Synergistic n → π* and n → π* interactions in C-terminal modified prolines: effect on Xaa-Pro / 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 → π* interaction on their DAH side strengthened the n → π* interaction on the Pro side, which was experimentally determined by measuring the of the Xaa-Pro amide bond.