Carbohydrates_1_Monos_2025

Carbohydrates Overview

Monosaccharides

• Basic unit of carbohydrates that cannot be broken down into simpler sugars.

• The term for a monosaccharide is glycose.

Structure of Monosaccharides

• Basic structure: Monosaccharides consist of a functional group on each carbon atom.

• Chirality: High proportion of chiral centers (asymmetric carbons).

• Common forms include:

  • Aldoses: contain an aldehyde group.

  • Ketoses: contain a ketone group.

Interconversion & Stability

• Monosaccharides can easily change between linear and cyclic forms.

• Mutorotation: The phenomenon of changing specific rotations due to the interconversion of forms.

• Tautomeric forms: e.g., fructose has two tautomeric forms in solution.

Photosynthesis & Carbohydrates

• General reaction: CO2 + H2O + light energy → Sugar + O2

• Carbohydrates are termed “hydrated carbon” due to the general formula Cx(H2O)y (e.g., glucose: C6H12O6).

• Classified into:

  • Monosaccharides

  • Oligosaccharides (2-10 monosaccharide units)

  • Polysaccharides (over 10 monosaccharide units)

Dietary Significance

• Carbohydrates represent:

  • ~90% of dry matter in plants.

  • ~80% of caloric intake for humans.

Sugar Composition in Foods

• Apple: 14.5g sugar

  • Monosaccharides: glucose (1.2g), fructose (6.0g)

  • Disaccharides: sucrose (3.8g)

  • Polysaccharides: starch (1.5g), cellulose (1.0g)

• Grape: 17.3g sugar

  • Monosaccharides: glucose (5.4g), fructose (5.3g)

  • Disaccharides: sucrose (1.3g)

• Honey: 82.3g sugar

  • Monosaccharides: glucose (28-35g), fructose (34-41g)

Functional Groups in Carbohydrates

• Functional groups on each carbon, contributing to the reactive properties.

• Saccharose group showcases:

  • For aldoses: R = H

  • For ketoses: R = CH2OH

D-Glucose Characteristics

• D-Glucose is defined as:

  • A polyhydroxy aldehyde and hexose.

  • Contains aldehyde at C1 and primary –OH at C6.

  • Secondary –OH groups at C2, C3, C4, and C5.

Sugar Classification by Configuration

• D-sugars: –OH group on highest numbered chiral carbon is on the right side (D-configuration).

• Each chiral center exhibits 2^n arrangements, where n is the number of chiral centers.

D-Fructose Characteristics

• D-Fructose: A ketose with three chiral centers (labeled as "fruit sugar").

• Forms a disaccharide with D-Glucose known as sucrose.

• Important in high-fructose corn syrup and honey.

Fischer and Rosanoff Projections

• Fischer projection represents the 2D arrangement of the molecule's 3D structure on paper.

• Rosanoff projection visualizes the structure of aldoses and ketoses appropriately.

Hemiacetals and Acetals

• Formation processes:

  1. Aldehydes react with hydroxyl groups to form hemiacetals.

  2. Hemiacetals can further react to form acetals.

• Ring closures lead to pyranose (6-membered) and furanose (5-membered) structures.

Anomers

• Anomers: Diastereomers that differ at the anomeric carbon.

• Distinction between D and L forms based on the orientation of –OH groups in cyclic forms.

  • Alpha (α) anomer has –OH below the ring, Beta (β) has –OH above the ring.

Dynamic Equilibrium of Hexoses

• Hexoses in solution exhibit a dynamic equilibrium between open-chain, 5-membered, and 6-membered ring forms.

  • Pyranose forms are thermodynamically favored.

  • Open-chain forms are a minor fraction but allow reactions as if they exist entirely as aldehydes or ketones.

Other Monosaccharides

• Deoxysugars: Sugars missing –OH at any carbon but the carbonyl (e.g., deoxyribose in DNA).

• L-Rhamnose: Associated with pectin.

• Glucosamine: An aminodeoxy sugar relevant in animal and bacterial polysaccharides.