Arrange the three residues, glycine, phenylalanine and proline, in the decreasing order of backbone flexibility

Arrange the three residues, glycine, phenylalanine and proline, in the decreasing order of backbone flexibility

(1) Gly > Phe > Pro

(2) Pro > Gly > Phe

(3) Phe > Pro > Gly

(4) It is not possible to comment

Backbone flexibility in amino acids depends on the torsional freedom of the main chain (Φ, Ψ angles).

Analyzing each residue:

1. Glycine (Gly)

   • Most flexible due to the absence of a bulky side chain (R = H).
   • Can adopt a wide range of Φ and Ψ angles.
   • Appears frequently in turns and flexible regions.

2. Phenylalanine (Phe)

   • Has a bulky benzyl side chain but does not significantly restrict backbone flexibility.
   • Still relatively flexible compared to rigid residues like Proline.

3. Proline (Pro)

   • Least flexible because its side chain forms a ring with the backbone, restricting the Φ angle (~ -60°).
   • Has limited conformational freedom, often found in turns and cis-peptide bonds.

Arranging in decreasing order of flexibility:

Glycine > Phenylalanine > Proline

Thus, the correct answer is:
(1) Gly > Phe > Pro