Mixed ligand ruthenium(II) complexes of 5,6-dimethyl-1,10-phenanthroline: The role of ligand hydrophobicity on DNA binding of the complexes - Analytical Methods & Tools for Lyophilization

Mixed ligand ruthenium(II) complexes of 5,6-dimethyl-1,10-phenanthroline: The role of ligand hydrophobicity on DNA binding of the complexes
Received 23 March 2006; revised 13 July 2006; accepted 22 July 2006. Available online 1 August 2006.
Palanisamy Uma Maheswaria, Venugopal Rajendirana, Mallayan Palaniandavara, , , Reji Thomasb and G.U. Kulkarnib
Inorganica Chimica Acta
Volume 359, Issue 14 , 1 November 2006
ScienceDirect
Copyright ? 2006 Elsevier B.V. All rights reserved.
aSchool of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, India
bChemistry and Physics of Materials Unit, Jawaharlal Nehru Centre For Advanced Scientific Research, Bangalore 560 064, India
Abstract
A series of mixed ligand Ru(II) complexes of 5,6-dimethyl-1,10-phenanthroline (5,6-dmp) as primary ligand and 1,10-phenanthroline (phen), 2,2'-bipyridine (bpy), pyridine (py) and NH3 as co-ligands have been prepared and characterized by X-ray crystallography, elemental analysis and 1H NMR and electronic absorption spectroscopy. The X-ray crystal structure of the complex [Ru(phen)2(bpy)]Cl2 reveals a distorted octahedral coordination geometry for the RuN6 coordination sphere. The DNA binding constants obtained from the absorption spectral titrations decrease in the order, tris(5,6-dmp)Ru(II) > bis(5,6-dmp)Ru(II) > mono(5,6-dmp)Ru(II), which is consistent with the trend in apparent emission enhancement of the complexes on binding to DNA. These observations reveal that the DNA binding affinity of the complexes depend upon the number of 5,6-dmp ligands and hence the hydrophobic interaction of 5,6-dimethyl groups on the DNA surface, which is critical in determining the DNA binding affinity and the solvent accessibility of the exciplex. Among the bis(5,6-dmp)Ru(II) complexes, those with monodentate py (4) or NH3 (5) co-ligands show DNA binding affinities slightly higher than the bpy and phen analogues. This reveals that they interact with DNA through the co-ligands while both the 5,6-dmp ligands interact with the exterior of the DNA surface. All these observations are supported by thermal denaturation and viscosity measurements. Two DNA binding modes ? surface/electrostatic and strong hydrophobic/partial intercalative DNA interaction ? are suggested for the mixed ligand complexes on the basis of time-resolved emission measurements. Interestingly, the 5,6-dmp ligands promote aggregation of the complexes on the DNA helix as a helical nanotemplate, as evidenced by induced CD signals in the UV region. The ionic strength variation experiments and competitive DNA binding studies on bis(5,6-dmp)Ru(II) complexes reveal that EthBr and the partially intercalated and kinetically inert [Ru(phen)2(dppz)]2+ (dppz = dipyrido[3,2-a:2',3'-c]phenazine) complexes revert the CD signals induced by exciton coupling of the DNA-bound complexes with the free complexes in solution.
Graphical abstract
The DNA binding affinities of the mixed ligand complexes of the types [Ru(5,6-dmp)2(diimine)]2+and [Ru(diimine)2(5,6-dmp)]2+depend upon the number of 5,6-dmp ligands. The latter exhibit hydrophobic interaction with DNA and promote aggregation of the complexes on the DNA helix as a helical nanotemplate, as evident from induced CD signals.
Keywords: Ru(II) complexes; Mixed ligands; Circular dichroism; Emission; Exciton coupling
You can view the abstract online. A subscription is required to view the full text or it can be purchased online.