SYNTHESES OF MACROCYCLIC ENZYME MODELS .8. CONFORMATIONAL MOBILITY AND MOLECULAR RECOGNITION BY THE INTERNAL CAGE OF KYUPHANE

Temperature-dependent H-1 NMR measurements in CDCl3-CS2 (3:7 v/v) and DMF-d7 in concert with MM2 conformational analysis reveal that the molecular framework of "Kyuphane" (1), a cage-type cubical molecule with six faces each consisting of a 2,11,20,29-tetraaza[3.3.3.3]paracyclophane ring, is significantly more rigid than the corresponding noncage host (2), but still conformationally flexible. A slow rate of interconversion among degenerated conformers of the lowest energy C(i) conformation accounts for the observed NMR line broadening. Hosts 1 and 2 are soluble in acidic aqueous media below pH 4 and behave as polycationic species. Electrostatic field solvation analysis of the tetraprotonated salt of Kyuphane suggests a square-planar proton placement (11a) to predominate in solution. Line broadening for the tetracation appears to arise from both conformer interconversion and proton exchange. Guest recognition behavior of these hosts under acidic conditions was studied by means of H-1 NMR and fluorescence spectroscopy. Kyuphane demonstrates a pH-dependent guest-binding ability due to changes in the specific microenvironmental polarity of its three-dimensional cavity upon variable protonation of the nitrogen atoms. The host also shows size-sensitive and regioselective molecular discrimination originating from the semirigid geometry of the hydrophobic cavity and the specific protonation geometry. The specific molecular discrimination evidenced by 1 was analyzed by MM2 molecular mechanics and applied to selective transport of hydrophobic molecules between organic phases across an aqueous phase in which 1 was present as a carrier. It is noteworthy that the proton NMR signals of guest molecules naphthalene-2,6-disulfonate, 8-anilinonaphthalene-1-sulfonate, and 6-p-toluidinonaphthalene-2-sulfonate completely disappear upon complexation with Kyuphane, whereas the identical guest naphthalene-2,6-disulfonate shows normal upfield shifts of its NMR proton signals upon complexation with 2.