Recall the levels of protein structure

Proteins have four main levels of structure:

• Primary structure: the linear sequence of amino acids joined by peptide bonds.
• Secondary structure: local folding such as $\alpha$-helix and $\beta$-pleated sheet, mainly stabilized by hydrogen bonding.
• Tertiary structure: overall 3D folding of a single polypeptide chain.
• Quaternary structure: arrangement of multiple polypeptide subunits.

Effect of heating on proteins

Heating generally causes denaturation of proteins. Denaturation disrupts weaker interactions such as:

• hydrogen bonds
• ionic interactions
• hydrophobic interactions

Therefore, heating can destroy:

• secondary structure
• tertiary structure
• quaternary structure

What remains unaffected?

The primary structure is held together by strong covalent peptide bonds. Ordinary heating does not break peptide bonds during denaturation.

So, the structure unaffected by heating is the primary structure.

Option-wise check

• A: secondary structure $\rightarrow$ affected by heating ❌
• B: tertiary structure $\rightarrow$ affected by heating ❌
• C: primary structure $\rightarrow$ unaffected by heating ✅
• D: quaternary structure $\rightarrow$ affected by heating ❌

Final answer

$\boxed{\text{C: primary structure}}$