chem week 11 leture 1

Introduction to Carbohydrates

• Carbohydrates are the first biomolecules in this series.

• They have various roles, including being good energy and food sources.

• D-glucose is the most common monosaccharide and represents carbohydrates.

Definition of Carbohydrates

• Traditionally, carbohydrates are hydrates of carbon (Cx(H2O)_y).

• D-glucose has a carbon backbone with H and OH groups around the carbons.

• More correctly, carbohydrates are polyhydroxyaldehydes or polyhydroxyketones, or compounds hydrolyzable to these.

Types of Carbohydrates (Saccharides or Sugars)

• Monosaccharides: one sugar unit (mono- = one).

• Disaccharides: two sugar units (di- = two).

• Oligosaccharides: several sugar units.

• Polysaccharides: many sugar units; often referred to as complex carbohydrates.

Major Sources of Carbohydrates

• Plants are major sources.

• Cellulose: provides structure to plants and is an important source of insoluble fiber for humans.

• Starches: major energy source in foods.

• Glucose and sucrose: examples of simple saccharides.

• Photosynthesis synthesizes most carbohydrates: H2O + CO2 \xrightarrow{h\nu} Carbohydrates + O_2 where h\nu represents light.

• Carbohydrates provide energy and carbon for non-photosynthetic cells in plants, animals, and microorganisms.

Importance in Living Systems

• Ribose and deoxyribose: found in nucleic acids (DNA and RNA), determining genetic makeup and protein production.

• Complex carbohydrates: structural elements of bacteria, plants, and exoskeletons of arthropods/crustaceans.

• Coat animal cell surfaces, providing biological specificity.

• Important in lubricating skeletal joints and cell adhesion.

Monosaccharides

• Polyhydroxyaldehydes or polyhydroxyketones.

• Colorless, crystalline solids, highly water-soluble due to numerous alcohol groups that can hydrogen bond with water molecules.

• Most are sweet.

• Generic names: aldoses (containing an aldehyde group) and ketoses (containing a ketone group).

• Examples:

  • Aldopentose: five-carbon chain with an aldehyde group.

  • Ketoheptose: seven-carbon chain with a ketone group.

• Suffix -ose indicates a sugar; prefixes indicate the number of carbons (tri- for three, tetr- for four, pent- for five, etc.).

• D-glucose is an aldohexose.

Fischer Projections

• Represent the three-dimensional structure around each carbon.

• Horizontal lines: groups coming towards you.

• Vertical lines: groups going back.

Examples with Glyceraldehyde

• One of the simplest carbohydrates.

• Shows the aldehyde group.

Glucose Structure

• Fischer projection shows groups coming towards you.

• Model demonstrates the three-dimensional shape and ring closure.

• Glucose has four chiral carbons.

D and L Sugars

• Naturally occurring monosaccharides are mostly D sugars.

• In a Fischer projection with the aldehyde/ketone at the top, if the OH on the last chiral carbon is to the right, it is a D sugar.

• D sugars have the R configuration at the last chiral carbon.

• If the OH is to the left on the last chiral carbon, it is an L sugar, having the S configuration.

• L sugars are enantiomers of D sugars; they are non-superimposable mirror images.

• D/L nomenclature only refers to the last chiral carbon and doesn't describe the other stereogenic centers.

Common Naturally Occurring Sugars

• Aldopentoses: D-ribose, D-arabinose, D-xylose, D-lyxose.

• Aldohexoses: D-allose, D-altrose, D-glucose, D-mannose, D-gulose, D-idose, D-galactose, D-talose.

• These sugars are diastereomers.

• Diastereomers have different properties and shapes, leading to different sweetening properties.

• Memorization of these sugars is not required; structures will be provided in exams.

Reactions of Monosaccharides

• Aldehydes/ketones react with alcohols to form hemiacetals or hemiketals.

• Nucleophilic addition of an alcohol to the carbonyl group, breaking the pi bond.

Intramolecular Nucleophilic Addition

• Sugars with both hydroxyl and carbonyl groups undergo intramolecular reactions, forming hemiacetals or hemiketals.

• In D-glucose, the alcohol at carbon five attacks the carbonyl group.

• All substituents on the left side of the Fischer projection are on top in the closed ring.

Ring Closure

• The alcohol can attack the carbonyl carbon from either side.

• The original carbonyl carbon is the anomeric carbon.

• Two arrangements: OH up or down.

• Beta anomer: OH up and CH_2OH cis.

• Alpha anomer: OH down and CH_2OH trans.

• Ketoses can also ring close (e.g., D-fructose).

• D-fructose can form five-membered (furanose) or six-membered (pyranose) rings.

• All substituents on the left in the Fischer projection (except for the oxygen involved in ring closure) are drawn on the top (up) in the ring.

Examples of Ring Structures

• Aldopentoses (D-ribose and D-deoxyribose) form furanose rings.

Terminology

• Six-membered ring: pyranose.

• Five-membered ring: furanose.

• Anomeric carbon: the carbon that was the carbonyl group.

• Alpha: anomeric OH group is down.

• Beta: anomeric OH group is up.

• Alpha and beta isomers are diastereomers (enantiomers).

Haworth Projections and Chair Conformations

• Represent the ring looking from the side.

• In beta-D-glucopyranose, all non-hydrogen groups are equatorial in the chair conformation.

• Having all large groups equatorial makes beta-D-glucopyranose the most stable monosaccharide conformer.

Formation of Disaccharides, Oligosaccharides, and Polysaccharides

• Monosaccharides react with alcohol groups to yield acetals or ketals.

• The linkage is a glycosidic linkage or glycosidic bond.

• This linkage is from the anomeric carbon in one sugar to a hydroxyl group on another sugar.

Disaccharides Examples

• Maltose: two glucose units joined by an alpha-1,4-glycosidic linkage.

• Cellobiose: two glucose units joined by a beta-1,4-glycosidic linkage.

• Lactose: galactose and glucose joined by a beta-1,4-glycosidic linkage (found in milk).

• Sucrose: glucose and fructose are linked at both anomeric carbons; it's abundant but not as sweet as fructose.

Polysaccharides

• Hundreds or thousands of simple sugars join to form polymers with glycosidic linkages.

• Common linkages are 1,4 or 1,6.

• Amylopectin: component of starch with alpha-1,4-glycosidic linkages and occasional alpha-1,6-glycosidic linkages; important energy source.

• Cellulose: glucose units joined by beta-1,4-glycosidic linkages; indigestible fiber.

• Amylose: simpler than amylopectin, without alpha-1,6-glycosidic linkages.

• Glycogen: glucose units for energy storage in animals.

• Chitin: structural polysaccharide in crustacean shells.

Carbohydrates in Biological Processes

• Carbohydrates on red blood cell surfaces define blood group type.

• Blood group A: L-fucose, D-galactose, and N-acetyl-D-glucosamine.

• Different combinations give blood groups B and O.

• Carbohydrates are important for cell-cell communication and virus transfer in the respiratory system.