Carbohydrate Structural Isomerism

Monosaccharides

• Monosaccharides are the basic unit of carbohydrates, water-soluble white crystalline solids with a sweet taste.
• They cannot be hydrolyzed into a simpler form of carbohydrates as they are already in simplest form.
• Examples include glucose, fructose, galactose, ribose (in RNA), and Deoxyribose (in DNA).

Classification of Monosaccharides

• Based on the number of carbon atoms present in their structures:
  • Trioses: 3-carbon monosaccharides
  • Tetroses: 4-carbon monosaccharides
  • Pentoses: 5-carbon monosaccharides (e.g., ribose and ribulose, xylose and xylulose)
  • Hexoses: 6-carbon monosaccharides
  • Heptoses: 7-carbon monosaccharides

Structural Isomerism

• Aldose and ketose sugars are structural isomers, having the same number and kinds of atoms, but different structural or spatial configurations.
• Examples include erythrose (aldose) and erythulose (ketose), which are 4-carbon monosaccharides.

Ring Structure

• Glucose and fructose do not exist in open-chain structures, but cyclize into rings, forming hemiacetals and hemiketals.
• Hexoses form when the second to last –OH group reacts with a C=O.
• Aldohexoses form 6-membered rings, and ketohexoses and aldopentoses form 5-membered rings.

Haworth Structures

• The 6-membered ring is not planar but rather exists in the chair formation.
• Haworth structures show the ring structure of sugars.

Stereoisomers

• Stereoisomers are molecules with the same structural formula but with different spatial configuration.
• Types of stereoisomers:
  • Enantiomers: have four different atoms or groups of atoms attached.
  • Epimers: isomers that differ due to the H and OH configuration of carbons 2 or 3 or 4.
  • Diastereoisomers: have a different spatial configuration, but are not enantiomers or epimers.
  • Anomers: have an additional asymmetric carbon added after cyclization.

Optical Isomerism

• The presence of asymmetric carbons or chirality influences the optical activity of compounds.
• Examples include aldose and ketose sugars, which have different optical activity due to the presence of asymmetric carbons.

Carbohydrates

• Carbohydrates are essential components of all living organisms, including humans, plants, animals, bacteria, and viruses.
• They contain an aldehyde (-CHO) or ketone (-C=O) group with two or more hydroxyl (-OH) groups in their structures.

Monosaccharides

• Monosaccharides are the basic units of carbohydrates.
• They are water-soluble white crystalline solids with a sweet taste.
• Examples include glucose, fructose, galactose, ribose (in RNA), and deoxyribose (in DNA).
• They cannot be hydrolyzed into a simpler form of carbohydrates as they are already in their simplest form.

Classification of Monosaccharides

• Classified based on the number of carbon atoms in their structures:
  • Trioses: 3-carbon monosaccharides
  • Tetroses: 4-carbon monosaccharides
  • Pentoses: 5-carbon monosaccharides
  • Hexoses: 6-carbon monosaccharides
  • Heptoses: 7-carbon monosaccharides

Isomerism in Monosaccharides

• Ketoses are isomers of aldoses, with the same number and kinds of atoms but different structural or spatial configurations.
• Isomers of carbohydrates can be classified into two classes:
  • Structural isomers
  • Optical isomers (stereo-isomers)

Structural Isomerism

• Commonly seen differences are on carbons 1 and 2, with no variation in spatial arrangement.
• Examples include erythrose (aldose) and erythulose (ketose), which are 4-carbon monosaccharides.

Ring Structure of Carbohydrates

• In solution, glucose and fructose do not exist in open-chain structures, but rather cyclize into rings, forming hemiacetals and hemiketals.
• Hexoses form when the second to last –OH group reacts with a C=O.
• Aldohexoses form 6-membered rings, and ketohexoses and aldopentoses form 5-membered rings.

Haworth Structures

• Haworth showed that glucose and fructose cyclize into rings, forming hemiacetals and hemiketals.
• The 6-membered ring is not planar, but rather exists in the chair formation.

Stereoisomerism

• Same structural formula but with different spatial configuration.
• Enantiomers – four different atoms or groups of atoms are attached.

Assessment Details

• Practical Assess: Practicals 1 to 8 (10%)
• Make-up Assessment Dates: To be announced
• Practical report Submissions (15%)
• Tutorial Quiz Submissions (5%)
• Main Exam (3 hours): Date to be advised
• Supplementary Exam (3 hours): Date to be advised

Introduction to Carbohydrates

• Carbohydrates are essential components of all living organisms
• Carbohydrates contain an aldehyde (-CHO) or ketone (-C=O) group with two or more hydroxyl (-OH) groups in their structures
• Examples include: Glyceraldehyde, Dihydroxyacetone, Glucose, Fructose

Classification of Carbohydrates

• General classification: monosaccharides, disaccharides, oligosaccharides, polysaccharides, based on the number of monomeric units present

Monosaccharides

• Monosaccharides are the basic unit of carbohydrates
• They are water-soluble white crystalline solids with a sweet taste
• Examples include: glucose, fructose, galactose, ribose (in RNA), Deoxyribose (in DNA)
• They cannot be hydrolyzed into a simpler form of carbohydrates as they are already in simplest form

Classification of Monosaccharides

• Trioses: 3-carbon monosaccharides
• Tetroses: 4-carbon monosaccharides
• Pentoses: 5-carbon monosaccharides
• Hexoses: 6-carbon monosaccharides
• Heptoses: 7-carbon monosaccharides

Isomerism in Monosaccharides

• Ketoses are isomers of aldoses, i.e. same number and kinds of atoms, but different structural or spatial configurations
• Isomers of carbohydrates are classified into two different classes:
  • Structural isomers
  • Optical isomers or stereo-isomers

Enantiomers: D and L Configurations

• D (dextro) and L (levo) of glyceraldehyde contain a single asymmetric carbon – and are mirror images
• Enantiomers have the same structural formula but with different spatial configuration

Epimers

• Epimers – isomers that differ due to the H and OH configuration of carbons 2 or 3 or 4
• Examples include: D-glucose and D-mannose are epimers at C-2, and D-glucose and D-galactose are epimers at C-4

Diastereoisomers

• D-Glucose and D-mannose are epimers at C-2, and D-glucose and D-galactose are epimers at C-4
• Note: there is no epimeric relationship between D-galactose and D-mannose, their differences are at more than 1 carbon (i.e. 2 and 4); hence they are diastereoisomers – (neither epimers, nor enantiomers)

Anomers

• Anomers - Following cyclisation, there is an additional asymmetric carbon added
• The C-1 in a ring structure can become the asymmetric centre of the ring, resulting in the alpha- and beta-configurations of the sugar