Photosensitized samarium(III) and erbium(III) complexes of planar N,N-donor heterocyclic bases: crystal structures and evaluation of biological activity

Samarium(III) and erbium(III) complexes, namely [Sm(dpq)(DMF)(2)(H2O)Cl-3] (1), [Sm(dppz)(DMF)(2)(H2O)Cl-3] (2), [Er(dpq)(DMF)(2)Cl-3] (3) and [Er(dppz)(2)Cl-3] (4) (dipyrido[3,2-d:2',3'-f]quinoxaline (dpq in 1 and 3), dipyrido(3,2-a:2',3'-c]phenazine (dppz in 2 and 4), N,N'-dimethylformamide (DMF) and water (H2O)), have been synthesized and structurally characterized. The X-ray crystal structures of complexes 1-4 show discrete mononuclear Ln(III)-based structures. Sm(III) in [Sm(dpq)(DMF)(2)(H2O)Cl-3] (1) and [Sm(dppz)(DMF)(2)(H2O)Cl-3] (2) adopts an eight-coordinated distorted square antiprism structure with a bidentate N,N-donor dpq/dppz ligand, three Cl- anions, two DMF molecules and one water molecule. The Er(III) complexes, [Er(dpq)(DMF)(2)Cl-3] (3) and [Er(dppz)(2)Cl-3] (4), show a seven-coordinated monocapped octahedral structure where Er(III) is coordinated to a bidentate dpq/dppz ligand, two DMF molecules and three Cl- anions. The crystal lattices of the complexes show intermolecular pi-pi stacking interactions between the planar dpq and dppz ligands. Considering the planarity and photosensitizing ability of the coordinated dpq and dppz ligands, the complexes were studied for their binding interaction with DNA and proteins and their photoinduced DNA cleavage activity. They display significant binding propensity to CT-DNA (K-b similar to 10(4) M-1) in the order: 2, 4 (dppz) > 1, 3 (dpq). Complexes 1-4 bind DNA through groove binding and partial intercalation. All the complexes also show binding propensity (K-BSA similar to 10(5) M-1) to bovine serum albumin (BSA) protein. Complexes 1-4 efficiently cleave supercoiled (SC) ds-DNA to its nicked circular (NC) form upon exposure to UV-A light of 365 nm via formation of singlet oxygen (O-1(2)) and a hydroxyl radical (HO center dot) as reactive oxygen species in a photoredox pathway.