Carbohydrate Stereochemistry: Fischer Projection

Carbohydrates and Their Structures

• Anomeric Carbon: α formation means the -OH group is opposite to the terminal -CH2OH; β formation means it is on the same side.
• Ring Structures: Pyranose indicates a six-membered hemiacetal ring, while furanose refers to a five-membered hemiacetal ring.
• Aldopentoses: These also form cyclic hemiacetals due to their structure.

Mutarotation

• Definition: Mutarotation is the change in specific rotation during the equilibration of α- and β-anomers in solution.
• Example: Dissolving either α-D-glucose or β-D-glucose in water leads to a specific rotation equilibrium of +52.7°, with compositions of 64% β and 36% α forms.

Reduction to Alditols

• Reduction Process: Monosaccharides can have their carbonyl group reduced to a hydroxyl group by agents like NaBH4 or H2 with a transition metal catalyst.
• Alditols: Resulting from such reductions; common example: Sorbitol, found in fruits and used as a sugar substitute, is 60% as sweet as sucrose.

Oxidation of Aldoses

• Aldonic Acids: The aldehyde group in aldoses can be oxidized to form carboxylate anions under basic conditions.

Cyclic Structures

• Formation: Aldehydes and ketones react with alcohols to create hemiacetals, forming five- or six-membered rings.
• D-Ribose: Predominantly occurs in the furanose form in nature.
• D-Fructose: Forms a furanose cyclic structure as a 2-ketohexose.

Haworth Projections and Chair Conformations

• Cyclic Hemiacetals Representation: D-Glucose forms cyclic hemiacetals depicted in Haworth projections.
• Chair Conformations: Provides a more accurate representation of pyranose structures, showing ring orientations and substituent positions.

Anomers and Stereochemistry

• Anomeric Carbon: C-1 for aldoses and C-2 for ketoses; stereoisomers differing at the anomeric carbon are termed anomers.
• β-D-Glucopyranose: In its structure, the group orientations on carbons 1-5 are specified as up, down, up, down, and up.

Carbohydrate Terminology

• Carbohydrate Definition: Comprises polyhydroxyaldehydes or polyhydroxyketones, or compounds yielding them upon hydrolysis.
• Monosaccharides: Simplest carbohydrates not reducible to simpler forms, following the general formula ( C_n(H_2O)_n ).
• Classification: Based on the carbonyl group type; aldoses contain aldehyde groups, ketoses contained ketone groups.

Amino Sugars

• Characteristics: Contain an -NH2 group instead of -OH; include D-glucosamine, D-mannosamine, and D-galactosamine.
• Biological Role: Amino sugars help detoxify compounds in the liver.

Disaccharides and Polysaccharides

• Polysaccharides: Composed of many monosaccharide units linked by glycosidic bonds; starch is a notable polymer of D-glucose consisting of amylose and amylopectin.
• Maltose: A disaccharide made of two D-glucopyranose units linked by α-1,4-glycosidic bonds, classified as a reducing sugar.

Connective Tissue and Hyaluronic Acid

• Connective Tissue: Comprises various forms like cartilage and bone, primarily made of collagen and acidic polysaccharides, notably hyaluronic acid.
• Hyaluronic Acid Functions: Abundant in embryonic tissues and synovial fluid, it maintains tissue elasticity and hydration.

Cellulose and Heparin

• Cellulose Structure: Linear polymer with up to 3000 units of D-glucose linked by β-1,4-glycosidic bonds; indigestible by humans due to lack of specific enzymes.
• Heparin: A variable polysaccharide with anticoagulant properties, synthesized in mast cells, and has molecular weight ranging from 6,000 to 30,000 g/mol.

Test your understanding of carbohydrate stereochemistry and Fischer projections, a 2D representation of tetrahedral stereocenters. Learn about monosaccharides and how to interpret horizontal and vertical bonds.