Carbohydrates

Chapter 1: Introduction to Carbohydrates

  • Carbohydrates serve as a source of energy and have other important functions.

  • Carbohydrates are involved in molecular recognition and intercellular communication.

  • Carbohydrates are named as such because their formulas suggest they are hydrates of carbon.

  • Carbohydrates are actually polyhydroxy alcohols or polyhydroxy aldehydes/ketones.

Chapter 2: Digestibility of Carbohydrates

  • Carbohydrates are the most abundant molecular building blocks in cells.

  • Subtle chemical differences in the way small sugars are connected result in different characteristics and functions.

  • Not all carbohydrates can be readily broken down and digested.

  • Starch, found in rice and other plant foods, can be broken down into maltose (glucose molecules linked together).

  • Cellulose, found in wood, cannot be digested by humans due to the lack of necessary enzymes.

  • Termites can digest cellulose because they have cellulases or enzymes that break down cellulose into glucose.

Chapter 3: Structural Variations of Carbohydrates

  • Carbohydrates have varied structures and forms.

  • Differences in properties of carbohydrates are due to stereochemistry or the orientation of groups in space.

  • Carbohydrates are composed of saccharides, which means sugars.

  • Monosaccharides are simple sugars that cannot be hydrolyzed.

  • Examples of monosaccharides include glucose, mannose, and galactose.

Chapter 1: Introduction to Carbohydrates

  • Carbohydrates are composed of monosaccharides

  • General formula of monosaccharides is CnH2nOn

  • Carbohydrates were mistakenly thought to be hydrates of carbon

Chapter 2: Classification of Monosaccharides

  • Monosaccharides can be classified as aldoses or ketoses

  • Aldoses contain an aldehyde group

  • Ketoses contain a ketone group

  • Monosaccharides are classified by the number of carbon atoms they have

  • Glyceraldehyde is an example of an aldose with three carbon atoms

  • Dihydroxyacetone phosphate is a ketose with three carbon atoms

Chapter 3: Stereochemistry of Monosaccharides

  • Glyceraldehyde exists as a pair of enantiomers due to its chiral center

  • Fisher projection is used to represent the structure of carbohydrates

  • Horizontal lines represent bonds projecting forward, vertical lines represent bonds projecting to the rear

  • D-glucose is an aldose with hydroxyl groups on carbon 2, 3, 4, 5, and 6

  • D-fructose is a ketose with hydroxyl groups on carbon 2, 3, 4, 5, and 6

  • D-monosaccharides have the hydroxyl group on the penultimate carbon on the right

  • L-monosaccharides have the hydroxyl group on the penultimate carbon on the left

Chapter 4: Enantiomers and Diastereomers

  • Aldose tetroses have four carbon atoms

  • Erythrose and threose are examples of aldose tetroses

  • Erythrose and threose are enantiomers

  • Erythrose and erythrose are diastereomers

  • D-sugars are naturally occurring sugars

Relationship between stereoisomers and enantiomers

  • Stereoisomers have different arrangements of groups or substituents around chiral centers

  • Enantiomers are mirror images of each other

  • The substances discussed in the transcript are stereoisomers but not enantiomers

Cyclization of sugars

  • Glucose, for example, forms a cyclic compound in solution

  • Cyclization occurs due to interaction between the functional group and distant carbons C1 and C5

  • The cyclization results in the formation of cyclic hemiacetals

Hemiacetals and hemiacetols

  • Hemiacetals are formed by the reaction between an alcohol and an aldehyde

  • Hemiacetals have a hydroxyl group and a carbon-oxygen ether bond

  • Hemiacetals are geminal hydroxy ethers

  • Hemiacetols and ketos are terms learned in organic chemistry

Formation of hemiacetals in glucose

  • Glucose undergoes intramolecular reaction to form a cyclic hemiacetal

  • The reaction involves the attack of the oxygen at carbon number five on carbon number one

  • Two cyclic structures are formed: alpha-D-glucopyranose and beta-D-glucopyranose

  • The structures resemble pyran and are called pyranoses

Anomers and diastereomers

  • Anomers are carbohydrates with different configurations at their anomeric carbons

  • Alpha and beta glucopyranose are examples of anomers

  • Anomers are diastereomers, which are stereoisomers that are not mirror images

Representation of hemiacetals

  • Hemiacetals are represented as planar pentagons or hexagons

  • The anomeric carbon is usually on the right, and the hemiacetal oxygen is to the back right

  • The designation beta means the OH on the anomeric carbon is cis to the terminal CH2OH

  • The designation alpha means the OH is trans to the terminal CH2OH

Chapter 1: Introduction

  • Pyranose and furanose are terms used to describe different ring structures in carbohydrates.

  • Pyranose refers to a six-membered ring, while furanose refers to a five-membered ring.

  • The terms pyranose and furanose are derived from the resemblance of these ring structures to spiron and furan, respectively.

Chapter 2: Structure of Furanose

  • Furanose has a five-membered ring structure.

  • The CH2 group is removed to create a bond, resulting in a furanose structure.

  • Furanose is a five-membered hemiacetal.

Chapter 3: Representation of Furanose

  • Furanose structures are close to being planar but not entirely.

  • Cyclopentanes in organic chemistry have an envelope confirmation, similar to furanose.

  • Pyranose structures are more accurately represented as a strain three-chair conformation, similar to cyclohexane chair flipping.

Chapter 4: Difference between Furanose and Pyranose

  • Furanose has two double bonds, while pyranose has no double bonds.

  • Hayworth representations are used to depict pyranose structures.