Carbohydrate Chemistry

• Carbohydrates, also known as "hydrates of carbon," are aldehydic or ketonic derivatives of polyhydroxy alcohols with hemiacetal glycosidic linkages.
• The general empirical formula for carbohydrates is Cn(H2O)n.
• Carbohydrates are the primary energy source for the body.
• Brain cells, red blood cells, kidney medulla, lens, cornea, testes, and exercising muscles rely almost entirely on glucose for energy.
• Carbohydrates are the most abundant organic matter, playing essential roles in all life forms.
  • Carbohydrates store energy, serve as fuels, and act as metabolic intermediates
  • Ribose and deoxyribose sugars form the structural backbones of RNA and DNA.
  • Polysaccharides are structural components of bacterial and plant cell walls, with cellulose being the main constituent of plant cell walls.
  • Carbohydrates linked to proteins and lipids facilitate cell interactions and environmental element interactions.
• Common carbohydrate sources include milk, bread, popcorn, potatoes, and maize.
• Carbohydrates are found in both plant and animal tissues.
• In animal cells, carbohydrates are mainly present as glycogen and glucose, while in plant cells, they exist as cellulose and starch.

Naming Carbohydrates

• The simplest carbohydrate has 3 carbons
• Use greek prefixes like tri-, tetra-, penta-, hexa- with the suffix -ose to denote a carbohydrate's carbon number.
  • A triose has 3 carbons, while a hexose has 6 carbons.
• This naming doesn't specify the exact molecular nature; 24 different hexoses exist, including glucose.

Functional Groups

• Sugars have two major functional groups: aldehyde or ketone (carbonyls) and alcohol.

• Sugars have multiple alcohol groups, but only one aldehyde or ketone group.

• Add "aldo-" or "keto-" prefixes to the carbohydrate name based on the functional group.

  • "Aldo-" indicates an aldehyde, and "keto-" indicates a ketone.

• Formaldehyde is the simplest aldehyde but not a carbohydrate

• Glycolaldehyde is the simplest molecule with C=O and -OH groups, but not a carbohydrate

• D-glyceraldehyde is the simplest carbohydrate molecule.

  • "Aldotetrose" contains a 4-carbon chain.
  • "Aldopentose" contains a 5-carbon chain.
  • "Aldohexose" contains a 6-carbon chain.

• "Ketotriose" contains a 3-carbon chain.

  • "-ose" ending means a carbohydrate.

• Ketoses cannot have a ketone functional group at the beginning of the chain; it's usually on the second carbon.

• Some ketoses are named using "-ul-" before "-ose," like D-xylulose from D-xylose.

• Dihydroxyacetone, D-fructose, D-ribulose, and D-xylulose are biologically significant ketoses.

Isomers

• Molecules that cannot be superimposed on their mirror images are optical isomers (stereoisomers).

• A chiral (asymmetric) carbon atom causes optical isomerism.

• Glyceraldehyde, the simplest carbohydrate with a chiral carbon, exists in two isomeric forms that are mirror images

• Glyceraldehyde has one asymmetric carbon; thus, this sugar has two stereoisomers.

• D-Glyceraldehyde and L-glyceraldehyde (or D- and L- glucose; for example) are enantiomers, or mirror images of each other.

• Molecules with multiple asymmetric carbons are diastereoisomers, isomers that are not mirror images.

• D-glucose and D-mannose differ in configuration only at C-2.

• Sugars differing in configuration at a single asymmetric center are epimers.

  • D-glucose and D-mannose are epimeric at C-2; D-glucose and D-galactose are epimeric at C-4.
  • D-mannose and D-galactose are not epimers because they differ in configuration about two of their C atoms.

• Configuration: The three-dimensional arrangement of groups around the chiral carbon atom.

• Stereoisomers differ from each other in configuration.

• The D,L system denotes stereochemistry widely used by biochemists.

• D-Glucose is the only aldose that commonly occurs in nature as a monosaccharide.

• D-glyceraldehyde, D-ribose, D-mannose, and D-galactose are essential components of larger biological molecules.

• L Sugars are less abundant than D sugars.

Reactions

• Alcohols react with carbonyl groups in aldehydes and ketones to form hemiacetals and hemiketals, respectively.

• Monosaccharide hydroxyl, aldehyde, or ketone groups react intramolecularly to form cyclic hemiacetals and hemiketals

• Haworth projection formulas represent the configurations of substituents on sugar rings.

• A six-membered ring sugar is a pyranose, analogous to pyran.

• A five-membered ring sugar is a furanose, analogous to furan.

• Cyclic forms of glucose and fructose are glucopyranose and fructofuranose.

• Monosaccharide cyclization renders the carbonyl carbon asymmetric

• Isomers differing only in configuration about that carbon atom are anomers: designated as α or β

• The carbonyl carbon becomes an anomeric carbon.

• In the α anomer, the OH substituent on the anomeric carbon is on the opposite side of the sugar ring.

• The opposite anomer is the β form

• Anomers of D-glucose have different physical and chemical properties

  • α-D-glucose and β-D-glucose have specific rotations of +112.2° and +18.7°, respectively.
  • When dissolved in water, the specific rotation changes to +52.7° at equilibrium, known as mutarotation.

• The equilibrium mixture in glucose consists of 63.6% α anomer and 36.4% β anomer.

• Interconversion between anomers occurs through the linear form of glucose.

• Haworth projections are convenient

• They do not accurately portray pyranose and furanose ring conformations.

• With C-C-C tetrahedral bond angles of 109° and C-O-C angles of 111°, pyranose and furanose rings cannot be planar.

• Pyranose rings adopt puckered conformations like the chair and boat conformations.

• Substituents can be equatorial (coplanar with the ring) or axial (parallel to the ring's axis).

• Bulky substituents are more stable in equatorial positions; chair conformations are more stable than boat conformations.

• α-D-glucose can adopt a conformation with all bulky groups in the equatorial position

• α-D-glucose is abundant and central to carbohydrate metabolism, due to stability.

Function

• Carbohydrates provide energy (4 kcal/gm).
• Carbohydrates form structural components of membranes.
• Carbohydrates provide the structural basis for DNA and RNA (Ribose/Deoxyribose)
• Carbohydrates provide the structural basis for nucleosides and nucleotides.
• Carbohydrates are a carbon skeleton source for some amino acids
• Carbohydrates are structural elements in cell walls of arthropods, bacteria, and plants
• Carbohydrates link to many proteins and lipids
• Carbohydrates are the basis of some intracellular messenger systems.

Classification

Generally, carbohydrates are classified into three major groups

• Simple sugars – Monosaccharides & disaccharides
• Oligosaccharides
• Polysaccharides

Monosaccharides

• Molecules having only one actual or potential sugar group
• Monosaccharides are the building blocks of all carbohydrates
• Monosaccharides cannot be further hydrolyzed.
• Monosaccharides: also called simple sugars
• Monosaccharides include: Aldoses (Aldo sugars), Ketoses (Keto sugars)
  • Aldoses (Aldo sugars): Monosaccharides containing the aldehydic group
  • Ketoses (Keto sugars): Monosaccharides containing the ketonic group
• Glyceraldehyde is the aldo sugar with three carbons (an aldotriose).
• Dihydroxyacetone is the keto sugar with three carbon atoms (a ketotriose)

Glucose

• Formed in the body from hydrolysis of cane sugar, starch, lactose and maltose
• Found in blood, fruits, honey, and urine.
• The structure can be depicted as a ring or chain

Fructose

• Found naturally in honey tomatoes, and applies
• Obtained from hydrolysis of cane sugar in the body
• Molecular formula C6H12O6
• Known as the sweetest monosaccharide
• Prepared from sucrose hydrolysis

Galactose

• An element of glycoproteins and glycolipids
• Produced in mammary glands, hydrolyzed and used to make lactose in Milk

Mannose

• Obtained on the hydrolysis of plant gums and mannosans
• a constituent of the prosthetic polysaccharide of albumins, mucoproteins, and globulins.
• Pentoses and hexoses exist in either ring and open chain forms

Classification by number of carbon atoms

• The simplest sugars might be further divided into tetroses, trioses, hexoses or heptoses, pentoses, and ketoses
• Aldehydes and ketones are also differentiated based on whether the ketone and aldehyde groups are present - (C3H6O3) Trioses : Glyceraldehyde, and Dihydroxyacetone - (C4H8O4) Tetroses: Erythrose, and Erythrulose - (C5H10O5): Ribose, and Ribulose

Trioses

• Trioses are formed throughout the metabolic breakdown of the hexoses in the body i.e dihydroxyacetone and glyceraldehydes.

Pentoses

• Pentoses are vital constituents of many coenzymes and nucleic acids
• Also formed as transitional throughout certain metabolic processes
  • Example: nucleic acids and coenzymes NAD, Ribose that as structural element of ATP, and flavor protein. Arabinose, rybulose and xlylose are example constituents

Disaccharides

• Disaccharides comprise two monosaccharides connected by a glycosidic linkage (C-O-C).
• Cn (H2O)n-1 is the general formula for disaccharides.
• Disaccharides forms are lactose, maltose and sucrose

Maltose

• Formed as a transitional product of the action of amylases on starch and it contains two glucose residues in alpha 1, 4 linkages.
• It can be seen in a detectable amount in many germinating tissues and seeds where starch is being broken down.

Lactose

• Lactose resides in milk.
• On hydrolysis, it produces D-galactose and D-glucose. it is a reducing disaccharide, as it has a free anomeric carbon on the glucose residue.

Sucrose

• Cane sugar or sucrose is a disaccharide of fructose and glucose
• The hydrolysis of sucrose to D-glucose and D-fructose is often known as inversion as it is accompanied by a net change in optical rotation
• The mixture formed is known as invert sugar.
• Certain enzymes like invertases catalyze the reaction.
• Tremendously abundant in plants and is generally known as table sugar.

Trehalose

• Trehalose possesses two d- glucose resdues and is a mon-reducing disaccharide just like that.
• Trehalose consistss of linked by a-(1→1) glycosidic bond.

Oligosaccharides

• Are arbitrarily defined as carbohydrates that contain three to ten monosaccharide units per molecule joined by glycosidic linkages
• On hydrolysis they yield monosaccharides
• Trisaccharides, where plenty of trisaccharides occur free in nature
  • Raffinose can be seen abundantly in sugar beets
  • Melezitose can bee seen in sap of coniferous trees

Polysaccharides

• The majority of carbohydrates are found in nature take place as polysaccharide of high molecular weight
• Polysaccharides are complex carbohydrates that are formed by the method of polymerization of a huge number of monosaccharide monomers
• The other name for polysaccharides is also known as glycans.
• They are lengthy polymers of monosaccharide units joined by glycosidic linkages which might be unbranched or branched
• After completion of hydrolysis of the saccharides i they give derivatives and or monosaccharides Polysaccharides consist of two types
  • Homosaccharides :
    • They contain only of one type of monosaccharides as the repeating unit and hydrolysis
    • Gives only one type of sugar
  • Heteropolysaccharides:
    • consist of mixed disaccharides as the repeating unties
    • give mixtures of monosccharides
    • Made of amino acids and sugar acids.

This lesson covers the fundamentals of carbohydrate chemistry, including functional groups, isomers, and configurations. It also explores the formation of hemiacetals and hemiketals, offering a comprehensive introduction to carbohydrate structures and reactions.