During the denaturation of proteins, which of these structures will remain intact?

We need to identify which structural level of protein remains intact during denaturation.

Proteins have four structural levels: the primary structure, which is the linear sequence of amino acids held together by peptide bonds (covalent bonds); the secondary structure, consisting of local folding patterns ($$\alpha$$-helix, $$\beta$$-sheet) stabilized by hydrogen bonds; the tertiary structure, referring to the overall 3D shape stabilized by hydrogen bonds, disulfide bridges, hydrophobic interactions, and ionic bonds; and the quaternary structure, which is the arrangement of multiple polypeptide subunits.

Denaturation involves the disruption of the secondary, tertiary, and quaternary structures of a protein by external agents (heat, acids, bases, organic solvents, etc.) that break the weak interactions (hydrogen bonds, ionic bonds, hydrophobic interactions) and sometimes disulfide bonds maintaining the higher-order structures.

During denaturation, the primary structure remains intact because the peptide bonds (covalent bonds) between amino acids are strong and are not broken by denaturing agents, while the higher-order structures are disrupted.

Hence, the correct answer is Option A.