22 Carbohydrates II - MEDCHEM Notes

Recommended Reading

• Organic Chemistry with Biological Applications, John McMurry
  • Chapter 21
  • Section 21.4 – Configurations of the Aldoses
  • Section 21.5 – Cyclic Structures of Monosaccharides: Anomers
  • Section 21.8 – Disaccharides
  • Section 21.9 – Polysaccharides and their Synthesis
  • Section 21.10 – Some Other Important Carbohydrates

Learning Outcomes

• Mutarotation: Explain the interconversion of anomers.
• Monosaccharides Reactions: Recall including oxidation and acetal formation.
• Glycosidic Linkage: Define and identify in various sugars.
• Disaccharides: Recall structures, including maltose, cellobiose, lactose, and sucrose, noting glycosidic linkages.
• Lactose Intolerance & Galactosemia: Explain biochemical basis and pathological consequences.
• Structural Features: Identify ribose, 2'-deoxyribose, and glucosamine; explain biological relevance.
• Recognition Role: Explain carbohydrates' roles in cell recognition and signaling.

D-Glucose Anomers

• D-Glucose has two pure solid diastereomers (anomers) which differ in:
  • Melting Points:
  • $ext{α-D-Glucose: } M<em>p = 150^ ext{o}C, [ ext{α}]</em>D = +19^ ext{o}$
  • $ext{β-D-Glucose: } M<em>p = 146^ ext{o}C, [ ext{α}]</em>D = +112^ ext{o}$

Mutarotation

• Definition: Interconversion of α- and β-anomers in solution.
• Stability: Anomers are stable in solid form; in solution, they equilibrate after ring opening to linear form.
• Equilibrium: 50:50 ratio yields a specific rotation of $65.5^ ext{o}$ with true equilibrium at $52.2^ ext{o}$.
• Stability of form: The excess form is more stable due to equatorial position of anomeric hydroxyl group.

Reactions of Monosaccharides

• Oxidation of Aldoses: E.g., glucose to aldonic acids.
  • Prefer Br2 solution over Tollen’s reagent for oxidation to prevent decomposition.

Glycoside Formation

• Mechanism: Hemiacetals react with alcohol and acid catalyst to yield stable acetals (glycosides).

Disaccharides

• Defined: Composed of two sugars linked by glycosidic bonds.

• Common Linkage: 1→4 bond between C1 of one sugar and an –OH of another.

• Maltose:

  • Structure: Two $ext{α-D-Glucose}$ units via a $ext{α(1→4)-glycosidic bond}$.
  • Source: Hydrolysis of starch.

• Cellobiose:

  • Structure: Two $ext{β-D-Glucose}$ units via a $ext{β(1→4)-glycosidic bond}$.
  • Source: Partial hydrolysis of cellulose.

Lactose

• Structure: Naturally occurring disaccharide in milk; contains a $ext{β(1→4)-linkage}$ between galactose and glucose.
• Intolerance: Low lactase levels cause lactose build-up; bacteria ferment lactose leading to gas and acids.

Galactosemia

• Genetic disorder from the buildup of galactose-1-phosphate due to enzyme deficiency.
  • Symptoms: Jaundice, lethargy, organ damage; inability to digest lactose leads to severe health issues.

Sucrose

• Common table sugar, composed of glucose and fructose.
• Not a reducing sugar; does not undergo mutarotation.
• Role in Tooth Decay: Metabolized by bacteria into acids that erode tooth enamel.

Polysaccharides

• Structure: Long chains of sugar units linked through glycosidic bonds.
  • Cellulose: Mostly $ext{β(1→4)}$ linked glucose units; gives rigidity to plants.
  • Starch: Mostly $ext{α(1→4)}$ linked chains; digested by human enzymes.

Cellulose Metabolism

• Enzyme: Cellulase (absent in humans); ruminants utilize microbes to digest cellulose-containing diets.

Glycogen

• Storage form of glucose in animals; similar to amylopectin but more branches.
• Regulated via insulin and glucagon to maintain blood glucose levels.