Carbohydrates: Oligosaccharides and Polysaccharides

Hemiacetals and Acetals

  • Cyclic hemiacetals react with alcohols to form acetals and water.
  • If the alcohol is from another sugar, an acetal is formed.
  • The reaction is reversible in the presence of a strong acid (equilibrium).
  • The new bond formed is called a glycosidic bond or glycosidic linkage.
  • The acetal product is called a glycoside.
  • C_{anomeric} - O - R is an acetal.

Stability of Acetals vs Hemiacetals

  • Acetals are more stable than hemiacetals.
  • Hemiacetals exist in equilibrium with the open-chain monosaccharide form, even in neutral aqueous conditions.
  • Acetals are stable in neutral conditions; a strong acid is needed for conversion to hemiacetals.
  • Repeating sugar units linked by acetals are chemically stable without strong acids.
  • Glycosides are non-reducing sugars because there is no hemiacetal.

Higher Saccharides

  • Repeating units of sugars are joined by acetals, containing a glycosidic bond.
  • Very stable chemically unless a strong acid is present.
  • In a biological setting, acetal hydrolysis requires enzymes.
  • Nomenclature:
    • Monosaccharide: 1 sugar unit
    • Disaccharide: 2 sugar units
    • Trisaccharide: 3 sugar units
    • Oligosaccharide: 2-10 sugar units
    • Polysaccharide: >10 sugar units
  • Higher saccharides can be branched, with varied linkage stereochemistry.

Glycosidic Linkage

  • Two acetal anomers are chemically possible (alpha or beta).
  • Different bond connectivity is possible depending on which sugar alcohol reacts.

Glucose Disaccharides

  • Maltose: two D-glucopyranose units linked by an a-1,4-glycosidic bond.
  • Cellobiose: two D-glucopyranose units linked by a b-1,4-glycosidic bond.

Reducing vs Non-Reducing Saccharides

  • A non-reducing sugar does not contain a hemiacetal.
  • Maltose is a reducing sugar because the hemiacetal will exist in equilibrium with the ring-opened form (aldehyde + alcohol).
  • Sucrose is a disaccharide of D-glucopyranose and D-fructofuranose linked by an a-1,2-glycosidic bond and is a non-reducing sugar.

Polysaccharides

  • Amylose: D-glucose linked by a-1,4-glycosidic bonds (up to 4,000 units).
  • Cellulose: D-glucose linked by b-1,4-glycosidic bonds (up to 4,000 units).
  • Cellulose has a flat 3D shape, forming straight ‘molecular rods’ stabilized by hydrogen bonds, which give it strength, and it is indigestible without the enzyme amylase.

Carbohydrates and Molecular Recognition

  • Carbohydrates, when attached to a protein or lipid, form a cell surface molecular recognition code based on 3D shape.
  • Glycosidic bonds can vary in linkage position and stereochemistry, creating many possible 3D shapes.
  • Initial interactions between human cells and invading microorganisms are often governed by cell surface carbohydrates

Molecular Recognition - Blood Group Example

  • Blood groups A, B, and O have different carbohydrates on the surface of red blood cells.

Case Study: Honey

  • Honey mainly contains monosaccharides (glucose and fructose).
  • Honey bees collect nectar containing sucrose and break it down into monosaccharides using invertase that cleaves the a-1,2- glycosidic bond.
  • Honey has a unique chemical signature based on the plant's nectar, pollen, season, and region.

Mānuka Honey

  • Mānuka has cultural significance to Māori.
  • Mānuka extracts were traditionally used as a rongoā rākau (traditional medicine).
  • Mānuka honey contains leptosperin (a gentiobiose glycoside, made of 2 x D-glucose with a b-1,6-glycosidic linkage) and methylglyoxal.
  • The level of methylglyoxal (MGO) contributes to the antimicrobial activity.
  • MGO is highly reactive and can form imines with amines.

Adulterated Honey

  • Fake honey contains added colorants, sweeteners, or other foreign substances.
  • Adding synthetic methylglyoxal or high fructose corn syrup (HFCS) is a concern.
  • DNA testing and carbon isotope ratio analysis can be used to detect adulteration.