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Carbohydrates Sugars or Saccharides. One of the most abundant compounds of living cells In living cells (plants) -> carbohydrates are made by photosynthesis Monosaccharides : C 3 -C 9 Oligosaccharides : 2-10 units. Monosaccharides (biological sugars –ose).

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CarbohydratesSugars or Saccharides

  • One of the most abundant compounds of living cells

  • In living cells (plants) -> carbohydrates are made by photosynthesis

  • Monosaccharides : C3-C9

  • Oligosaccharides : 2-10 units


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Monosaccharides (biological sugars –ose)

Most frequently found in nature: - hexoses (six-carbon sugars) -> glucose and fructose

- pentoses (five-carbon sugars) -> ribose

Carbohydrates

2 Enantiomers

S, R Configuration

Fischer Projection

Highest Oxidation state


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Monosaccharides (biological sugars –ose)

Most frequently found in nature: - hexoses (six-carbon sugars) -> glucose and fructose

- pentoses (five-carbon sugars) -> ribose

Carbohydrates

Pyranoside

Furanoside

Aldoses

Ketoses


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Monosaccharides

D-Aldoses in the range of C3-C6

Carbohydrates


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Monosaccharides

D-Ketoses in the range of C3-C6

Carbohydrates


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Monosaccharides

Stereochemistry

Carbohydrates

D-Erythrose + L-Erythrose -> Enantiomers

D-Erythrose + D-Threose -> Diastereoisomers

D-Erythrose + L-Threose -> Diastereoisomers

D-glucose + L-glucose -> Enantiomers


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Monosaccharides

Enolization and Isomerization

Epimerization

Epimerization

Carbohydrates

Base-catalysed in water:

-> Epimerization + isomerization

Isomerization


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Monosaccharides

Cyclic hemiacetals + hemiketals

Carbohydrates


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Monosaccharides

Cyclic hemiacetals + hemiketals -> glucose

Carbohydrates


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Monosaccharides

Cyclic hemiacetals + hemiketals -> glucose

Carbohydrates

a

b

D

L

α-Sugars have the -CH2OH group and the anomeric hydroxyl group trans.

β-Sugars have the -CH2OH group and the anomeric hydroxyl group cis.


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Monosaccharides

Cyclic hemiacetals + hemiketals -> glucose

Carbohydrates

Glucopyranoside more stable -> almost 100% in pyranoside form


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Monosaccharides

Cyclic hemiacetals + hemiketals -> ribose

Carbohydrates

In solution mainly in pyranoside form (76%)

In nucleotides (or other combinations) mostly found in furanose form


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Monosaccharides

Cyclic hemiacetals + hemiketals -> fructose

Carbohydrates


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Monosaccharides

Cyclic hemiacetals + hemiketals -> fructose

Carbohydrates

In solution mainly in pyranoside form (67%)

In combinations mostly found in furanose form


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Monosaccharides

The anomeric center

Carbonyl group is planar -> attack of OH from either side possible -> 2 epimeric structures (anomers)

Both anomers are in equillibrium in solution

Carbohydrates

Epimers:

α -> anomeric center + highest chiral center -> different configuration (R,S convention)

Hydroxyl (OH) on anomeric C

Down -> α-D-sugars and β-L-sugars

β -> anomeric center + highest chiral center -> same configuration (R,S convention)

Up -> β-D-sugars and α-L-sugars

Epimer (Anomer)

Epimer (Anomer)


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Monosaccharides

The anomeric center

Carbonyl group is planar -> attack of OH from either side possible -> 2 epimeric structures (anomers)

Both anomers are in equillibrium in solution

Carbohydrates

Epimers:

β -> anomeric center + highest chiral center -> same configuration (R,S convention)

α -> anomeric center + highest chiral center -> different configuration

R,R-> β

S,R-> α


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Monosaccharides

The anomeric center

Carbonyl group is planar -> attack of OH from either side possible -> 2 epimeric structures (anomers)

In solution -> all 4 forms in equillibrium !!!

Carbohydrates

Aldohexoses

Ketose


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Monosaccharides

The anomeric center

Esterfication -> freezes sugar in its anomeric form (no interconvertion between α and β any more)

Carbohydrates


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Monosaccharides

Alditols

Reduction of sugars -> reducing agent (NaBH4) -> reduces aldehydes + ketons

Reduction occurs at small amount of open chain form -> shift in equilibrium -> total reduction achieved

Reduction of Aldoses -> 1 product (primary alcohol) -> Alditols

Reduction of Ketoses -> 2 products (secondary alcohol) -> Alditols

Carbohydrates


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Monosaccharides

Glycosides (replace suffix –ose with –oside) -> Reaction at C1 (anomeric C)

Hemiacetals + Alcohol -> Acetals (Glycoside)

Hemiketals + Alcohols -> Ketals (Glycoside)

Carbohydrates

OH on the anomeric C1


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Monosaccharides

Glycosides (replace suffix –ose with –oside)

Hemiacetals + Alcohol -> Acetals (Glycoside)

Hemiketals + Alcohols -> Ketals (Glycoside)

Carbohydrates


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Monosaccharides

Glycosides (replace suffix –ose with –oside) -> Reaction at C1

Hemiacetal/Hemiketal + Alcohol -> O- Glycoside -> Polysaccharides

Hemiketal/Hemiketal + Amine -> N-Glycoside

Carbohydrates


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Monosaccharides

Glycosides (replace suffix –ose with –oside)

Hemiacetal/Hemiketal + Alcohol -> O- Glycoside -> Polysaccharides

Hemiketal/Hemiketal + Amine -> N-Glycoside

Carbohydrates

Template for aspirin

Nucleotides of RNA and DNA


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Monosaccharides

Glycosides (replace suffix –ose with –oside)

Glucose + MeOH -> Acetal -- reaction works directly

Glucose + complex alcohol -> not that easy

Carbohydrates

Example: Production of salicin

-> Template for Aspirin


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Monosaccharides

Glycosides (replace suffix –ose with –oside)

Glucose + MeOH -> Acetal -- reaction works directly

Glucose + complex alcohol -> not that easy

Carbohydrates

Example: Production of salicin

-> Template for Aspirin


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Monosaccharides

Cyclic Acetals + Ketals -> protecting groups

2 OH groups are cis conformation -> reaction with keton

Carbohydrates

C6


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Monosaccharides

Modified sugars -> in cell membranes

Carbohydrates


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Oligosaccharides -> a few monomers

Carbohydrates

Found in dietary products


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Polysaccharides

Carbohydrates

In Plant cell wall -> cotton

In animal and plant cells


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Polysaccharides

Glycogen -> mammalian sugar storage

Carbohydrates


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Polysaccharides

Starch: amylose + amylopectine

-> main plant food reserve

Carbohydrates


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Polysaccharides - Hydrolysis

  • Chemically: - under harsh conditions: with hot acid -> monosaccharides (glucose)

  • - under mild conditions: with acid -> oligosaccharides (randomly)

  • 2. Enzymatic: very specific hydrolysis

  • -> α-amylase -> hydrolysis α 1 -> 4 bonds in starch (mainly maltose + glucose)

  • -> α-1,6-glucosidase -> hydrolysis α 1 -> 6 bonds in starch

  • -> lactase -> hydrolysis lactose

  • (high activity of enzyme in infants – low activity in adults -> intolerance)

  • -> cellulase -> hydrolysis β 1 -> 4 bonds -> animals do not have it (need bacteria)

Carbohydrates


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Oxidation of sugars

Under mild conditions -> with cupric ion (Fehling’s solution) or Br2 -> oxidation of aldehyde group -> acid (aldonic acid)

If sugars are glycosidic linked (acetal formation) -> aldehyde group not available for oxidation -> no reducing sugars

If sugar are free -> reducing sugars

Carbohydrates


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Oxidation of sugars

Under strong conditions -> with HNO3 -> oxidation of aldehyde group + alcohol groups (primary alcohol) -> diacid (aldaric acid)

Carbohydrates


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Oxidation of sugars

Determination of glucose level in blood or urine -> oxidation of glucose –> H2O2 involved in second reaction -> colorimetric or voltametric sensor (Biosensor)

Diabetes mellitus: Insulin deficiency

Insulin regulates blood sugar (glucose) level

If glucose level is high -> insulin level increases -> prevents break down of glycogen into glucose and conversion of fat or protein into glucose -> blood glucose level will sink again

If glucose level is low -> insulin level drops -> more glucose produced -> blood glucose level will rise

Carbohydrates


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Oxidation of sugars

Also oxidized by most enzymes

Terminal OH group oxidized in Aldoses -> Uronic acids

Carbohydrates

Pectins: in cell walls of fruits

-> acid solutions form gels -> jam making


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Oxidation of sugars

Terminal OH group oxidized in Aldoses -> Uronic acids

Carbohydrates

Pain reliefer

Even stronger pain reliefer

Morphine metabolism in human body -> make it more soluble -> excretion


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Oxidation of sugars

Vitamin C: most animals can synthesize Vitamin C – Humans and primates cannot NOT

-> synthesized in liver from glucose

Essential for: - formation of structural proteins in skin, bones, ligaments

- cofactor in synthesis reactions of amino acids and modifications

- antioxidants

Carbohydrates

Deficiently (scurvy) -> muscular pain, skin lesions, fragile blood vessel, bleeding gums, tooth loss


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Aminosugars

Replacement of one OH group by a amino group

In nature -> done by enzymes -> 2-amino-2-deoxy sugars

Carbohydrates

Chitin: insect skeleton and shells of crustaceans


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Aminosugars

Replacement of one OH group by a amino group

In nature -> done by enzymes -> 2-amino-2-deoxy sugars

Bacterial cell walls -> glycycosidic bond cleaved by lysozyme

Carbohydrates

Lysozyme

β-lactam antibiotics -> inhibit peptidase linking peptide bond during biosythesis of cell wall


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Aminosugars

Replacement of one OH group by a amino group

In nature -> done by enzymes -> 2-amino-2-deoxy sugars

Carbohydrates

Blood groups:

Determined by glycoproteins