Which of the following bonds is not found in fibrous proteins?

The correct answer is Option (3) → Phosphodiester linkage

In fibrous proteins the polypeptide chains run parallel and are held together by hydrogen bonds and disulphide bonds. Each chain itself is made of amino acids joined by peptide bonds. A phosphodiester linkage joins nucleotides in nucleic acids, not amino acids in proteins.

Option (1) Disulphide linkage — Found in fibrous proteins. –S–S– bonds hold the parallel chains together.

Option (2) Hydrogen bond — Found in fibrous proteins. Hydrogen bonds also hold the parallel chains together.

Option (3) Phosphodiester linkage — Not found in fibrous proteins. It links the 5′ and 3′ carbon atoms of the pentose sugars of nucleotides in DNA and RNA.

Option (4) Peptide bond — Found in fibrous proteins. The –CO–NH– bond joins the amino acids in every polypeptide chain.

NCERT (2026-27 edition):
(a) Fibrous proteins – “When the polypeptide chains run parallel and are held together by hydrogen and disulphide bonds, then fibre-like structure is formed. Such proteins are generally insoluble in water.”

10.5.2 Structure of Nucleic Acids – “Nucleotides are joined together by phosphodiester linkage between 5′ and 3′ carbon atoms of the pentose sugar.”