Synergistic n → π* and n N → π* Ar interactions in C-terminal modified prolines: effect on Xaa-Pro cis / trans equilibrium.

Carbonyl-carbonyl (CO⋯CO) n → π* interaction often coexists with a hydrogen bond (HB) or another n → π* interaction. Although the interplay between HB and n → π* interaction was previously studied, there is no systematic investigation that shows a synergistic relationship of n → π* with another noncovalent interaction. Herein, we have studied a set of proline-diacylhydrazine (Pro-DAH) molecules and observed that increase in the strength of the n N → π* Ar interaction on their DAH side strengthened the n → π* interaction on the Pro side, which was experimentally determined by measuring the K trans / cis of the Xaa-Pro amide bond. Overall, we describe a simple C-terminal modification strategy to stabilize the trans -Pro geometry that could be useful to stabilize PPII helices and collagen triple helices that require Pro to adopt the trans amide geometry.