Energetics of an nπ* Interaction that Impacts Protein Structure

Abstract
The trans/cis ratio of the amide bond in N-formylproline phenylesters correlates with electron withdrawal by a para substituent. The slope of the Hammett plot (ρ = 0.26) is indicative of a substantial effect. This effect arises from a favorable n → π* interaction between the amide oxygen and ester carbonyl. In a polypeptide chain, an analogous interaction can stabilize the conformation of trans peptide bonds, α-helices, and polyproline type-II helices.