A polypeptide segment containing 15 amino‐acid residues can adopt either an α-helix or a β-sheet conformation in aqueous solution. The sequence is rich in alanine and leucine, with no charged side chains.
Answer the following questions about the noncovalent interactions that govern the stability of the two secondary structures.
(a) Identify the principal type of noncovalent interaction that stabilizes the peptide backbone within an α-helix and describe how the geometry of this interaction is established along the helical axis.
(b) Explain how the orientation of the carbonyl and N–H groups in a β-sheet differs from that in an α-helix and discuss the effect of this difference on the strength and directionality of the same type of interaction identified in part (a).
(c) Predict the effect of substituting every fourth residue with proline on the stability of the α-helix. Provide a molecular-level rationale that distinguishes between intramolecular and intermolecular consequences of this substitution.
(d) The peptide exists primarily as an α-helix at 25°C but converts to a β-sheet at 80°C. Account for this temperature dependence in terms of the relative strengths and entropic contributions of the relevant noncovalent interactions.