VIVO and CuII complexation by ligands based on pyridine nitrogen donors

The binary and ternary systems formed by (VO)-O-IV and Cu-II ions with ligands (L) based on the pyridine ring (1,10-phenanthroline (phen), 2,2'-bipyridine (bpy), 2,2': 6',2 ''-terpyridine (terpy), 2,2'-bipyrimidine (bpm) and 2,3-bis(2-pyridyl)pyrazine (bpp)) were studied, combining spectroscopic (EPR and UV-vis), pH-potentiometric and computational (DFT calculations) methods. In the systems with (VO)-O-IV, the formation of mono-chelated complexes with equatorial-equatorial and equatorial-axial (phen, bpy, bpm and bpp) or equatorial-equatorial-equatorial and equatorial-axial-equatorial coordination (terpy) and bis-chelated species with cis-octahedral geometry, with a water or a hydroxido ion in the fourth equatorial position, is demonstrated. Phen, bpy, bpm and bpp form also a dinuclear [(VO)(2)L-2(H2O)(2)(OH)(2)](2+) complex with an anti-coplanar arrangement of the two (VO)-O-IV ions and a ferromagnetic coupling between the metal ions. Due to the low basicity of the nitrogen donors, the potentially tetradentate 2,2'-bipyrimidine and 2,3-bis(2-pyridyl)pyrazine behave like simple bidentate ligands, and in the ternary systems with 2,2'-bipyridine the expected dinuclear species, in which the former ligands would act as a bridge between the two metal ions using all four nitrogen donors, are not formed. The interaction of phen and Cu-II-phen complexes with human serum albumin (HSA) was also studied at pH 7.4 by circular dichroism and EPR spectroscopy, the formation of several HSA-Cu-II-phen containing species being confirmed and their binding constants determined.