21 In-Depth Notes on Carbohydrates

Introduction to Carbohydrates

• Wide class of polyhydroxylated aldehydes and ketones, collectively known as sugars.
• Main process: Synthesized by green plants via photosynthesis.
• The name is derived from glucose, the first carbohydrate obtained in pure form.
  • Molecular formula: $C<em>6H</em>{12}O<em>6$, historically viewed as a hydrate of carbon: $C</em>6(H<em>2O)</em>6$
• Approximately 50% of dry weight of Earth's biomass consists of glucose polymers.

Classification of Carbohydrates

Simple Carbohydrates

• Monosaccharides: simplest form (e.g., glucose, fructose) that cannot be hydrolyzed into simpler sugars.

Complex Carbohydrates

• Disaccharides: consist of two monosaccharides linked by glycosidic bonds (e.g., sucrose = glucose + fructose).
• Polysaccharides: multiple monosaccharides linked together (e.g., cellulose = thousands of glucose units).

Fischer Projections

• A method to represent tetrahedral carbons on a flat surface.
• Key elements:
  • Horizontal lines project out of the plane (toward the viewer).
  • Vertical lines project back behind the plane.

Aldose and Ketose

• Monosaccharides classified as:
  • Aldoses: contain an aldehyde group (e.g., glucose).
  • Ketoses: contain a ketone group (e.g., fructose).
• Suffix "-ose" indicates a carbohydrate, prefixes such as "aldo-" or "keto-" differentiate the types.
• The number of carbons denoted using terms like tri-, tetr-, pent-, hex-.

D and L Sugars

• Most naturally occurring monosaccharides follow D configuration, with hydroxyl (-OH) group at the bottom right in Fischer projections.
• D-glyceraldehyde: reference point for D sugars.
• L sugars: mirror images of D sugars; have an S configuration with -OH pointing left.

Stereoisomers of Monosaccharides

• Aldopentoses (e.g., ribose, arabinose, xylose, lyxose) have $2^3 = 8$ stereoisomeric forms due to 3 chiral centers.
• Aldohexoses (e.g., glucose, mannose, galactose) have $2^4 = 16$ stereoisomeric forms from 4 chiral centers.

Cyclic Forms of Monosaccharides

• Monosaccharides undergo intramolecular nucleophilic addition forming cyclic hemiacetals:
  • D-Glucose primarily exists in a six-membered pyranose form in aqueous solution.
  • Cyclization occurs when OH from C5 attacks C1 carbonyl group.
• Chair Conformation: most stable form of cyclic sugar representation, can also illustrate axial and equatorial positions.

Converting Fischer to Haworth Projections

• When converting from Fischer to Haworth:
  1. The aldehyde forms a hemiacetal.
  2. Groups on the right in Fischer go below the ring; those on the left go above.
• For example, D-glucose:
  • C5 -OH becomes part of the ring; C6 -CH2OH is above.

Anomers

• Anomers arise during cyclization (e.g., D-glucose has two anomers).
  • They differ in configuration at the anomeric carbon (formerly the carbonyl).

Epimers of Glucose

• D-Mannose: C2 epimer of D-glucose.
• D-Galactose: C4 epimer of D-glucose.

Ketose Sugars

• D-Fructose: a ketose that can cyclize to form both five-membered furanose and six-membered forms.
• More common to encounter the furanose form in sucrose as a component.

Summary of Key Concepts

• Carbohydrates are essential biomolecules, classified based on their structure (simple vs complex).
• Fischer projections give insight into stereochemistry with respect to carbonyl groups and chiral centers.
• Cyclical forms change depending on hydroxy group interactions within the structure, influencing their biological activity and properties.