Equilibrium studies on the protonation and Cu(II) complexation by an hexaaza macrocycle containing p -xylyl spacers. The crystal structure of the hexaprotonated ligand and the kinetics of decomposition of the Cu(II) complexes
Abstract The protonation constants of the ligand 3,6,9,16,19,22-hexaazatricyclo[22.2.2.211,14]triaconta-1(26),11(12),13,24,27, 29-hexaene (L, BPXD) have been determined in water at 25°C by potentiometric procedures and indicate a lower basicity than that corresponding to the analogue ligand with m-xylyl spacers between the diethylenetriamine subunits (BMXD). The crystal structure of a salt of H6L6+ containing Br− and NO3 − as anions has been solved by X-ray diffraction procedures and reveals that the macrocycle adopts an almost planar configuration. The protonated amine groups are involved in a complex network of hydrogen bonds with the anions and water molecules, with two anions being placed close to the cavity of the macrocycle. Despite the lower basicity of BPXD, the stability of the mononuclear CuL2+ and HCuL3+ complexes is several log units higher than that corresponding to the analogous BMXD complexes, which suggests the possibility of a different coordination mode for both closely related ligands. Actually, the whole set of stability constants for the mono and binuclear Cu(II)–L complexes can be rationalised by considering that the macrocycle acts as tetradentate in the mononuclear species. In contrast, the kinetic parameters for decomposition of the mono and binuclear Cu(II)–L complexes in acid solutions are only slightly different from those previously determined for the BMXD complexes.
Molecular recognition. 1. Crystal structures of hexaazamacrocyclic amines containing p-xylylene spacers and their adducts with acids
The macrocyclic amine, 1,5,9,18,22,26-hexaaza [11.11]-p-cyclophane (1) contains two dipropylenetriamine units which make the molecule highly basic. Owing to this basicity, 1 can be highly protonated in acidic media and can form supramolecular assemblies with different anions. Two new supramolecular structures arise from self-assembly of the salt of 1 with fumaric acid and of the N-methyl derivative of 1.
Molecular diversity of skin mucus lectins in fish.
Among lectins in the skin mucus of fish, primary structures of four different types of lectin have been determined. Congerin from the conger eel Conger myriaster and AJL-1 from the Japanese eel Anguilla japonica were identified as galectin, characterized by its specific binding to beta-galactoside. Eel has additionally a unique lectin, AJL-2, which has a highly conserved sequence of C-type lectins but displays Ca(2+)-independent activity. This is rational because the lectin exerts its function on the cutaneous surface, which is exposed to a Ca(2+) scarce environment when the eel is in fresh water. The third type lectin is pufflectin, a mannose specific lectin in the skin mucus of pufferfish Takifugu rubripes. This lectin showed no sequence similarity with any known animal lectins but, surprisingly, shares sequence homology with mannose-binding lectins of monocotyledonous plants. The fourth lectin was found in the ponyfish Leiognathus nuchalis and exhibits homology with rhamnose-binding lectins known in eggs of some fish species. These lectins, except ponyfish lectin, showed agglutination of certain bacteria. In addition, pufflectin was found to bind to a parasitic trematode, Heterobothrium okamotoi. Taken together, these results demonstrate that skin mucus lectins in fish have wide molecular diversity.