CHEMLEC 2 M1: Carbohydrates

• Carbohydrates, known as "sugars" and "starches," are synthesized in green plants through photosynthesis. This process utilizes solar energy to convert carbon dioxide and water into glucose and oxygen. Within the body, carbohydrates serve as a vital source of energy, particularly during exercise, where they are utilized in the form of glucose. The chemical formula for carbohydrates is represented as Cn(H2O)n, denoting them as "hydrates of carbon."
• Monosaccharides, the building blocks of carbohydrates, are polyhydroxy aldehydes or ketones. This means they are molecules containing more than one hydroxyl group and a carbonyl group either at the terminal carbon atom (aldose) or at the second carbon atom (ketose). They cannot be broken down into simpler units by hydrolysis reaction and typically contain 3 to 7 carbon atoms. Additionally, they are water soluble, appearing as white, crystalline solids. Examples of monosaccharides include glucose and fructose.
• Disaccharides, composed of two molecules of simple sugars, consist of monosaccharides linked by a glycosidic bond, forming carbohydrates containing two monosaccharide units covalently bonded to each other. Hydrolysis of disaccharides produces two monosaccharide units. They typically manifest as crystalline, water-soluble substances. Examples of disaccharides include sucrose (table sugar), lactose (milk sugar), maltose (malt sugar), and cellobiose.
• Oligosaccharides encompass any carbohydrate composed of three to ten units of simple sugars. Many oligosaccharides are created by partially breaking down more complex carbohydrates, such as polysaccharides. Hydrolysis of oligosaccharides produces several monosaccharide molecules. Examples of oligosaccharides include raffinose and stachyose.
• Polysaccharides, also known as glycans, represent the predominant form of natural carbohydrates. When composed of one type of sugar or one sugar derivative, they are termed homopolysaccharides or homoglycans. Those derived from more than one sugar are termed heteropolysaccharides or heteroglycans. Examples of polysaccharides include starch, cellulose, glycogen, and chitin.
• Monosaccharides can be classified based on the type of carbonyl group present:

  1. Aldose: This refers to a monosaccharide containing an aldehyde group.
  2. Ketose: This refers to a monosaccharide containing a ketone group.
• The number of carbon atoms in a monosaccharide's chain is indicated by a Greek numerical prefix followed by "-ose" for both aldoses and ketoses:

  • Triose: Contains three carbon atoms.
  • Tetrose: Contains four carbon atoms.
  • Pentose: Contains five carbon atoms.
  • Hexose: Contains six carbon atoms.
• Chirality in chemistry refers to a property of a molecule where a carbon atom is bonded to four different groups, resulting in the molecule having non-superimposable mirror images, known as stereoisomers. In the context of carbohydrates, all except dihydroxyacetone possess one or more chirality centers. The naturally occurring isomer used as a reference for the assignment of configurations to other sugars is D-glyceraldehyde. This molecule has a single chiral carbon and serves as the standard for assigning D- or L- configuration to other sugars based on their relationship to D-glyceraldehyde.
• Chirality, in the context of chemistry, pertains to the study of the three-dimensional structure of molecules, particularly focusing on molecules with non-superimposable mirror images, known as enantiomers. These enantiomers exhibit left-handed and right-handed forms, which arise due to the spatial arrangement of atoms around chiral centers. The study of chirality is essential in various fields, including organic chemistry, biochemistry, and pharmacology, due to its profound implications on molecular interactions and biological activity.
• Enantiomers are molecules that have the same molecular and structural formulas but differ in the orientation of atoms in space
• Chiral molecules is a molecule whose mirror images arenot superimposable
• Achiral molecule is a molecule whose mirror images are superimposable
• Chirality it should have an atom, usually a carbon that is bonded to four different groups in a tetrahedral orientation (chiral center)
• In 1891, Emil Fischer made the arbitrary assignments of D- and L- to the enantiomers of glyceraldehyde. The configuration of the chirality center farthest from the carbonyl group determines whether a monosaccharide is D or L
• A chiral center is typically depicted as the point of intersection between vertical and horizontal lines, commonly represented by a carbon atom. It must be bonded to four different groups arranged tetrahedrally to exhibit chirality.
• All naturally occurring sugars are D sugars.
• The Haworth Projection Formula, developed by Walter Norman Haworth, is a two-dimensional structural notation used to represent the three-dimensional structure of cyclic forms of monosaccharides.
• Dexter means right

  Laevous means left
• A dextrorotatory is a chiral compound that rotates the plane of polarized light in a clockwise direction (to the right)
• Levorotatory compound is a chiral compound that rotates the plane of polarized light in a counterclockwise direction (to the left).
• BIOCHEMICALLY IMPORTANT MONOSACCHARIDES

  1. D-glycerlaldehyde

  2. Dihydroxyacetone

  3. D- glucose

  4. D- galactose

  5. D- fructose

  6. D- ribose

  All six of these monosaccharides are water-soluble,

  white, crystalline solids
• D-Glyceraldehyde and Dihydroxyacetone

  The simplest of the monosaccharides, these two trioses are important intermediates in the process of glycolysis a series of reactions whereby glucose is converted into two molecules of pyruvate. D-Glyceraldehyde is a chiral molecule, but dihydroxyacetone is not.
• D-glucose the most abundant and nutritionally important monosaccharide for humans is glucose. It is an aldohexose, commonly known as grape sugar, blood sugar, or dextrose. Normal blood glucose concentration ranges from 70 to 100 mg/dL (1 dL is equal to 100 mL). Glucose, along with all other naturally occurring monosaccharides, exists predominantly in the D configuration. It serves as the primary energy source for cells.
• D-Galactose, often referred to as "brain sugar," is a component of the disaccharide lactose. Individuals with galactosemia lack an enzyme necessary for metabolizing galactose, leading to its accumulation in the body and resulting in conditions such as cataracts and cirrhosis. Additionally, D-Galactose plays a role in the chemical markers that distinguish various blood types, including A, B, AB, and O.
• D-Fructose, also known as levulose and "fruit sugar," is a component of the disaccharide sucrose. Among all sugars, D-fructose is considered the sweetest-tasting. As a ketohexose, it is naturally found in honey and is almost twice as sweet as table sugar, while containing the same number of calories per gram.
• D-Ribose serves as a component in various complex molecules, including ribonucleic acids (RNAs) and energy-rich compounds like adenosine triphosphate (ATP). Additionally, the compound 2-deoxy-D-ribose holds significance in nucleic acid chemistry as it is a component of DNA molecules.
• Tautomerism is a phenomenon where a single chemical compound can exist in two or more interconvertible structures. These structures differ in the relative position of one atomic nucleus, typically hydrogen.
• Hemiacetal formation occurs when aldehydes or ketones react with alcohols, resulting in the formation of hemiacetals. In a hemiacetal, an alcohol group and an ether group are attached to the same carbon atom.
• Intramolecular hemiacetal formation occurs when the aldehyde or ketone functional group and the hydroxyl group are part of the same molecule. This proximity allows for the formation of cyclic hemiacetals. Typically, these interactions result in the formation of five- or six-membered rings.
• Anomers refer to the two stereoisomeric forms of cyclic monosaccharides that arise from the hemiacetal carbon atom, known as the anomeric carbon. This carbon atom is bonded to both an OH group and the oxygen atom in the heterocyclic ring. Cyclic monosaccharide formation typically results in the production of two anomeric forms: the alpha (α) and beta (β) anomers. These anomers differ only in the positions of the substituents attached to the anomeric carbon atom.
• Pyranose and furanose are terms used to describe the cyclic forms of monosaccharides:

  • A cyclic monosaccharide containing a six-atom ring, resembling the structure of the cyclic ether pyran, is called a pyranose.
  • Similarly, a cyclic monosaccharide containing a five-atom ring, resembling the structure of the cyclic ether furan, is called a furanose.
• ALDOHEXOSE

  PYRANOSE: -OH of C5 to C1

  FURANOSE: -OH of C4 to C1

  KETOHEXOSE (HEMIKETAL)

  • PYRANOSE: -OH of C6 to C2
• OXIDATION TO PRODUCE ACIDIC SUGARS

  
  

  • Monosaccharide oxidation can produce acidic sugars.
  • Weak oxidizing agents, such as Tollens and Benedict’s solutions, oxidize the aldehyde end of an aldose to give an aldonic acid.
  • Oxidation of the aldehyde end of glucose produces gluconic acid
  • Oxidation of the aldehyde end of galactose produces galactonic acid
  • With Tollens solution, glucose reduces Ag+ ion to Ag
  • With Benedict’s solution, glucose reduces Cu2+ ion to Cu+ ion.
  • A reducing sugar is a carbohydrate that gives a positive test with Tollens and Benedict’s solutions.
• OXIDATION TO PRODUCE ACIDIC SUGARS

  ● Strong oxidizing agents can oxidize both ends of a monosaccharide at the same time (the carbonyl group and the terminal primary alcohol group) to produce a dicarboxylic acid.

  ● Such polyhydroxy dicarboxylic acids are known as aldaric acids.

  ● For glucose, this oxidation produces glucaric acid
• REDUCTION TO PRODUCE SUGAR ALCOHOL

  • The carbonyl group present in a monosaccharide (either an aldose or a ketose) can be reduced to a hydroxyl group, using hydrogen as the reducing agent.
  • For aldoses and ketoses, the product of the reduction is the corresponding polyhydroxy alcohol, which is sometimes called a sugar alcohol.
  • The reduction of D-glucose gives D-glucitol. It has a common name of D-sorbitol.
• Phosphate ester formation involves the reaction of the hydroxyl groups of a monosaccharide with inorganic oxyacids to form inorganic esters:

  • Esterification of the hemiacetal group (carbon 1) and the primary alcohol group (carbon 6) in glucose results in the formation of compounds known as glucose 1-phosphate and glucose 6-phosphate, respectively.
• GLYCOSIDE FORMATION

  • The general name for monosaccharide acetals is glycoside.
  • A glycoside is an acetal formed from a cyclic monosaccharide by replacement of the hemiacetal carbon OH group with an OR group.
  • Glycoside produced from glucose is called a glucoside
  • For galactose, it is called a galactoside.
• DISACCHARIDES

  • It is a carbohydrate in which two monosaccharides are bonded together.
  • In disaccharide formation, one of the monosaccharide reactants functions as a hemiacetal, and the other functions as an alcohol
• GLYCOSIDE FORMATION

  • The bond that links the two monosaccharides of a disaccharide (glycoside) together is called a glycosidic linkage.
  • A glycosidic linkage is the bond in a disaccharide resulting from the reaction between the hemiacetal carbon atom OH group of one monosaccharide and an OH group on the other monosaccharide.
• Maltose


  Often called malt sugar