Carbonyl-carbonyl (CO···CO) n→π* interaction often coexists with hydrogen bond or another n→π* interaction. However, cooperativity of n→π* interactions with other interactions or among themselves is not studied. Herein, we have synthesized a set of proline-diacylhydrazine (Pro-DAH) hybrid molecules and established a synergistic relationship between n→π* interaction in their Pro side and n_N→π*_Ar interaction in the DAH side by 1D, 2D NMR and theoretical studies.¹ Wennemers and coworkers showed that hydrogen bond donors at the γ-position (S-chirality) of the proline ring polarized the C-terminal CO via hydrogen bonding, which enhanced CO···CO interaction from the N- to C-terminal CO leading to trans proline stabilization.² When such a modified proline was incorporated in an oligo-Pro, it stabilized all-trans amide in PPII helix. These studies inspired us to study the effect of incorporating a DAH unit at the C-terminus of an oligo-Pro. We observed that these peptides adopted all-trans amide polyproline II (PPII) helical conformations in propanol as well as aqueous medium. This is unusual as propanol is known to stabilize oligo-Pro in all-cis amide PPI structure. We propose that this synergistic n→π* and n_N→π*_Ar interactions at the C-termini of these oligo-Pro induced cooperative CO···CO interactions by polarizing neighboring CO groups that enforced all the proline rings to be locked in trans conformation. As a result, this cascade effect or domino effect of n → π* interactions from the C-terminus to the N-terminus enforces the overall stabilization of polyproline in PPII helical geometry. Our study indicates that just by controlling the amide geometry of the C-terminal Proline, we can control the secondary structure of a proline oligomer.