CHEM1010 W8 L1
Monosaccharides
Monosaccharides are commonly known as sugars.
Carbohydrates are also referred to as saccharides.
Carbohydrates are an important class of naturally occurring substances found in both plant and animal matter.
They are a crucial part of our daily nutrition.
Carbohydrates, or sugars (monosaccharides), are composed of hydrates of carbon, with the empirical formula Cx(H2O)y (x number of carbon, y number of water).
Polyhydroxy aldehydes are known as aldoses. These contain many hydroxyl groups and an aldehyde group.
Polyhydroxy ketones are known as ketoses. These contain many hydroxyl groups and a ketone group.
Examples:
Glucose (D-glucose): An aldose with an aldehyde group.
Fructose (D-fructose): A ketose with a ketone group.
Galactose (D-galactose): An aldose with an aldehyde group.
The designation of D refers to the configuration of the chiral center furthest from the carbonyl group, which will be discussed later.
Fischer Projection
Fischer projection is a two-dimensional representation of three-dimensional organic molecules, especially useful for carbon hydrates.
Devised by Emil Fischer in 1891.
Example: Glyceraldehyde (aldotriose).
It has three carbons (tri-), and an aldehyde group (aldose).
Possesses a chiral carbon, making the structure chiral.
Dihydroxyacetone (keto trials) is symmetric, therefore it's aCarol.
Fischer structure is mainly used for chiral structures.
Drawing a Fischer Structure
Carbonyl group on the top.
Hydroxy and hydrogen groups are drawn as a cross with the carbon in the middle.
Hydroxyl or proton on the chiral center point outwards.
Configurations
If the hydroxyl group (OH) is on the left, it is the L configuration (e.g., L-glyceraldehyde).
If the hydroxyl group is on the right, it is the D configuration (e.g., D-glyceraldehyde).
L-glyceraldehyde turns light anticlockwise (optical activity of degrees).
D-glyceraldehyde turns light clockwise (optical activity of degrees).
Reference Point
The position of the carbon stereo center furthest away from the carbonyl group (aldehyde or ketone) determines the D or L configuration.
For a molecule with multiple chiral centers, identify the chiral carbon furthest from the aldehyde group.
If the hydroxy group is on the left, it’s an L structure; if on the right, it’s a D structure.
Example: L-glucose and D-glucose.
Drawing a Fischer structure:
Orient the structure so the ketone or aldehyde group is at the top.
Rotate the carbon 3 and 4 bond to ensure all hydroxy and proton groups point outwards.
Then determine if it is a D or L configuration.
The chiral center furthest from the carbonyl group is carbon 4.
If the hydroxy group is on the right-hand side, the configuration is D.
Cyclic Structure
Sugars can form cyclic or ring structures, creating a new sterile center termed the anomeric center.
Example: Formation of a cyclic hemiacetal from 5-hydroxypentanal.
The carbon that bonds with the hydroxy group on the end becomes a chiral center, called the anomeric carbon.
Cyclic Glucose
The hydroxyl group can react with the aldehyde group in two ways.
If it reacts from the top then the formula hydroxyl group ponts the bottom, it will create alpha glucose, with the hydroxyl group pointing downwards.
If it reacts from the bottom, the forming hydroxyl group points upward, therefore, forming beta glucose.
The carbonium group can react to give alpha or beta structures of cyclic sugars.
Ring Formation
Forms either a six-membered ring (pyranose) or a five-membered ring (furanose).
Six-membered ring with oxygen: pyran.
Five-membered ring with oxygen: furan.
Paranose form or D-fructose paranose (similar to pyran structure).
Fuchserrate furan nodes (similar to furan structure).
Reaction can occur from the top or bottom, leading to alpha-D-fructofuranose or beta-D-fructofuranose structures.
If the hydroxy group on the number five carbon reacts with carbonic group then basically your ring will be smaller and form five membrane ring which is furanose.
Sugar Derivatives
If a hydroxyl group is replaced by an amine group, it forms an amino derivative of glucose (e.g., glucosamine).
D-glucosamine is found in many natural products, such as chitin in shellfish and cell membranes.
It is useful for cartilage repair.
Oligosaccharides
Disaccharides
Two sugars joined together via a glycosidic bond.
Examples:
Sucrose (table sugar): glucose + fructose.
Lactose: galactose + glucose.
Sweetness Levels
Fructose: 100 (sweetest).
Sucrose: 50.
Lactose and glucose: less sweet.
Hydrolysis
Disaccharides can react with water and hydrolyze in the presence of acid to form two monosaccharides.
Sucrose hydrolyzes into glucose and fructose.
Lactose hydrolyzes into galactose and glucose.
Cyclodextrin
Cyclic molecule composed of glucose units.
Hydrophilic outer shell and hydrophobic interior.
Can host hydrophobic molecules, such as pyrene, in its core.
Types:
Alpha-cyclodextrin: six glucose units (pore size ~5 angstroms).
Beta-cyclodextrin: seven glucose units (pore size ~6 to 6.5 angstroms).
Gamma-cyclodextrin: eight glucose units (pore size ~7 to 8 angstroms).
Different pore sizes allow hosting different sizes of hydrophobic molecules, forming super molecules for various applications.
Polysaccharides
Polysaccharides are natural polymers made of many monosaccharide units joined by bonding arrangements similar to those in disaccharides and oligosaccharides.
Important polysaccharides: starch, glycogen, and cellulose, all formed from repeating glucose units.
Structure and Properties
Starch has alpha 1-4 glycosidic linkages and can be digested by mammals.
Cellulose has beta linkages and cannot be digested by mammals.
This small difference in linkage leads to different properties (e.g., humans can digest starch in flowers but not cellulose in tree leaves).
Examples of Polysaccharides
Cotton consists of cellulose.
Potatoes and flowers consist of starch (glucose joined differently).
Alginate (found in seaweed): one of the hydroxy methyl group has been oxidized to carboxy acid.
Chitin (found in shellfish): hydroxyl group replaced by an acetamide group.
Conclusion
The lecture covered sugars, oligosaccharides, and polysaccharides.
Fischer projection is a challenging but important concept.
Oligosaccharides and polysaccharides are generally straightforward.
Next lecture will cover amino acids and peptides proteins.