A hydrogen-bonding-modulated molecular rotor: environmental effect in the conformational stability of peptidomimetic macrocyclic cyclophanes.

The conformational behavior of 16- to 18-membered ring peptidomimetic p-cyclophanes 1a,b-3a,b has been studied by NMR. The cycles bearing 16 and 17 atoms showed a dynamic process within the NMR time scale, produced by the rotation of the aromatic p-diphenylene moiety with respect to the macrocyclic main plane. The temperature dependence of 1H NMR spectra has been studied in order to get activation parameters of the energetic barrier for the process (VT-NMR and line shape analysis). The rate of the movement clearly depends on the macrocyclic ring size and the nature of the peptidomimetic side chain. Entropic and enthalpic contributions to the free energy of activation are discussed. The rotation of the aromatic ring is closely related to the intramolecular hydrogen bonding pattern, as suggested by temperature factors of NH chemical shifts (DeltadeltaNH/DeltaT) and molecular modeling. The interconnected roles of the solvation and the intramolecular H-bonds have been established by measurements (VT-NMR and DeltadeltaNH/DeltaT) in environments of different polarities and H-bonding abilities. We concluded that the conformational stability of the systems directly depends on the stability of the intramolecular H-bonding pattern. We finally showed how one of these peptidomimetics behaves as a methanol-dependent artificial molecular rotor. In this simple molecular device, the well-defined molecular rotation is tuned by the competition between intramolecular hydrogen bonds and interactions with the solvent.