharides• e.g. Hyaluronic acide.g. Hyaluronic acid

• contains from 300 to 100,000 repeating units.

• is most abundant in embryonic tissues, connective tissues (synovial fluid) the lubricant of joints

-the vitreous of the eye: provides a clear, elastic gel that maintains the retina in its proper position

O

HOOH

COO-

OHO

NH

CH2OH

CH3C O

O O

The repeating unit of hyaluronic acid

4

13

1

3

4

D-glucuronic acid N-Acetyl-D-glucosamine

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Acidic Acidic PolysaccharidesPolysaccharides• HeparinHeparin: a heterogeneous mixture of

variably sulfonated polysaccharide chains, ranging in molecular weight from 6,000 to 30,000 g/mol.

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Acidic Acidic PolysaccharidesPolysaccharides• Heparin (cont’d)

• Heparin is synthesized and stored in mast cells of various tissues, particularly the liver, lungs, and gut.

• The best known and understood of its biological functions is its anticoagulant activity (blood thinner).

• It binds strongly to antithrombin III, a plasma protein involved in terminating the clotting process.

Chair Conformations

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