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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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Carbohydrates

Monosaccharides (biological sugars –ose)

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

- pentoses (five-carbon sugars) -> ribose

S, R Configuration 2 Enantiomers

Fischer ProjectionHighest Oxidation state

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Carbohydrates

Monosaccharides (biological sugars –ose)

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

- pentoses (five-carbon sugars) -> ribose

Aldoses

Ketoses

Pyranoside Furanoside

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CarbohydratesMonosaccharides

D-Aldoses in the range of C3-C6

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CarbohydratesMonosaccharides

D-Ketoses in the range of C3-C6

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CarbohydratesMonosaccharides Stereochemistry

D-Erythrose + L-Erythrose -> Enantiomers

D-Erythrose + D-Threose -> Diastereoisomers

D-Erythrose + L-Threose -> Diastereoisomers

D-glucose + L-glucose -> Enantiomers

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CarbohydratesMonosaccharides

Enolization and Isomerization

Isomerization

Epimerization Epimerization

Base-catalysed in water:-> Epimerization + isomerization

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CarbohydratesMonosaccharides

Cyclic hemiacetals + hemiketals

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CarbohydratesMonosaccharides

Cyclic hemiacetals + hemiketals -> glucose

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CarbohydratesMonosaccharides

Cyclic hemiacetals + hemiketals -> glucose

O

CH2OH

O

CH2OH

O

CH2OH

OH

D L

O

CH2OHOH

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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CarbohydratesMonosaccharides

Cyclic hemiacetals + hemiketals -> glucose

Glucopyranoside more stable -> almost 100% in pyranoside form

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CarbohydratesMonosaccharides

Cyclic hemiacetals + hemiketals -> ribose

In solution mainly in pyranoside form (76%)

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

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CarbohydratesMonosaccharides

Cyclic hemiacetals + hemiketals -> fructose

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CarbohydratesMonosaccharides

Cyclic hemiacetals + hemiketals -> fructose

In solution mainly in pyranoside form (67%)

In combinations mostly found in furanose form

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CarbohydratesMonosaccharides

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

Epimer (Anomer)

Epimer (Anomer)

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

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CarbohydratesMonosaccharides

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

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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CarbohydratesMonosaccharides

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 !!!

Aldohexoses

Ketose

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CarbohydratesMonosaccharides

The anomeric center

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

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CarbohydratesMonosaccharides

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

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CarbohydratesMonosaccharides

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

Hemiacetals + Alcohol -> Acetals (Glycoside)

Hemiketals + Alcohols -> Ketals (Glycoside)

OH on the anomeric C1

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CarbohydratesMonosaccharides

Glycosides (replace suffix –ose with –oside)

Hemiacetals + Alcohol -> Acetals (Glycoside)

Hemiketals + Alcohols -> Ketals (Glycoside)

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CarbohydratesMonosaccharides

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

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

Hemiketal/Hemiketal + Amine -> N-Glycoside

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CarbohydratesMonosaccharides

Glycosides (replace suffix –ose with –oside)

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

Hemiketal/Hemiketal + Amine -> N-Glycoside

Template for aspirin

Nucleotides of RNA and DNA

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CarbohydratesMonosaccharides

Glycosides (replace suffix –ose with –oside)

Glucose + MeOH -> Acetal -- reaction works directly

Glucose + complex alcohol -> not that easy

Example: Production of salicin-> Template for Aspirin

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CarbohydratesMonosaccharides

Glycosides (replace suffix –ose with –oside)

Glucose + MeOH -> Acetal -- reaction works directly

Glucose + complex alcohol -> not that easy

Example: Production of salicin-> Template for Aspirin

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CarbohydratesMonosaccharides

Cyclic Acetals + Ketals -> protecting groups

2 OH groups are cis conformation -> reaction with keton

C6

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CarbohydratesMonosaccharides

Modified sugars -> in cell membranes

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CarbohydratesOligosaccharides -> a few monomers

Found in dietary products

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CarbohydratesPolysaccharides

In Plant cell wall -> cotton

In animal and plant cells

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CarbohydratesPolysaccharides

Glycogen -> mammalian sugar storage

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CarbohydratesPolysaccharides

Starch: amylose + amylopectine -> main plant food reserve

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CarbohydratesPolysaccharides - Hydrolysis

1. 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)

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CarbohydratesOxidation of sugarsUnder 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

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

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CarbohydratesOxidation of sugarsDetermination 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

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CarbohydratesOxidation of sugarsTerminal OH group oxidized in Aldoses -> Uronic acids

Pectins: in cell walls of fruits

-> acid solutions form gels -> jam making

Also oxidized by most enzymes

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CarbohydratesOxidation of sugarsTerminal OH group oxidized in Aldoses -> Uronic acids

Pain reliefer

Even stronger pain reliefer

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

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CarbohydratesOxidation 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

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

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CarbohydratesAminosugarsReplacement of one OH group by a amino group

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

Chitin: insect skeleton and shells of crustaceans

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CarbohydratesAminosugarsReplacement 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

Lysozyme

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

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CarbohydratesAminosugarsReplacement of one OH group by a amino group

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

Blood groups:Determined by glycoproteins