Bond Linking Monomers in Polymers: Every living thing is made up of various proteins, enzymes, certain peptide hormones, carbohydrates, nucleic acids, polyphenolics etc. are important biomolecules. These are called macromolecules, that is, very large molecules. These macromolecules, also called natural polymers, are made up of smaller molecules which are called monomers.

These monomer units are linked together by chemical bonds. These chemical bonds are designated as specific names such as peptide bonds in proteins glycosidic bonds in carbohydrates. These natural macromolecules’ size and molecular weight are used to classify them. As stated above, these are formed by linking together monomeric units, that is to say, by polymerisation of monomers. 

What are Monomers?

A monomer is a simple molecule having two or more binding sites that create covalent bonds with other monomer molecules to produce a macromolecule. Simple sugars, amino acids, nucleotides, fatty acids and glycerol are the four main types of monomers. Each of these monomer types is important in the survival and development of life, and each can be synthesised abiotically. 

From Monomer to Polymer

A polymer is a material made up of many small molecules joined together by means of a chemical bond to form a larger molecule. Depending on its chemical composition, various types of bonds are used to hold the monomeric unit together. The nature of bonds connecting the monomer units of polymers is dependent on the chemical nature of the constituent monomers. Each complex biomolecule is composed of a small monomer. A list of significant polymers are given below:

Polymer	Monomer	Bonds involved
Starch	Glucose	α 1-4 or α 1-6 glycosidic bonds
Polypeptide chain	Amino Acid	Peptide bond
Protein	Amino Acid	Peptide bond Disulphide bond H-bond
Lipid	Fatty acid + Glycerol	Ester bond
DNA	Deoxyribonucleotide: Sugar + Phosphate + Nitrogenous base (A,T,G,C)	Phosphodiester bond Glycosidic bond Hydrogen bond
RNA	Ribonucleotide: Sugar + Phosphate + Nitrogenous base(A,U,G,C)	Phosphodiester bond Glycosidic bond

Fig: Various Biomolecules

Important Bonds Present in Biomolecules

The chemical bonds can be categorised into two in the case of biomolecules:

1. Primary Bonds: These are covalent bonds formed due to sharing of electrons. They may form by a reversible or irreversible chemical reaction. These are the permanent bonds that either consume or release energy while forming. E.g., Glycosidic bond, Peptide bond, ester bond.
2. Secondary Bonds: These are formed due to electrostatic attraction developed due to the orientation of molecules in space. In biological molecules, secondary bonds are the transient forces of attraction that form when particular atoms or groups approach near together. These bonds are primarily responsible for maintaining biological molecules’ secondary, tertiary, and higher structures. Proteins and nucleic acids contain most of them. E.g., Hydrogen bond, hydrophobic interaction, disulphide bond, ionic interaction.

Major bonds present in biomolecules are discussed below:

Peptide Bond

The amino acids are held together by peptide bonds to form polypeptide chains. A peptide bond is an amide bond (-CONH) formed by the –NH₂ and –COOH groups of neighbouring amino acids. The molecule that results is known as an amide. An amide group, or peptide group, is a four-atom functional group with the formula C (=O)NH. A water molecule is eliminated when a peptide bond is formed, and the reaction is also called a condensation reaction.

Polypeptide chains of amino acids make up proteins. Peptide bonds can form at both N-terminal and C-terminal. An amide is generated as a result of the reaction.

Glycosidic Bond

A glycosidic bond is produced when two neighbouring monosaccharides join together to form disaccharides or polysaccharides. A water molecule is eliminated whenever a glycosidic link is formed. The reaction is often referred to as dehydration or condensation reactions. It can be classified into various types:

1. O-glycosidic bond: It’s the most common glycosidic bond in which the carbonyl group of carbohydrates interacts with the hydroxyl group of another chemical to form an O-glycosidic bond.
2. N-Glycosidic Bond: A chemical reaction occurs when the carbonyl group of a carbohydrate molecule reacts with the amino group (-NH₂) of a non-carbohydrate molecule to create this bond.
3. Alpha-Glycosidic bond: The atoms that make up an alpha-glycosidic bond are all orientated in the same plane, making them stereochemically similar. It is found in starch.
4. Beta-Glycosidic bond: The two bond-forming atoms in beta-glycosidic bonds are orientated in opposing planes and have differing stereochemistry. It is found in cellulose.

Phosphodiester Linkage

A phosphodiester bond is a covalent link in which the two OH groups of the phosphoric acid, (HO)₂P(=O)₂, form two separate ester bonds between the OH containing carbon number 5’ on one sugar unit and OH of carbon 3’ of another sugar unit, that is –C-OH, of two sugar molecules. Nucleic acids are the most common examples (DNA and RNA). DNA contains a C₅ sugar called 2-deoxyribose. The OH group of the phosphoric acid and the OH group of the sugar react together to form a P-O-C type linkage.

Two sugar units + (HO)₂P(=O)₂ react, giving out two water molecules and forming two phosphodiester bonds.,,the   

In RNA, the sugar is ribose. In deoxyribose, the oxygen at the C-2 position is missing. Note that this connection is also formed via a condensation event between the hydroxyl groups of two sugars and a phosphate group. It is called an ester linkage because the P atom still has P=O in addition to the C-O-P bond; recall that the ester group is of the type O=C-O-, which is generally written as –COO unit. The unit containing phosphate plus sugar is the backbone of nucleic acids. 

During polymerisation reaction of nucleotides, the hydroxyl group on the phosphate group attaches to the position 3′ carbon of sugar of one nucleotide to form an ester bond to the phosphate of another nucleotide.

The removal of a water molecule causes the formation of a phosphodiester bond. DNA polymerases catalyse the addition of extra nucleotides to the growing polynucleotide chains. The 3′-end has a hydroxyl group at the 3′-carbon of sugar, and the 5′-end has a hydroxyl group or phosphate group at the 5′-carbon of sugar, and the synthesis continues from the 5′ to the 3′-end in the nucleotide addition process, which produces nucleotide chains.

Ester Bond

It’s a covalent bond that’s required in a variety of lipids. Between an acid and an alcohol, an ester bond or ester linkage is created. When a molecule with a carboxylic group combines with another molecule containing a hydroxyl group, an ester bond is produced. The carboxylic group loses hydrogen and oxygen, whereas the hydroxyl group of the alcohol loses hydrogen. A water molecule is produced as a result, and the two carbons are joined by an oxygen bridge, generating the -COC- bond.

Hydrogen Bond

The strongest secondary link is the hydrogen bond, which is nearly as strong as covalent bonds. The hydrogen bond is very important to maintain the structural integrity of larger molecules such as protein, carbohydrate, and nucleic acid. Hydrogen bonds are prevalent in proteins, and they may be found in many parts of the chain, such as the backbone and side chains. H-bond may be intramolecular or intermolecular in carbohydrates to sustain a big 3-D structure.

The two strands of DNA are held together by weak hydrogen bonds formed between the nitrogen bases. The hydrogen bonds that link the nitrogen bases are quite specific. Guanine forms three hydrogen bonds with cytosine on the opposite strand, whereas adenine only forms two hydrogen bonds with thymine in the other strand. Two nitrogenous bases from opposite strands bonded by H-bond are called base pairs.

Hydrophobic Interactions

These are the non-polar molecules’ interactions with one another. Hydrophobic interaction is the clumping of hydrophobic molecules. The tertiary and quaternary structure of proteins is maintained through hydrophobic interactions.

Disulphide Bond

A disulphide bond is a covalent connection that joins the thiol groups in the side chains of two cysteine residues to produce a single cysteine residue. The two cysteine residues that had been held apart by the intervening amino acids are now joined by this bond. This bond is responsible for directing protein folding and stabilising the tertiary structure of proteins.

Ionic Interactions

The secondary forces of attraction that emerge between the charged groupings are these. They have a role in protein folding as well.

Summary

A monomer is a simple molecule that comprises two or more binding sites. It helps in forming covalent bonds with other monomer molecules to produce a macromolecule. A polymer is a material formed by several minor molecules joined together via a chemical bond to create a larger molecule. The chemical bonds in biomolecules can be divided into two parts namely, primary and secondary bonds. Primary bonds are formed due to sharing of electrons. Some of the examples of primary bonds are the Peptide bond, ester bond, glycosidic bond, etc. Secondary bonds are formed due to electrostatic attraction formed due to the orientation of molecules in space. Some of the examples of secondary bonds are hydrogen bonds, hydrophobic interactions, ionic interactions, etc.

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FAQs

Q.1. What are the different types of chemical bonds found in biomolecules?
Ans: The different types of chemical bonds are Covalent bonds, hydrogen bonds and Ionic interactions etc.

Q.2. What elements are the most important in biomolecules?
Ans: Carbon, hydrogen, oxygen, nitrogen, sulphur, and phosphorus.

Q.3. Which bond involves interaction between non-polar compounds?
Ans: Hydrophobic interaction involves non-polar compounds.

Q.4. Which bond is present in Protein?
Ans: Peptide bond links two amino acids, whereas disulphide bond, ionic interaction and hydrogen bonds stabilise the tertiary structure of the protein.

Q.5. Which type of bond holds the DNA strands together?
Ans: Hydrogen bond links the strands of DNA molecules together.

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