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

Phosphodiester

The correct answer is option 3. Phosphodiester.

Let us delve into each type of bond mentioned and their relevance to fibrous proteins:

1. Disulfide Bonds

Disulfide bonds are covalent bonds formed between two cysteine residues within a protein chain. Cysteine is an amino acid that contains a thiol \((-SH)\) group. When two cysteine residues come close together in the protein structure, the thiol groups can undergo oxidation, forming a disulfide bond \((-S-S-)\). Disulfide bonds play a crucial role in stabilizing the tertiary and quaternary structures of proteins. In fibrous proteins, such as keratin and collagen, disulfide bonds contribute to their structural stability and strength. For example, in keratin (found in hair and nails), disulfide bonds between adjacent protein chains create strong cross-links that give keratin its tough, insoluble properties.

2. Hydrogen Bonds

Hydrogen bonds are weak electrostatic attractions between a hydrogen atom covalently bonded to an electronegative atom (such as oxygen or nitrogen) and another electronegative atom nearby. Hydrogen bonds are critical for stabilizing the secondary and tertiary structures of proteins. In fibrous proteins, hydrogen bonds help maintain the regular, extended conformation of the protein chains. For example, in collagen (a fibrous protein in connective tissues), hydrogen bonds between adjacent chains contribute to the triple helical structure and overall stability of the protein.

3. Peptide Bonds

Peptide bonds are covalent bonds formed between the carboxyl group of one amino acid and the amino group of another amino acid, resulting in the formation of a peptide bond \((-CO-NH-)\). Peptide bonds form the backbone of all proteins, including fibrous proteins. In fibrous proteins, peptide bonds link amino acid residues in a linear sequence, forming long, repetitive protein chains. These chains can then fold into specific structures (such as alpha helices or beta sheets) depending on the protein's function and environment.

4. Phosphodiester Bonds

Phosphodiester bonds are covalent bonds that link nucleotides in DNA and RNA molecules. They form between the phosphate group of one nucleotide and the hydroxyl group on the sugar of another nucleotide, creating the sugar-phosphate backbone of the nucleic acid. Phosphodiester bonds are specific to nucleic acids (DNA and RNA) and are not found in proteins, including fibrous proteins. They play a crucial role in the structure and function of nucleic acids by forming the linear backbone that supports the sequence of nucleotide bases.

Conclusion: Based on the detailed explanations:

Disulfide bonds, hydrogen bonds, and peptide bonds are all found in fibrous proteins and play essential roles in their structure and stability. Phosphodiester bonds are unique to nucleic acids (DNA and RNA) and are not present in proteins, including fibrous proteins. Therefore, the bond that is not found in fibrous proteins is Phosphodiester. This distinction underscores the different structural requirements and roles of bonds in proteins versus nucleic acids.