Part (i): Peptide bond
Step 1: Proteins are polymers of \(\alpha\)-amino acids, and the link that strings these monomers together is the peptide bond.
Step 2: Chemically it is an amide group \(-\text{CO}-\text{NH}-\). It appears when the \(-\text{COOH}\) of one amino acid reacts with the \(-\text{NH}_2\) of another, throwing out a molecule of water (a dehydration/condensation step). Two amino acids so joined give a dipeptide; repeating the step gives tri-, tetra- and finally polypeptides.
Step 3: Thus the peptide bond forms the backbone of every protein chain.
Part (ii): Denaturation
Step 1: A protein in its natural, working form is called native protein. Denaturation is the loss of this natural form and of biological function.
Step 2: Agents such as heat, strong acids or bases, alcohol, or salts of heavy metals disrupt the hydrogen bonds and disulphide/electrostatic interactions holding the coiled secondary and folded tertiary structure. The chain unfolds, but the covalent peptide bonds of the primary structure are not broken, so the amino-acid sequence survives.
Step 3: Everyday evidence: boiling an egg turns the clear runny albumin white and solid, and milk turns to curd. Because the folded shape cannot re-form, the change is usually permanent (irreversible).