Cu(ii)-induced twisting of the biphenyl core: exploring the effect of structure and coordination environment of biphenyl-based chiral copper(ii) complexes on interaction with calf-thymus DNA

Three new clip-like receptors (H2L1, H2L2, and H2L3) have been synthesized based on a biphenyl moiety consisting of two amide and Schiff base linkages, and used to prepare the corresponding copper(ii) complexes, [Cu-2(HL1)(py)(2)(H2O)](ClO4) (1), [Cu-2(L-2)(py)(2)] (2), and [Cu-2(HL3)(py)(2)](ClO4) (3). Interestingly, upon complexation with Cu2+, the two arms of the biphenyl moiety of the ligands are forced to approach each other, thus considerably reducing the dihedral angle between the two phenyl rings with respect to the conformation assumed in the free ligands. The reason behind the formation of this type of sterically hindered structures has been explained using theoretical calculations. The solution state structures of the copper complexes were found to be almost the same as those detected in the solid state. Furthermore, the interaction between the complexes with calf-thymus DNA (ct-DNA) has been investigated. Among the three, complex 3 exhibits the strongest interaction with ct-DNA because of its structure and coordination environment, whereas complex 1 does not interact with ct-DNA. The binding mode of complex 3 with DNA was found to be mainly intercalative in nature and the interaction has been explored using optical melting and viscometric study. The binding process of 3 is forced mainly by electrostatic interactions, whereas for 2, non-electrostatic interactions appear to play a vital role. It is worth mentioning that the interaction between complex 3 and ct-DNA is governed by a high binding constant, a negative enthalpy change and a positive entropy change. CD measurements have also been performed to investigate the structural changes of ct-DNA.