CHEM 191 - Module 4 Lecture 3: Carbohydrates

CHEM 191 Module 4 Lecture 3: Chemistry of Carbohydrates

Learning Objectives

• Interpret Fischer projections, Haworth projections, and chair diagrams of monosaccharides.
• Designate monosaccharides as D or L.
• Understand the mechanism for hemiacetal formation.
• Understand the equilibrium (mutarotation) between open chain and cyclic hemiacetal forms of monosaccharides.
• Identify the anomeric carbon and assign the anomer stereochemistry as α or β.
• Define and recognize a ‘reducing sugar’.

Importance of Carbohydrates

• Dietary Roles: Carbohydrates provide energy, regulate blood glucose, spare proteins and fats for energy use, and supply dietary fiber.
• Chemical Formula: Most carbohydrates have the formula $C<em>m(H</em>2O)<em>n$, making them 'hydrates' of carbon (e.g., glucose $C</em>6H<em>{12}O</em>6$ which is (C cdot H2O)6). Deoxy sugars are exceptions to this formula.
• In Vivo Synthesis: Carbohydrates like ribose are essential for nucleotide production. Cell-surface carbohydrate-based polymers are important for adhesion and molecular recognition.

Classes of Carbohydrates

• Monosaccharides: Single sugar units that cannot be broken down into simpler sugars.
  • Polyhydroxy aldehydes (aldoses)
  • Polyhydroxy ketones (ketoses)
  • Polyols (alditols)
  • Polyhydroxy acids
• Saccharides: Carbohydrates are known as ‘saccharides’.
• Classification: Based on the number of saccharide units.
• Di- to Polysaccharides: Covered in Lecture 4.

Fischer Projections

• Representation: 2D drawing of a 3D chiral molecule around its stereocenter.
• Glyceraldehyde: The simplest aldose ($C<em>3H</em>6O_3$) with a chiral carbon.
• (R)-glyceraldehyde: Shown as the D-form in a Fischer projection.
• Drawing Conventions:
  • Chiral carbon is represented as a cross piece.
  • For aldoses, the aldehyde is at the top and $CH_2OH$ at the bottom.
  • The longest carbon chain runs top-to-bottom.
  • Bonds up and down project into the page, while bonds to the side project out.

D and L Nomenclature

• D-isomer: The non-hydrogen atom on the horizontal bond points to the right.
• L-isomer: The non-hydrogen atom points to the left.

Larger Aldoses

• Stereoisomers: An aldose with 'n' asymmetric carbons has $2^n$ possible stereoisomers (e.g., glucose with 4 asymmetric carbons has $2^4 = 16$ stereoisomers).
• D/L Assignment: Determined by the chiral carbon next to the $CH_2OH$ group.
• Monosaccharide Prevalence: Almost all natural monosaccharides are D-sugars.

Hemiacetals and Acetals

• Importance: The chemistry of carbohydrates is dominated by the chemistry of hemiacetals and acetals.

Cyclic Hemiacetals

• Formation: Occurs when the same molecule contains both an aldehyde or ketone and an alcohol.
• Anomeric Carbon: The aldehyde/ketone carbon in the acyclic molecule becomes a new stereocenter in the cyclic molecule.
• Haworth Projection: A way to visualize the stereochemistry of cyclic saccharides, with the anomeric carbon on the right-hand side and the hemiacetal ring oxygen in the back right position.

Mutarotation

• Definition: The equilibrium between cyclic hemiacetal diastereoisomers via the open-chain form of the monosaccharide.
• Anomers: Cyclic hemiacetal diastereoisomers.
• α Anomer: The OH is on the opposite side to $CH_2OH$ ('down' in the Haworth projection).
• β Anomer: The OH is on the same side as $CH_2OH$ ('up' in the Haworth projection).

Measuring Mutarotation

• Method: Use polarimetry to measure specific rotation.
• Process:
  1. Dissolve pure crystalline β-D-galactose in water, measure immediately (gives +151°).
  2. The reaction starts towards equilibrium, measure optical rotation (+80°).
  3. Start with pure crystalline α-D-galactose, add water, measure immediately (gives +53°), also turns into +80° over time
  4. Calculate the ratio at equilibrium (72:28 α:β anomers).

Mutarotation Mechanism

• Reaction: Aldehyde + alcohol to hemiacetal (and vice versa).
• Catalysis: Occurs faster with $H^+$ catalysis but can occur slowly at neutral pH.
• Equilibrium: Hemiacetals are always in equilibrium with an aldehyde/ketone + alcohol under basic, neutral, or acidic conditions.
• Reaction Rate: The reaction can occur in neutral conditions without acid, but it is much slower.

Cyclic Hemiacetal Ring Sizes

• Dominance: For most hexose (C6) monosaccharides, cyclic hemiacetal forms dominate (open chain acyclic <1%).
• Ring Sizes: 6-membered (pyranose) and 5-membered (furanose) rings are possible.
• D-glucose in solution:
  • α-D-glucopyranose (36%)
  • β-D-glucopyranose (64%)
  • α-D-glucofuranose (<0.05%)
  • β-D-glucofuranose (<0.05%)

Chair Representation of a Sugar

• Geometry: Carbons in glucose are tetrahedral.
• Stability: Chair form is a very stable structure.
• Axial vs. Equatorial Bonds: Axial (a) and equatorial (e) bonds.
• Substituent Stability: Larger substituents are more stable in an 'equatorial' position.

Reducing Sugars

• Definition: Sugars containing a functional group that can act as a reducing agent, thereby being oxidized.
• Hemiacetal Presence: Any sugar with a cyclic hemiacetal is a reducing sugar because of the equilibrium with an aldehyde.
• Silver Mirror Test: Reducing sugars can reduce $Ag^+$ to Ag (metal), forming a silver mirror.
• Stability: Polysaccharides containing only acetals are much more stable and are not “reducing sugars.”