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By photosynthesis◦ The most abundant organic molecules◦ >90% dry matter of plants

Significant contribution to the properties of foods Initial source of all food Provide◦ Energy (~4.5 Cal/g), Carbon and fiber

Cx(H2O)y 70-80% human energy needs (US~50%) Monomers and polymers

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Sweetness Precursors of aroma compounds Precursors of colored compounds Provide texture◦ Cellulose◦ Starch◦ Pectin

Build viscosity at low concentration◦ Starch◦ Gums◦ Cellulose fragments

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•Definition

•Polyhydroxyaldehyde or polyhydroxyketonehaving 2 or more hydroxyl groups

•Aliphatic polyhydroxy aldehydes, ketones and

their derivatives

•Food Carbohydrates: sugars, starches, gums, fiber, gels...

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◦ aliphatic: Open chain organic compounds and those cyclic compounds that resemble them (i.e. not aromatic)

OH

C

CH3

H

C

H

H

C

H

CH3 COOH

OH

OHHO

CH 2OHO OHAliphatic

molecules

Aromatic molecule

OH

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Cannot be broken down by mild acid hydrolysis C3-9 (esp. 5 and 6) Polyalcohols with aldehyde or ketone functional

group Many chiral compounds◦ C has tetrahedral bond angles

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◦ Categorized by chain length

Category # of Glycose Units Food Example

Monosaccharide 1 Sugars

Disaccharide 2 Sugars

Oligosaccharide 3 - 10 Sugars

Polysaccharide >10 Starch, Gums

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Classified as no. of carbons (typically 3-8) and carbonyl group◦ Aldehyde: aldose (carbonyl at C-1, -ose) ◦ Ketone: ketose(carbonyl at C-1, -ulose)◦ hexoses (6 carbons): most common◦ Major one: aldoses◦ Triose, tetrose, pentose, hexose◦ The most simple sugar: carbon 3 (glyceraladehyde,

dihydroxyacetone)

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Chiral carbon

OHHOHH

OHHHHO

CCCCCC

its mirror image is an "optical

isomer"

e.g. glucose

H2OH

HO 4 chiral centers e.g. at C2 carbon: This

structure has a non-superimposable mirror imageCHO

(CHOH) 3CH2OH

OHHC2

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Different compounds having same molecular composition

Constitutional isomers Stereoisomers

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Stereoisomer◦ 3-D orientation difference of atoms

Enantiomer◦ Non superimposable mirror image◦ Have same physicochemical properties except Optical

rotation Diastereomer: stereoisomer not Enantiomer◦ Cis, trans◦ Anomer, apimer• Different Physicochemical properties

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A carbon is chiral if it has four different groups

Chiral compounds have the same composition but are not superimposable

Display in Fisher projection

CH2OHH OH

CHO

CH2OHOH H

CHO

D-glyceraldehyde L-glyceraldehydeENANTIOMERS 18

An anomer is one of a special pair of diastereomericaldoses or ketoses ◦ At carbon of hemiacetal◦ α, β-form◦ Different Physicochemical properties

An epimer is one of a special pair of diastereomericaldoses or ketoses except on glyceraldehyde◦ mannose, glucose

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Optical activity

sample

Light source Polar meter

right: (+)left: (-)

Polarized light

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[α] = αㆍ100/(dㆍc)

α= observed rotationd = width (dm)c = conc of sample (g/100mL)

example (25°C, sodium lamp)[α]25

D = -13.52°

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“D” or “L” is up to the last chiral carbon’s conformation

◦ D conformation (i.e. D-glucose)◦ Most sugars: D conformation

H

H2OH

last chiral carbon C

CC

OCC

OH

HO

OHHO

H

HH

CH2OHH

12

3

4 5

6

5 OHHOHH

OHHHHO

CCCCCC

HO1

234

6

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4 chiral centers —> 16 stereoisomers

OH

OHOH

CH2OHO OH

OHOH

OHCH2OH

O OH

D-mannose D-galactose D-gulose

OHOHOH

CH2OHO OH

OHHOHHHHO

CCCCCCH2OH

HO

HO H

OHH

HHO

CCCCCCH2OH

HO

HO

H OH

HOHH

OH

CCCCCCH2OH

HO

HO

H OH

HH

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◦Hemiacetal is formed when Carbonyl group was attacked by nucleophile◦ Monosugars: Aldehyde and alcohols hemiacetal(pyranose, furanose)

formation

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Hemiacetal: has -OH and -OR group at the same carbon

Acetal: has two OR group at the same carbon

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The most stable form: ring (cyclic) form

HC

CC

OCC

OH

HO

OHHO

H

HH

CH 2 OHH

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4 5

6-D-glucopyranose (glucose)—an aldose

a hexosean aldohexose

—4C1 chair conformation

C CO

CCH

H

COH

H

OHH

OHOH

CH2OH1

2

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6H

-D-fructopyranose (fructose)—a ketose

a hexulosea ketohexose

—2C5 chair conformation28

Anomer is formed

◦ nucleophilic C5 -OH can attack either up or down carbonyl group

HOH 2CH•••O

CC C

C

COH

H

CH2OH

C

CC

C

O

O•••HC

OH

OHHO

CH2OHO OH

OH

OHHO

CH2OHO

OH

CCCCCCH2OH

OH

H O

OHOH

HOH

HH

H

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“” or “” form anomeric carbon

OH

OHOH

CH2OHO

OHOH

OHOH

CH2OHO OH

or or

anomer anomer

HCC

C

OCC

OH

HO

OHHO

H

H

H

CH2OHH

12

3

4 5

6

HCC

C

OCC

OH

HO

OHHO

H

HH

CH2OHH

12

3

4 5

6

trans cis

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Various OH groups can be nucleophile

CH2OH

OHOHOH 2C

HO-D-fructofuranose

HO

OHHO CH2OH

O OH

-D-fructopyranose

HO

CC

CCCCH2OH

O

OHOH

HO

H2OH

HH

HHCC

CCCCH2OH

O

OHOH

HO

H2OH

HH

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Nomenclature:◦ Pyranose: hexagonal (six-membered) ring (a very

stable form due to optimal bond angles)◦ Furanose: pentagonal (five-membered) ring

OH

OHHO

CH2OHO OH

-D-glucopyranose

OH

HOHO CH2OH

O OH

-D-fructopyranose

1

12

234

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56

5

6

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In aqueous acid or base C-O bond is broken: straight chain form

OH

OHHO

CH2OH

OO•••H

OH

OHHO

CH2OH

OOH

OHH

OHH

OHH

HHO

C

C

C

C

C

C H2OH

H

O

1

5 5

5

1

1

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Straight chain form (glucose and fructose):

OHH

OHH

OHH

HHO

C

C

C

C

C

C H2OH

H

O

OHH

OHH

O

HHO

C

C

C

C

C H OH2

C H OH21

2

3

4

5

6

D-glucose(an aldohexose)

D-fructose(an ketohexose)

1

2

3

4

5

6

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◦ Since carbonyl carbon is electron deficient Easy to be attacked by nucleophiles

1. if OH groups is nucleophile, rings formed2. formation of hemiacetal (alcohol adding to carbonyl group in nucleophilic attack)3. Reversible reaction is possible

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In solution, monosugars (e.g. D-glucose, D-fructose) are interchangeable◦ straight and ring form◦ different ring sizes◦ and anomeric isomers

mutarotation◦ Specific optical rotation of monocarbohydrate is change due

to the change of its structure◦ dynamic equilibrium◦ due to reversibility of reaction◦ Fig 3-2

straight chain

-furanose -furanose

-pyranose -pyranose

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Generally only a few isomers predominate

Straight chain form: lower than 1%!

0

10

20

30

40

50

60

70

80

% o

f all

isom

ers

D-glucose D-fructose D-mannose D-galactose

a-pyranoseb-pyranosea-furanoseb-furanose

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Straight form: the most reactive◦ free carbonyl susceptible to nucleophilic attack,

oxidation, reduction, isomerization◦ even small amounts allow reactions to proceed

Stereochemistry: effect on sweetness◦ different sugars and different isomeric forms vary

in sweetness!◦ -D-fructopyranose makes fructose sweeter than most sugars

Isomer stability◦ Affected by temp and solvent

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