Monosaccharides

Monosaccharides - Uses and Types of Monosaccharides

Monosaccharide or simple sugars are any of the basic compounds that serve as the building blocks of carbohydrates. The structure of monosaccharides is formed with more than one hydroxyl group (-OH) and a carbonyl group (-C=O) either at the terminal carbon atom known as aldose or at the second carbon atom known as a ketose. Molecules with such structures are called polyhydroxy aldehydes or ketones.



Monosaccharides are classified by the number of carbon atoms present in the molecule:

1) Dioses have two
2) Trioses have three
3) Tetroses four
4) Pentoses five
5) Hexoses six
6) Heptoses seven.

These different Monosaccharides can be found combined as xylem in woody materials or as arabinose from coniferous trees even in our body as ribose, a component of ribonucleic acids (RNA) and several vitamins.

Nomenclature:

The nomenclature of monosaccharides is regulated by international rules. The common names for the carbohydrates are used together with the type of anomalism, cyclic form, D- series, L- series and the rotatory power. A few examples for this kind of nomenclature are α-D (+)-glucopyranose, 2-amino-2-deoxy-D (+)-glucopyranose.

A few initials have been adopted to facilitate the writing some of the common names for that are: Glc: glucose; Gal: galactose; Man: mannose; Fuc: fucose; Xyl: xylose; Ara: arabinose; Rha: rhamnose; GlcN: glucosamine; GlcNAc: N- acetyl-glucosamine; Mur NAc: N-acetylmuramic acid; Glc AU: glucuronic acid; NeuAc: N-acetylneuraminic acid.

Uses of monosaccharides:

Several derivatives of monosaccharaides are important for different requirements.

Ascorbic acid (vitamin C) is derived from glucose. Important sugar alcohols (alditols), formed by the reduction of (i.e., in addition of hydrogen to) a monosaccharide, include sorbitol (glucitol) from glucose and mannitol from mannose; both are used as sweetening agents. Glycosides derived from monosaccharides are abundant and found almost everywhere, especially in plants.

Types of monosaccharides are as follows:

  • 1. Neutral monosaccharides

  • 2. Osamines

  • 3. Uronic acids

  • 4. Sialic acids

  • Neutral monosaccharides

    They include the carbohydrates which contain only an alcohol group with their ketone and aldehyde group. Example, D-glucose, D-galactose, D-mannose, D-xylose. The deoxys, which are monosaccharides having lost 1 or 2 oxygen atoms also are included in this type of monosaccharides.

     Osamines:

    They are derived from the neutral monosaccharides. The neutral monosaccharides hydroxyl (generally the one carried by carbon 2) is replaced by an amine group.



    Uronic acid

    Uronic acids are derived from aldoses by oxidation of the primary- alcohol group into a carboxylic group (and therefore maintain the aldehyde group). D-Glycuronic (also called “glucuronic”) acid.



    Sialic acids

    Sialic acids are derivatives of neuraminic acid which consists of a molecule of pyruvic acid which then is condensed with a molecule of D-monoamine.

    These acids are constituents of various glycoproteins and glycolipids.

    To understand the concept of monosaccharides better you must know the physical and chemical properties of monosaccharides. The students are advised to understand these properties properly as the questions asked will be based on these few properties of monosaccharides.

    Physical properties:

  • • Monosaccharides dissolve in water and give a sweet taste.

  • • They are all able to pass through a plasma membrane.

  • • The monosaccharides are soluble in water as when they dissolve in water they take up a ring-like form which is the main cause for their solubility in water.

  • • The lowering of water potential can be initiated by dissolving monosaccharides in water.

  • Chemical properties:

  • • Formation of ethers



  • • Alkylation


  • • Oxidation of monosaccharides


  • • The action of concentrated acids




  • • The action of Phenyl hydrazine


  • • Action of alcohols



  • A few derived compounds from monosaccharides are as follows:

    1. L-Ascorbic Acid (Vitamin C):

    A study of its structure revealed that it is the γ-lactone of a hexatonic acid, which itself derives from an aldohexose by oxidation of the aldehyde group to acid. It is further characterized by a double bond between 2 carbon atoms, each carrying a hydroxyl (enediol). L-Ascorbic Acid (Vitamin C) substance readily oxidizes to dehydroascorbic acid, which enables its participation in cellular oxidation-reduction processes.

    2. Polyalcohol (or Polyols):

    Reduction of the aldehyde or ketone group to the alcohol group gives us a carbohydrate called the Polyalcohol. There is a type of Polyalcohol or cyclic polyalcohol, called cyclitols. The representative of this group most frequently found in nature is myo-inositol, it is present either in the Free State or hexa-esterified by phosphoric acid or as a constituent of certain phospholipids, the phosphatidylinositol.

    A few more derivatives of Monosaccharides are mentioned below:

  • • Amino sugars such as:

  • Galactosamine
    Glucosamine
    Sialic acid
    N-Acetyl Glucosamine

  • • Sulfosugars such as:

  • Sulfoquinovose

  • • others

  • Ascorbic acid
    Mannitol
    Glucuronic acid.

    Isomerism of monosaccharides:

    The isomerism in monosaccharides is a very important phenomenon. But before we try to understand isomerism in monosaccharides we first need to understand the general meaning of the term isomerism.

    It is a phenomenon where two or more compounds have the same chemical formula but possess different structural formulas and different properties. Isomers are the compounds exhibiting isomerism.

    The isomerism in monosaccharides works in a similar way.

    When we consider the projection of the three-dimensional structure of any monosaccharide we find that there are 2 possibilities. For example if we take glyceraldehyde; in one case, the hydroxyl carried by the carbon next to the primary alcohol group (this carbon atom is called asymmetric because it carries 4 different types of substitutes and is indicated by an asterisk) is situated to the right of the plane formed by the carbon chain; this is the D-configuration; in the other case, the hydroxyl is situated to the left of this plane; this is the L-configuration.

    These two forms are mirror images which practically have the same chemical properties but their structure is different from each other.


    The aldoses and ketoses can be summarized and explained in the following way:

  • • Carbohydrates can be divided into two parts: polyhydroxy aldehydes, the aldoses, or polyhydroxy ketones, the ketoses.

  • • For a carbohydrate to be termed as an aldose it needs an aldehyde group.

  • • Aldehyde groups can be written as CH=O, is also often written as CHO.

  • • For a carbohydrate to be a ketose it needs to have a ketone group.

  • • The chiral centers are always marked with an asterisk mark or ‘*'.

  • • The assignment of molecules I very apparent in the acyclic form of the sugars.

  • • To find an aldose or a ketose, in a cyclic form first find the anomeric center (*). The substituents on that are to be taken care of; if one of them is an H then it is an aldose.

  • • The systems can be further classified on the basis of how many C atoms there are:

  • O 4 C = tetrose
    O 5 C = pentose
    O 6 C = hexose

    Cyclic structures of Monosaccharides:

    Knowing the cyclic structure of the monosaccharides is very important, as like the melting point or boiling point, the rotatory power is a constant characteristic of a substance, and this change must reflect a structural modification.

    The conversion of monosaccharides from linear to cyclic occurs due to their reaction with alcohols.


    Monosaccharides that contain five or more carbon atoms form cyclic structures, in aqueous solution. Two cyclic stereoisomers can form from straight-chain monosaccharide that will be known as anomers. An equilibrium mixture forms between the two anomers, and the straight-chain structure of a monosaccharide, in an aqueous solution. This process is known as mutarotation.

    The difference between α and β forms of sugars must seem trivial, but such structural differences are often very important in biochemical reactions. How energy is obtained from the starch in potatoes and other plants but not from cellulose, although both starch and cellulose are polysaccharides that are composed of glucose molecules linked together are explained by this.

    A few questions that can be asked in the theory exams can be as follows:

  • 1. Define each term.

  • • Mutarotation

  • • Anomer

  • • anomeric carbon

  • 2. How can you prove that mutarotation is exhibited by a solution of α-D-glucose?
    3. Explain the reduction of aldoses and the reduction of ketoses in detail.
    4. Mention a few structural importance of sugars and the basic physical properties of monosaccharides.
    5. Explain all the chemical properties of monosaccharides and make sure you write the reactions of every property as well with a proper description by using an example.

    To test if you understood the concept properly or not the students are advised to solve the few exercises given below:
  • • Draw the cyclic structure for β-D-glucose. Identify the anomeric carbon.

  • • Draw the cyclic structure for α-D-fructose. Identify the anomeric carbon.

  • • Draw the cyclic structure for α-D-mannose for a given aldohexose D-mannose that differs from D-glucose at the second carbon atom only in the configuration.

  • • Draw the cyclic structure for β-D-allose for a given aldohexose D-allose that differs from D-glucose at the third carbon atom, only in the configuration.

  • The types of multiple choice questions that can be asked are also given below:

  • 1. Glucose is a monosaccharide and is a

  • • Hexose

  • • Pentose

  • • Furanose

  • • Sucrose

  • 2. The simplest form of sugars is usually

  • • Colorless

  • • Water-soluble

  • • Crystalline

  • • all of the above

  • 3. The formula for monosaccharide is

  • • (CH2O)n

  • • CnH2n

  • • both A and B

  • • none of the above

  • 4. A monosaccharide switches from an open chain to a cyclic form through

  • • hydroxylation

  • • nucleophilic addition

  • • hydrogenation