Cyclic Forms of Monosaccharides and Enantiomers

Cyclic Forms of Monosaccharides

• D-Glucose has two forms: α-form and β-form.
• The α-form has the –OH group of carbon 1 and the CH2OH group of carbon 5 on opposite sides.
• The β-form has the –OH group of carbon 1 and the CH2OH group of carbon 5 on the same side.
• Anomers are cyclic monosaccharides that differ only in the position of the substituents on the anomeric carbon atom.
• Aldoses with 5 or more carbon atoms establish similar equilibria like D-glucose, but with different percentages of alpha, beta, and open-chain forms.
• Fructose and other ketoses with sufficient carbon atoms also cyclize.
• Pyranose is a cyclic monosaccharide containing a six-atom ring.
• Furanose is a cyclic monosaccharide containing a five-atom ring.

Properties of Enantiomers

• Enantiomers are optically active.
• Dextrorotatory compounds rotate the plane of polarized light clockwise.
• Levorotatory compounds rotate the plane of polarized light counterclockwise.
• There is no correlation between D, L, and +, -.
• Enantiomers react differently with other chiral molecules.
• Enantiomers have the same solubility in achiral solvents but different solubility in chiral solvents.
• Enantiomers have the same boiling points, melting points, and densities.
• Diastereoisomers have different physical and chemical properties, while enantiomers have the same physical and chemical properties.

Classification of Monosaccharides

• Monosaccharides are classified based on the number of carbon atoms and functional groups.
• Triose has 3 carbons, tetrose has 4 carbons, pentoses have 5 carbons, and hexoses have 6 carbons.
• Aldoses have one aldehyde group.
• Ketoses have one ketone group.

Biochemically Important Monosaccharides

• Glucose is the most abundant monosaccharide in nature.
• Glucose is the most important source of human nutrition.
• Glucose is also known as grape sugar or dextrose.
• Blood sugar levels range from 70–100 mg/dL.

Reactions of Monosaccharides

• Amino sugars are formed when one of the hydroxyl groups of a monosaccharide is replaced with an amino group.
• The C2 hydroxyl group is replaced by an amino group in naturally occurring amino sugars.
• Amino sugars are important building blocks of polysaccharides such as chitin and hyaluronic acid.

Disaccharides

• Two monosaccharides can react to form a disaccharide.
• The ether bond formed is a glycosidic linkage.
• Maltose (malt sugar) is composed of two D-glucose units linked via an α(1→4) glycosidic linkage.
• Humans can digest maltose easily because of an enzyme that can break α(1→4) linkages.

Structural Polysaccharides

• Cellulose is a linear polymer of D-glucose units linked via β(1→4) glycosidic linkages.
• Cellulose is found in plant cell walls and is important for structural support.
• Chitin is a linear polymer with all β(1→4) glycosidic linkages.
• Chitin has an N-acetyl amino derivative of glucose.
• Chitin gives rigidity to the exoskeletons of arthropods such as crabs, lobsters, shrimp, and insects.

Acidic Polysaccharides

• Acidic polysaccharides are polysaccharides containing a repeating disaccharide unit with an amino sugar and a sugar with a negative charge due to a sulfate or carboxyl group.
• Acidic polysaccharides are heteropolysaccharides, meaning they contain different monosaccharides in an alternating pattern.
• Hyaluronic acid is a highly viscous acidic polysaccharide that acts as a lubricant in joints and the vitreous humor of the eye.
• Heparin is an acidic polysaccharide that acts as a blood anticoagulant.