Which structure of protein remains intact after coagulation of egg white on boiling?

We need to identify which level of protein structure remains intact after coagulation (denaturation) of egg white on boiling. Since proteins have four levels of structure, it is helpful to recall their definitions: (i) Primary structure is the sequence of amino acids in the polypeptide chain, held together by covalent peptide bonds ($$-CO-NH-$$). (ii) Secondary structure refers to local folding patterns such as $$\alpha$$-helices and $$\beta$$-sheets, which are stabilized by hydrogen bonds between backbone atoms. (iii) Tertiary structure is the overall three-dimensional shape of the protein, stabilized by hydrophobic interactions, hydrogen bonds, ionic bonds, and disulfide bridges. (iv) Quaternary structure arises from the arrangement of multiple polypeptide subunits, stabilized by non-covalent interactions.

When egg white is boiled, the protein albumin undergoes denaturation, during which the secondary, tertiary, and quaternary structures are disrupted because the non-covalent interactions—hydrogen bonds, hydrophobic interactions, and ionic bonds—that maintain these structures are broken by heat.

However, the primary structure remains intact because it depends on strong covalent peptide bonds, and breaking these bonds requires hydrolysis (for example, with strong acids or specific enzymes) rather than just the temperature of boiling water (100°C).

Therefore, the correct answer is Option (1): Primary.