The Carbohydrate Structures and Nucleic Acids

I. Structure of Saccharides

  1. Monosaccharides

  2. Saccharides Reactions

  3. Oligosaccharides

  4. Polysaccharides

  5. Complex Saccharides

II. Structure of Nucleic Acids

  1. Basic Components of Nucleic Acids

  2. Nucleosides and Nucleotides

  3. DNA Chain Structures

  4. RNA Structures

I. Structure of Saccharides

Saccharides, commonly referred to as sugars, are specifically polyhydroxaldehydes and polyhydroxyketones that must contain a minimum of three carbon atoms in their chain. The classification of saccharides is based upon the number of saccharide units they contain:

  • Monosaccharides: Single sugar units

  • Oligosaccharides: Composed of 2 to 10 monosaccharide units

  • Polysaccharides: Long chains of monosaccharide units

  • Complex Saccharides: Various structures made from simpler saccharides

Chirality is a significant aspect in the structure of saccharides because it affects the properties and reactions of these molecules.

Monosaccharide Structure

Tautomers

  • Examples of tautomers include D-Glyceraldehyde (an aldose) and Dihydroxyacetone (a ketose).

Enantiomers

  • Enantiomers are stereoisomers that are mirror images of one another. For example:

    • D-Glyceraldehyde and L-Glyceraldehyde demonstrate this relationship.

Carbon Number and Configuration

  • When determining the configuration of carbon chains based on their stereochemical properties, if the priority of the substituent groups decreases in a clockwise manner when viewing the carbon atom, the configuration assigned is termed R. Conversely, if the priority decreases in a counterclockwise manner, the configuration is denoted as S.

Configurational Isomers

  • Enantiomers: Stereoisomers that are mirror images of each other.

  • Diastereomers: Stereoisomers that are not mirror images.

  • Anomers: Stereoisomers that differ at the anomeric carbon.

  • Epimers: Stereoisomers that differ at a single carbon (not the anomeric carbon).

Isomers

  • If two molecules have the same molecular formula, they can either be isomers or not based on the order of bonded atoms:

    • Constitutional Isomers: Different connectivity

    • Stereoisomers: Same connectivity but differ in spatial arrangement

      • Enantiomers: Non-superimposable mirror images

      • Diastereomers: Not mirror images of one another

Monosaccharide Classification

Tetr ose Diastereoisomers

  • Aldoses: Tetroses include molecules like D-Threose and D-Erythrose.

  • Ketoses: Include isomers like D-Erythrulose.

Examples

  • D-Galactose and D-Glucose are two types of diastereoisomers.

Monosaccharide Structure: Trioses, Tetroses, Pentoses, and Hexoses

  1. TRIOSE: D-Glyceraldehyde

  2. TETROSE: D-Threose, D-Erythrose

  3. PENTOSES: Examples include D-Ribose, D-Arabinose, D-Xylose, and D-Lyxose

  4. HEXOSES: Examples include D-Allose, D-Glucose, D-Galactose, and others

Hemiacetals Formation

  • Hemiacetals form when an aldehyde reacts with an alcohol. The projection formula illustrates that all hydroxyl (OH) groups positioned to the right in the Fischer projection appear below the "ring" structure, while those to the left are above the ring.

  • The ring closure can yield two stereoisomers:

    • α-D-Glucose

    • ß-D-Glucose

Anomers

  • Anomers are a subset of stereoisomers specific to cyclic forms of monosaccharides. The alpha and beta forms differ based on the orientation of the hydroxyl group on the anomeric carbon.

Cyclic Forms of Monosaccharides

  • In a crystalline state, only cyclic forms of monosaccharides are stable and exist in the form of α and β anomers like α-pyranose, β-pyranose, α-furanose, and β-furanose.

II. Structure of Nucleic Acids

Basic Components of Nucleic Acids

  • The fundamental components of nucleic acids include:

    1. Nucleotides

    2. Nucleosides

Nucleotide Structure

  • A nucleotide comprises a nitrogenous base, a sugar (either ribose in RNA or deoxyribose in DNA), and one to three phosphate groups.

Nucleotides Functionality

  • Nucleotides serve as the building blocks for nucleic acids and play roles in cellular metabolism, cellular signaling, and enzyme co-factors.

DNA Chain Structures

  • DNA is structured as a double helix, with antiparallel strands linked by hydrogen bonds between complementary base pairs (A-T and C-G).

RNA Structures

  • RNA typically exists in a single-stranded form, which can fold into complex three-dimensional shapes due to intramolecular base pairing.