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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.