Spectroscopic and voltammetric studies of copper(II) complexes of bis(pyrid-2-yl)-di/trithia ligands bound to calf thymus DNA

Abstract

Absorption and circular dichroic (CD) spectral, viscometric and electrochemical studies have been carried out on the interaction of [Cu(pdto)(ClO₄)] (ClO₄) (pdto = bis(pyrid-2-yl-3,6-dithiaoctane), [Cu(pttu)] (ClO₄)₂ (pttu = bis(pyrid-2-yl-3,6,9-trithiaundecane) and [Cu(pttn)] (ClO₄)₂ (pttn = bis(pyrid-2,yl)-3,6,9-trithianonane) with calf thymus DNA. The intense S(σ ) → Cu(II) charge transfer band of almost all these complexes exhibits a large hypochromism in the presence of DNA suggesting that all the complexes undergo ligand displacement in the presence of DNA leading to stabilisation of the Cu²⁺-DNA interaction. Spectral, viscosity and electrochemical studies suggest that the [Cu(pdto)]²⁺ complex binds to DNA presaturated with copper(II) perchlorate (Cu²⁺-DNA, [NP]/[Cu(ClO₄)₂] ~-2.2). via partial intercalation of the pyridine ring of the ligand. On the other hand, [Cu(pttu)]²⁺ and [Cu(pttn)]²⁺ engage in relatively weak binding with Cu²⁺-DNA via simple surface contact. CD spectral studies suggest negligible transconformational changes for Cu²-DNA from a more B-like to a more C-like structure on binding to all the complexes. The diaza analogue of [Cu(pdto)]²⁺ is stable in the presence of DNA due to the hydrogen bonding interaction of the coordinated NH with DNA. The residual charge on Cu(II), the coordination geometry and the ligand donor atom type play a key role in deciding the mode and extent of binding of complexes to DNA.