In the representation of the di-peptide shown below, the superscript ‘-1’ denotes the atom of the previous amino acid while ‘+1’ denotes the atom of the next amino acid.

The atomic coordinates of how many AND which of the following atoms are required to uniquely define the torsion angles, ϕ and ψ of the Ramachandran plot?
​

​​

Correct option is A

The Ramachandran plot is used to study the conformational angles of polypeptide chains. The torsion angles ϕ (phi) and ψ (psi) are defined by the backbone atoms of the peptide bond. These angles are crucial for determining the secondary structure of proteins.

1. Definition of ϕ (phi) angle:

• The ϕ angle describes the rotation around the N-Cα bond.
• It is measured using four atoms: C⁻¹, N, Cα, and C.
  • C⁻¹ (carbonyl carbon of the previous amino acid)
  • N (amide nitrogen of the current residue)
  • Cα (alpha carbon of the current residue)
  • C (carbonyl carbon of the current residue)

2. Definition of ψ (psi) angle:

• The ψ angle describes the rotation around the Cα-C bond.
• It is measured using four atoms: N, Cα, C, and N⁺¹.
  • N (amide nitrogen of the current residue)
  • Cα (alpha carbon of the current residue)
  • C (carbonyl carbon of the current residue)
  • N⁺¹ (amide nitrogen of the next residue)

Information Booster:

1. Peptide bonds are planar due to partial double-bond character, restricting rotation.
2. ϕ and ψ angles determine secondary structures like α-helices and β-sheets.
3. Glycine has a larger allowable Ramachandran space due to its small side chain (H).
4. Proline restricts ϕ rotation because of its cyclic structure.
5. Trans conformation is more stable than cis for peptide bonds (except in proline).
6. Steric hindrance affects the allowed regions of the Ramachandran plot.
7. Proteins fold into stable conformations by optimizing ϕ and ψ angles.