Evidence for stereospecific collision complexes of cyclo(tri‐ L ‐prolyl) with benzene

NMR spectra of cyclo(tri‐ L ‐prolyl), c‐(P) 3 , show large shifts of the H α resonance on adding C 6 D 6 to a solution of c‐(P) 3 in CD 2 Cl 2 . CPK models and observed coupling constants indicate a rigid c‐(P) 3 conformation, independent of solvent composition, suggesting that these shifts result from formation of stereospecific C 6 D 6 –c‐(P) 3 collision complexes in which the c‐(P) 3 H α lie near the face of the aromatic ring. The temperature dependence of the H α shifts and the solvent dependent shifts observed on adding toluene‐ d 8 or nitrobenzene‐ d 5 to the c‐(P) 3 solution suggest that preferred C 6 D 6 –c‐(P) 3 orientations result from attractive interactions between the electron‐rich aromatic ring and the electropositive H α 's and/or δ + nitrogen atoms in the peptide backbone. Reports of such interactions in increasingly diverse peptide model systems suggest that they may play a role in stabilizing protein structures.