CARBOHYDRATE CHEMISTRY

INTRODUCTION

  • Definition of Carbohydrates:
    • Composed of the elements: carbon (C), hydrogen (H), and oxygen (O).
    • Characterized by the hydrogen to carbon and oxygen ratio of 2:1.
  • Types of Carbohydrates:
    • Include sugars, starches, cellulose, and various other compounds found in living organisms.
    • Basic forms are simple sugars or monosaccharides.

COMPLEX CARBOHYDRATES

  • Combination of Simple Sugars:
    • Simple sugars can combine to form more complex carbohydrates.
    • Disaccharides:
    • Formed from the combination of two simple sugars.
    • Oligosaccharides:
    • Comprising two to ten simple sugars.
    • Polysaccharides:
    • Composed of a larger number of sugars.

BIOCHEMICAL DEFINITION

  • Definition:
    • Carbohydrates can be biochemically defined as polyhydroxyl aldehydes or polyhydroxyl ketones or compounds that yield them upon hydrolysis.

STRUCTURE OF CARBOHYDRATES

  • Types of Structural Representations:
    • Open Chain Structure:
    • The long straight chain form of carbohydrates.
    • Hemi-acetal Structure:
    • Formation occurs when the 1st carbon of glucose condenses with the -OH of the 5th carbon to create a ring structure.
    • Haworth Structure:
    • Features a pyranose or furanose ring structure.

CLASSIFICATION OF CARBOHYDRATES

  • Divided into three major groups based on the number of sugar units:

    1. Monosaccharides:
    • Simple sugars that cannot be hydrolyzed into smaller units.
    • Subdivisions based on number of Carbons:
      • 3 Carbons: Triose (examples: Glyceraldehyde, Dihydroxyacetone)
      • 4 Carbons: Tetrose (examples: Erythrose, Threose)
      • 5 Carbons: Pentose (examples: Arabinose, Ribose, Ribulose, Xylose, Xylulose, Lyxose)
      • 6 Carbons: Hexose (examples include Fructose, Galactose, Glucose, among others)
      • 7 Carbons: Heptose (example: Sedoheptulose, Mannoheptulose)
    1. Classification Based on Functional Group:
    • Aldose: Contains an aldehyde group.
    • Ketose: Contains a ketone group.

EXAMPLES OF PENTOSES

  • Key Pentoses:
    • D-Ribose:
    • A key component of ribonucleic acid (RNA).
    • D-Deoxyribose:
    • Lacks oxygen at position 2 and a component of deoxyribonucleic acid (DNA).
    • In nucleic acids, hydroxyl at carbon 1 is replaced with nucleotide bases.

HEXOSES

  • Aldo Hexoses:
    • Includes D-Allose, D-Altrose, D-Glucose, D-Mannose, D-Galactose, D-Idose, D-Talose.
  • Keto Hexoses:
    • Includes D-Tagatose, D-Fructose.

EPIMERS

  • Definition:
    • Structures differing only in the configuration of a hydroxyl group at one position (e.g., glucose and mannose).

HEPTOSES

  • Examples:
    • D-Sedoheptulose:
    • Similar structure to fructose with one additional carbon. Often found in carrots.
    • D-Mannoheptulose:
    • A monosaccharide often found in avocados.

CHAIN AND RING FORMS

  • Formation of Glucose Ring Structure:
    • Occurs when the oxygen from carbon 5 links with the carbonyl carbon (number 1), resulting in two configurations:
    • Alpha Glucose: Hydroxyl group on the opposite side of the -CH₂OH group.
    • Beta Glucose: Hydroxyl group on the same side as the -CH₂OH group.
  • Anomers:
    • Isomers differing in configuration about their carbonyl carbon atom. Natural glucose is denoted as D for dextrorotary, indicating it rotates polarized light to the right.
  • Furanoses vs. Pyranoses:
    • Five-sided rings (like ribose) are termed furanoses.
    • Six-sided rings (like glucose) are termed pyranoses.

OLIGOSACCHARIDES

  • Definition:
    • Polymers of monosaccharides containing two to ten residues connected by glycosidic bonds.
    • Types:
    1. Disaccharides:
      • Yield two monosaccharides upon hydrolysis.
      • Reducing Disaccharides:
        • Contain free aldehyde or ketone group (e.g., Maltose: Glucose + Glucose, Lactose: Galactose + Glucose).
        • Structural Formula:
        • Maltose: α-D-glucopyranosyl-(1 → 4)-α-D-glucopyranoside.
      • Non-Reducing Disaccharides:
        • Do not have free aldehyde or ketone group (e.g., Sucrose: Glucose + Fructose).
        • Structural Formula:
        • Sucrose: α-D-glucopyranosyl-(1 → 2)-β-D-fructofuranoside.
    2. Trisaccharides:
      • Example: Raffinose (Glucose + Fructose + Galactose), found in cotton seed and sugar beet.
    3. Tetrasaccharides:
      • Yield four monosaccharides upon hydrolysis. Example: Stachyose (Glucose + Fructose + Galactose + Galactose).

POLYSACCHARIDES

  • Definition:

    • Polymeric anhydrides of monosaccharides joined through glycosidic bonds, existing as long-chain or branched structures.

  • Classification Based on Function:

    1. Storage Polysaccharides:
    • Examples include starch and glycogen.
    1. Structural Polysaccharides:
    • Examples include cellulose and pectin.

  • Classification Based on Composition:

    1. Homopolysaccharides:
    • Upon hydrolysis gives single monosaccharide units.
    • Types:
      • Pentosans: Contain pentoses (C$5$H${10}$O$_5$).
      • Hexosans: Contain hexoses (C$6$H${12}$O$_6$).
    • Subdivisions:
      • Glucosans: Polymers of glucose (e.g., Starch, Glycogen).
      • Fructosans: Polymers of fructose (e.g., Inulin).
      • Galactans: Polymers of galactose (e.g., Galactan).
      • Mannans: Polymers of mannose (e.g., Mananas).
    1. Heteropolysaccharides:
    • Examples include hyaluronic acid and chondroitin sulfates.
    • Types:
      • Gums: Composed of arabinose, rhamnose, galactose, and glucuronic acid.
      • Agar: Sulphuric acid esters of galactans that include galactose and galacturonic acid.
      • Pectins: Fundamental unit is pectic acid, comprising arabinose, galactose, and galacturonic acid.

FUNCTIONS OF POLYSACCHARIDES

  1. Serve as structural components of cells.
  2. Function as a stored form of energy.
  3. Act as nutrients.

STRUCTURE AND PROPERTIES OF STARCH

  • Components:
    • Composed of amylose and amylopectin.
    • Amylose: Long-chain polysaccharides containing α-D-glucose molecules linked by 1-4 glycosidic linkages; reacts with iodine to produce a blue color.
    • Amylopectin: Branched-chain polysaccharide consisting of α-D-glucose linked by 1-4 glycosidic linkages and branched by 1-6 linkages; reacts with iodine to give a purplish color and forms a gel when mixed with hot water.
  • Cellulose:
    • A structural polysaccharide found in plant cell walls, composed of long chains of β-D-glucose linked by β-1-4 glycosidic bonds; has no branching and yields crystalline D-glucose upon hydrolysis.

FUNCTIONS OF CARBOHYDRATES

  1. Chief energy source, providing instant energy; glucose undergoes glycolysis/Krebs cycle resulting in ATP production.
  2. Source of energy storage; stored as glycogen in animals and starch in plants.
  3. Serve as intermediates in the biosynthesis of fats and proteins.
  4. Aid in the regulation of nerve tissue; primary energy source for the brain.
  5. Associate with lipids and proteins to create surface antigens, receptor molecules, vitamins, and antibiotics.
  6. Form structural and protective components (like in plant and microorganism cell walls).
  7. Important constituents of animal connective tissues.
  8. Participate in biological transport, cell-to-cell communication, and activation of growth factors.
  9. Rich-fiber carbohydrates help prevent constipation.
  10. Modulate the immune system.

PHYSICAL PROPERTIES OF CARBOHYDRATES

  • Stereoisomers:
    • Compounds with identical structural formulas but differing spatial configurations.
  • Asymmetric Carbon:
    • A carbon atom is asymmetric if it is attached to four different atoms/groups; the number of asymmetric carbon atoms (n) dictates the number of possible isomers, given by $2^n$.
  • Example:
    • Glucose having 4 asymmetric carbons produces 16 potential isomers.
  • Enantiomers:
    • Special types of stereoisomers that are mirror images of each other; examples include D-glucose (right-handed) and L-glucose (left-handed).
  • Optical Activity:
    • Characteristic of compounds with asymmetric carbon; certain compounds rotate plane-polarized light (e.g., (+) glucose rotates to the right while (-) glucose rotates to the left).
    • Dextro-rotatory (+) and levo-rotatory (-) are used to describe rotations; designations could also be D(+) or D(-), reflecting the direction of light rotation.
  • Racemic Mixture:
    • Contains equal concentrations of D and L isomers, resulting in no net optical activity.
  • Diastereomers:
    • Stereoisomers that are not mirror images; for example, mannose and galactose differ in configuration at C2, C3, or C4.
  • Anomerism:
    • The spatial configuration related to the first carbon atom in aldoses and the second carbon atom in ketoses.
  • Mutarotation:
    • The process of changing specific optical rotation; refers to equilibria of alpha and beta forms of D-glucose.

CHEMICAL PROPERTIES OF CARBOHYDRATES

  • Osazone Formation:
    • Reaction of carbohydrates with phenylhydrazine to form osazones.
  • Benedict's Test:
    • Qualitative test for reducing sugars.
  • Oxidation:
    • Carbohydrates can undergo oxidation reactions.
  • Reduction to Alcohols:
    • Carbohydrates can be reduced to form alcohols.