Question

Which of the following is correct concerning a peptide backbone? Choose one: O A. The peptide bond allows for free rotation due to its single bond character. OB. A Ramachandran plot shows the allowable and V angles for a peptide. O C. The torsional angle between the Ca and the Rgroup is called “ (phi). O D. The torsional angle between the amide nitrogen and the Ca is called (psi).

Answer 1

Answer: B

A: The peptide bond may look like it is a single bond, and therefore, free to rotate. However, the peptide bond has partial double bond character due to a resonance structure.

B: Plotting the torsional angles in this way graphically shows which combination of angles are possible. The torsional angles of each residue in a peptide define the geometry of its attachment to its two adjacent residues by positioning its planar peptide bond relative to the two adjacent planar peptide bonds, thereby the torsional angles determine the conformation of the residues and the peptide. Many of the angle combinations, and therefore the conformations of residues, are not possible because of steric hindrance. By making a Ramachandran plot, protein structural scientists can determine which torsional angles are permitted and can obtain insight into the structure of peptides.

C & D: The alpha carbon (Cα) in the center of each amino acid is held in the main chain by two rotatable bonds. The dihedral (torsion) angles of these bonds are called Phi and Psi (in Greek letters, φ and ψ). While the C_a (1) - C (O) - N (H) - C_a (2) atoms are all planar, ψ is the rotation angle about the C_a (1) - C bond and φ is the rotation angle about the N - C_a (2) bond.