Chiral and achiral macrocyclic copper(II) complexes: Synthesis, characterization, and comparative binding studies with calf-thymus DNA

The new chiral macrocyclic complexes [1,2-bis(1H-benzimidazol-2-yl)-1-(1,8-dihydro-1,3,5,8,10,12-hexaazacyclotetradecane)-2-hydroxyethanolate] copper(II) and -nickel(II) perchlorate, 3 and 4, respectively, were synthesized by the reaction of 1,2-bis(1H-benzimidazol-2-yl)ethane-1,2-diol (L) and (1,8-dihydro-1,3,5,8,10,12-hexaazacyclotetradecane)copper(II) and -nickel(II) diperchlorate complexes, 1 and 2, respectively. All complexes were characterized by various spectroscopic techniques. Molar-conductance measurements showed that all of the complexes are ionic in nature. In complexes 3 and 4, the metal center is encapsulated by the ligand L in a pentacoordinated environment. The optical-rotation values ([alpha](D)) of 3 and 4 at 25 degrees indicate that the complexes are chiral. Absorption- and fluorescence-spectral studies, cyclic voltammetry, and viscosity measurements have been carried out to assess the comparative binding of complexes 1 and 3 with calf thymus (CT)-DNA. Analysis of the results suggests that the new chiral complex 3 binds to CT-DNA through a partial intercalation mode that is different from the binding mode of parent achiral complex 1. The complexes 1 and 3 bind to CT-DNA with binding constants K(b) of 2.7 x 10(4) and 6.6 x 10(4) M(-1), respectively. Circular-dichroism (CD) studies have been further employed to ascertain the binding mode of complex 3, which is consistent with the other spectral studies.