Monosaccharides et Représentation

Monosaccharides

• Carbohydrates are one-third of the important classes, along with proteins and lipids.
• Carbohydrates (sugars) are also known as saccharides or hydrates of carbon (CO and H).
• Simple sugars are classified as either aldoses (e.g., glyceraldehyde) or ketoses (e.g., dihydroxyacetone).
• They are classified by the number of carbon atoms (triose, tetrose, pentose, hexose).
• Chiral molecules, such as sugars, can rotate polarized light. D and L isomers are mirror images (enantiomers).
• Epimers have the same chemical properties but differ only in the configuration of a single chiral carbon atom.

Representation, Nomenclature, and Reactions

• Aldehydes and ketones can react with alcohols to form hemiacetals or hemiketals
• The resulting structure is cyclic, and the different forms are called anomers based on the position of the hydroxyl group on the anomeric carbon (e.g., alpha or beta).

Cyclization of Monosaccharides

• Intramolecular reaction between aldehyde & alcohol group.
• Cyclic form is favored in solution
• Two molecules with different properties=anomers
• Cyclization occurs around C1 to C5 creating a stable ring.
• Nomenclature: furanose (5-membered rings) or pyranose (6-membered rings) according to ring size.

Oxidation of Glucose

• Oxidized to form different acids, e.g., glucaric acid (C1 and C6 oxidation), gluconic acid (C1 oxidation), and glucuronic acid (C6 oxidation).

Osamines

• One example is an amino sugar resulting from the substitution of an alcohol group with an amine group.
  • N-acetylglucosamine and N-acetylgalactosamine are examples
• These can be further conjugated with pyruvic acid.

Deoxyhexoses

• Important examples include L-rhamnose and L-fucose (resulting from the removal of an oxygen atom).

Reduction of Glucose

• Reduction occurs at the aldehyde group (-CHO)
• The product is Sorbitol or Mannitol.

Reactions of Osos with Acids

• All hydroxyl groups except the anomeric carbon are alcohols.
• Reaction between the hydroxyl groups and phosphoric acid results in phosphorylated sugars

Nucleotides

• Composed of ribose in its beta-D-ribofuranose form.
• C5 hydroxyl group is phosphorylated, and C1 is linked to a nitrogenous base (nucleoside)
• Phosphorylation of the nucleoside produces a nucleotide.
• Nucleotides are building blocks of DNA and RNA, and play roles in metabolic pathways as energy carriers (e.g., NAD+).

Fischer Projection

• It's a way to represent stereoisomers.
• Shown in the image are the different sugars and their possible isomer variations, both aldoses and ketoses.