Ternary iron(II) complexes of phenanthroline bases showing DNA binding and cleavage activity

Abstract

Ternary iron(II) complexes [Fe(L¹)B](PF₆)₂ (1-3), where L¹ is formed from FeCl₃ mediated cyclization of N,N'-(2-hydroxypropane-1,3-diyl)bis(pyridine-2-aldimine) (L²) and B is a phenanthroline base, viz. 1,10-phenanthroline (phen, 1), dipyrido[3,2-d:2',3'-f]quinoxaline (dpq, 2) and dipyrido[3,2-a:2',3'-c]phenazine (dppz, 3), are synthesized and complex 1 is structurally characterized by X-ray crystallography. The diamagnetic iron(II) complexes with L¹ and B showing respective tetradentate and bidentate chelating modes in a FeN₆ coordination geometry exhibit two metal-centered visible electronic bands near 520 and 570 nm in MeCN. The complexes bind to calf thymus DNA giving an order: phen (1) < dpq (2) < dppz (3). Control DNA cleavage experiments using supercoiled (SC) pUC19 DNA and distamycin-A suggest major groove binding for the dppz and minor groove binding for the other complexes. The redox active complexes cleave SC DNA to its nicked circular (NC) form with H₂O₂ involving hydroxyl radical as the cleavage active species. The complexes show photo-induced DNA cleavage activity at UV light of 365 nm (12 W) in absence of H₂O₂ via singlet oxygen (¹O₂) mechanism. Complexes 2 and 3 having respective photoactive dpq and dppz ligands show relatively poor DNA cleavage activity at visible light.