Carbohydrates Biochemistry Notes PDF

## **Carbohydrates** ### 2. **Diastereomers:** Pairs of isomers that have opposite configurations at one or more chiral centers but are NOT mirror images are diastereomers. Any 2 sugars in a row in figure above are diastereomers, such as glucose and galactose. ### 3. **Epimers:** It is a diastereo...

## **Carbohydrates** ### 2. **Diastereomers:** Pairs of isomers that have opposite configurations at one or more chiral centers but are NOT mirror images are diastereomers. Any 2 sugars in a row in figure above are diastereomers, such as glucose and galactose. ### 3. **Epimers:** It is a diastereomers, when two monosaccharides differ in configuration around only one specific carbon atom (chiral center, see below). | | Epimers | |---|---| | **Glucose** | CHO<br>HC-OH<br>HO-C-H<br>H-C-OH<br>H-C-OH"CH₂OH | C-2 epimers | | **Mannose** | CHO<br>HO-CH<br>HOCH<br>HCOH<br>HCOH<br>CH₂OH | | | **Galactose** | CHO<br>HCOH<br>HOCH<br>HO-CH<br>HCOH<br>CH₂OH | C-4 epimers | ### 4. **Anomers and Cyclic Structures (Howerth projection):** In fact, in aqueous solution, aldotetroses and all monosaccharides with five or more carbon atoms in the backbone occur predominantly as cyclic (ring) structures. How? #### a. **hemiacetals or hemiketals:** The formation of these ring structures is the result of a general reaction between alcohols and aldehydes or ketones to form derivatives called hemiacetals or hemiketals, which contain an additional asymmetric carbon atom, as shown in equations below. **Aldehyde** 1<br>R-C=O + HO-R² → 1<br>R-C-OR² H Alcohol Hemiacetal **Hemiketal** 1<br>R-C=O + HO-R<br>R² Ketone Alcohol → 1<br>R-C-OR³ R² Hemiketal **Acetal** 1<br>R-C-OR² + HO-R³ → 1<br>R-C-OR²OR³ + H₂O H H Acetal **Ketal** 1<br>R-C-OR³ + HO-R⁴ → 1<br>R-C-OR³OR⁴ + H₂O R² R² Ketal #### b. **Haworth projection formulas:** Fischer projection formulas give an unrealistic three-dimensional picture, because: - Each carbon atom must be viewed with both vertical bonds projecting behind the atom viewed making the chain folds back on itself and leads to collision. - They are making use of elongated bent lines to represent ordinary simple bonds: The British carbohydrate chemist Sir Norman Haworth used another way to show the stereochemistry of ring forms of monosaccharides. For these are six-membered ring compounds are called **pyranoses** because they resemble the six-membered ring compound **pyran**. For these are five -membered ring compounds are called **furanoses** because they resemble the five-membered ring compound **furan**. These hexagonal and pentagonal rings lying perpendicular to the plane of the paper, with thickened lines indicating the side of the ring closest to the reader. Haworth projection for D-glucose exists in solution as an intramolecular hemiacetal in which the free hydroxyl group at C-5 has reacted with the aldehydic C-1, making the latter carbon asymmetric and producing two stereoisomers, designated α or β, as shown in the figure below: ### **Note:** 1. When the OH group at the anomeric carbon is on the same side of a Fischer projection as the oxygen or the OH of the distant asymmetric carbon, the configuration at the anomeric carbon is α, as in α-D-glucose. When the anomeric hydroxyl is on the opposite side of the Fischer projection, the configuration is β, as in β-D-glucopyranose. 2. OH groups that lie on the right in the Fischer projection appear under the ring level in the Haworth projection, while those on the left appear above it. **Problem:** Write Haworth projection formulas for ketohexoses fructose? **Problem:** How many Haworth projection stereoisomers for D-glucose and D-fructose? ### **c. Anomers:** Isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal carbon atom are called **anomers**, designated as α or β. The hemiacetal (or carbonyl) carbon atom is called the **anomeric carbon**. The systematic names for the two ring forms of D-glucose are α-D-glucopyranose and β-D-glucopyranose. ### **d. Mutarotation:** The process in which the cyclic α and β anomers of a sugar in solution are in equilibrium with each other, and can be spontaneously interconverted. This mixture consists of about one-third (33%) α-D-glucose, two-thirds (66%) β-D-glucose, and very small amounts (much Less 1%) of the linear and five-membered ring (glucofuranose) forms, a process called mutarotation. ### **b. Conformational isomers:** Two conformation of a molecule are interconvertible without the breakage of covalent bonds, but related by rotation around single bond The six-membered pyranose ring is not planar, as Haworth perspectives suggest, but tends to assume either of two conformations "chair and boat". - **Axial bond** (projecting parallel to the vertical axis through the ring) - **Equatorial bond** (projecting roughly perpendicular to this axis) ### 1. **Oxidation reaction:** - The **reducing ability** of sugars with free anomeric carbons (the hydroxyl of anomeric carbon is free, not involved in bond), will reduce oxidizing agents, such as peroxide, ferricyanide and some metals (Cu and Ag). - These reducing sugars will reduce the cupperic solution (Cu⁺²) to cupperous solution (Cu⁺¹). - These **redox reactions** convert the sugar to a sugar acid. - **Glucose** is a reducing sugar - so these reactions are the basis for diagnostic tests for blood sugar - All **monosaccharides** are reducing sugars. **Aldehyde** RC-H + 2 Cu⁺² + 5 OH^- → **Carboxylate** RC-O- + Cu₂O↓ + 3 H₂O cupperic cupperous solution solution ### **Lactone:** - The carboxyl group of aldonic acid can react with an –OH group in the same molecule to form acyclic ester known as Lactone. (Vitamin C is a lactone derivative of D- glucouronic acid). - The carboxyl group of aldonic acid can react with an - OH group (hydroxyl) in the same molecule to form acyclic ester known as Lactone. (Vitamin C is a lactone derivative of D- glucouronic acid). ### 2. **Sugar Alcohols Reduction reaction:** - Reduction of the aldehyde and ketone groups of monosaccharide yields the sugar alcohols (alditols). **glucose** H C H-C-OH HO-C-H H-C-OH H-C-OH | CH₂OH D-Glucose → **glucitol** CH₂OH H-C-OH | HO-C-H H-C-OH H-C-OH CH₂OH D-Glucitol (sorbitol) - Sugar alcohols are used commercially in the processing of foods and pharmaceuticals. ### 3. **Amino Sugars:** - Amino sugars, including D-glucosamine and D-galactosamine (chondrosamine), contain an amino group (instead of a hydroxyl group) at the C-2 position. - They are found in many oligo- and polysaccharides, including chitin, a polysaccharide in the exoskeletons of crustaceans and insects. - Several **antibiotics** (eg, erythromycin) contain amino sugars believed to be important for their antibiotic activity. ### 4. **Esterification:** - Like all free -OH groups, those of carbohydrates can be converted to esters by reactions with acids. These often change sugar’s chemical and physical properties. - Phosphate and sulfate esters of carbohydrate molecules are among the most common ones found in nature. ### 5. **Fermentation reaction:** Occur by especial enzymes C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂ ### 6. **Glycosidic bond formation:** - The hydroxyl group of anomeric carbone (Hemiacetals or hemiketals) it is very active and can be reacting either with: 1. Another compound containing hydroxyl group such as methanol or another monosaccharide to form O-glycoside (acetals or ketals) with retention of the α- or β-configuration at the C-1 carbon. The new bond creates between the anomeric carbon atom and the oxygen atom of the alcohol is called a glycosidic bond. 2. Another compound containing amino group such as asparagines to form N-glycosidic bond. - Some glycosides include antibiotics such as streptomycin.