Carbohydrates have unique structures that determine their function. Stereochemistry plays a crucial role in how these molecules interact with each other and biological systems. Understanding chirality, anomers, and different representations is key to grasping carbohydrate chemistry.

Stereoisomers like enantiomers and epimers have identical chemical formulas but different spatial arrangements. This affects how they behave in reactions and biological processes. Knowing how to represent and identify these isomers is essential for predicting carbohydrate properties and interactions.

Chirality and Stereoisomers

Chiral Centers and Enantiomers

• Chirality refers to the property of a molecule being non-superimposable on its mirror image
• Chiral molecules contain at least one chiral center, typically a carbon atom with four different substituents
• Enantiomers are a pair of stereoisomers that are non-superimposable mirror images of each other
• Enantiomers have identical physical properties (melting point, boiling point, solubility) but differ in their interaction with plane-polarized light
• Enantiomers rotate plane-polarized light in opposite directions (dextrorotatory (+) and levorotatory (-))

D- and L-Sugars and Epimers

• D- and L-sugars are a way to classify the stereochemistry of monosaccharides based on the configuration of the chiral center furthest from the carbonyl group
• D-sugars have the hydroxyl group on the right side of the chiral center furthest from the carbonyl group in the Fischer projection (most common in nature)
• L-sugars have the hydroxyl group on the left side of the chiral center furthest from the carbonyl group in the Fischer projection
• Epimers are stereoisomers that differ in configuration at only one chiral center (glucose and galactose are epimers at C-4)

Carbohydrate Representations

Fischer Projections

• Fischer projections are a way to represent the stereochemistry of monosaccharides in a two-dimensional format
• Horizontal lines represent bonds coming out of the plane towards the viewer
• Vertical lines represent bonds going behind the plane away from the viewer
• The carbon chain is written vertically with the carbonyl group at the top (aldoses) or penultimate position (ketoses)
• The most oxidized carbon (carbonyl group) is placed at the top of the Fischer projection

Haworth Projections

• Haworth projections are a way to represent the cyclic structure of monosaccharides in a three-dimensional format
• The ring is drawn as a hexagon with the oxygen atom at the top right corner
• The carbon atoms are numbered clockwise from the anomeric carbon (C-1)
• Hydroxyl groups pointing upward are drawn above the plane of the ring (axial)
• Hydroxyl groups pointing downward are drawn below the plane of the ring (equatorial)
• The anomeric carbon (C-1) can have two configurations (alpha and beta) depending on the orientation of the hydroxyl group

Anomeric Configuration

Anomers and Mutarotation

• Anomers are stereoisomers that differ in configuration at the anomeric carbon (C-1) in the cyclic form of a monosaccharide
• The anomeric carbon is formed when the carbonyl group of a linear monosaccharide reacts with a hydroxyl group to form a cyclic hemiacetal
• Alpha (α) anomers have the hydroxyl group at C-1 pointing downward (axial) in the Haworth projection
• Beta (β) anomers have the hydroxyl group at C-1 pointing upward (equatorial) in the Haworth projection
• Mutarotation is the spontaneous interconversion between alpha and beta anomers in solution until an equilibrium mixture is reached (usually favoring the beta anomer)

Anomeric Effect and Glycosidic Bonds

• The anomeric effect is the tendency of electronegative substituents at the anomeric carbon to prefer the axial orientation over the equatorial orientation
• This is due to the stabilizing interaction between the lone pair of electrons on the endocyclic oxygen and the antibonding orbital of the C-1–O-1 bond
• Glycosidic bonds are formed when the hydroxyl group of one monosaccharide reacts with the anomeric carbon of another monosaccharide, releasing a water molecule
• The type of glycosidic bond (alpha or beta) depends on the configuration of the anomeric carbon involved in the bond formation
• Disaccharides (sucrose, lactose, maltose) and polysaccharides (starch, cellulose, glycogen) are formed through glycosidic bonds