Type of bonds

Macromolecules are held together by different types of chemical bonds. On the basis of energy required or released to break during bond formation or breakage, these bonds have been classified into two types, i.e., strong and weak bonds. Strong bonds preserve the structure of macromolecules and require greater amount of energy to break. Covalent bonds are strong type of bonds. Glycosidic, peptide, and nucleotide bonds are examples of covalent bonds. These are formed by sharing of electrons between two electronegative atoms.

Hydrogen bonding​ is very common in biological systems. Hydrogen bonding is very important in maintaining three-dimensional structure of nucleic acids, protein molecules and specially of water. In the absence of hydrogen bonding, water would have been in the gaseous state at room temperature and this would have made the existence of life impossible.

Ionic or electrostatic bonds are formed by transfer of electrons between oppositely charged atoms, e.g., between Na' and C1-. Ionic bonds are very strong (like covalent bond) in the absence of water. However, they are quite weak (like hydrogen bond) in the presence of water. Ionic bonds are very important in binding of enzyme and substrate.

a) Covalent bonds - Covalent bonds are strongest bonds formed by sharing of one or more electron pairs by the atoms. A water molecule consists of two hydrogen atoms and one oxygen atom joined by covalent bonds. The G' and & indicate a slight positive and negative charges on the molecules which is due to shifting of electrons to the more positively charged nucleus of oxygen.

b) Hydrogen bonds - For the formation of hydrogen bond a hydrogen atom is shared by two electronegative atoms, for example, the bond between an oxygen and nitrogen atoms and between two nitrogen atoms. Such bonds are predominantly found in proteins and nucleic acids.

c) Hydrogen bonds in water - Due to weak electrical charges, the water molecules are joined by hydrogen bonds transiently.

d) Hydrophobic bonds - A weak interaction force between water repelling, nonpolar residues and molecules, such as between fatty acid chains of membrane phospholipid or between aromatic bases in DNA.

e) Ionic bonds - These bonds are very strong in the absence of water. The force of attraction between the two charges, (+) 'IlQ and (-) is F = -- Where D= dielectric constant (1 for vacuum, 80 for water), r= distance between two GD molecules, q, and q, are the charges. Ionic bonds become weak in presence of water.

f) van der Waals force - A non specific, weak chemical interaction resulting from attractive forces produced two atoms or groups of atoms when they come near each other.

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