atoms to vibrate violently, and this disrupts the hydrogen and ionic bonds. Under these conditions, protein molecules become elongated, disorganised strands. Agents such as soap, detergents, acid, alcohol and some disinfectants disrupt the interchain bond and cause the molecule to be non-functional (Figure . ).
Christian Anfinsen explained denaturation of proteins by heat treatment leading to breakage of non-covalent bond. _SH _ S-S (b) Protein thermal lrreversible denaturatio (a) _ soluble Albumen Water Insoluble Native albumen Denaturation Crosslinking Figure . : Protein denaturation . .
Protein Bonding There are four types of chemical bonds C CH CH CH CH CH H C H C CH O H OH O Hydrogen bond CH CH S S CH CH CH CH C CH O O NH Ionic bond Disulfide bond Polypeptide backbone Hydrophobic Interactions and van der Waals Interactions Figure . : Protein bonding Hydrogen Bond It is formed between some hydrogen atoms of oxygen and nitrogen in polypeptide chain. The hydrogen atoms have a small positive charge and oxygen and nitrogen have small negative charge. Opposite charges attract to form hydrogen bonds.
Though these bonds are weak, large number of them maintains the molecule in 3D shape (Figure . ). Ionic Bond It is formed between any charged groups that are not joined together by peptide bond. It is stronger than hydrogen bond and can be broken by changes in pH and temperature.
- - . . PM Disulfide Bond Some amino acids like cysteine and methionine have sulphur. These form disulphide bridge between sulphur atoms and amino acids.
Hydrophobic Bond This bond helps some protein to maintain structure. When globular proteins are in solution, their hydrophobic groups point inwards away from water. . .
Test for Proteins The biuret test is used as an indicator for presence of protein as it gives a purple colour in the presence of peptide bonds (–C–N–). To protein solution, an equal quantity of sodium hydroxide solution is added and mixed. Then a few drops of . % copper (II) sulphate is added with gentle mixing.