Structure, antimicrobial activity, DNA- and albumin-binding of manganese(II) complexes with the quinolone antimicrobial agents oxolinic acid and enrofloxacin.

The reaction of MnCl2 with the quinolone antibacterial drug oxolinic acid (Hoxo) results to the formation of [KMn(oxo)3(MeOH)3]. Interaction of MnCl2 with the quinolone Hoxo or enrofloxacin (Herx) and the N,N'-donor heterocyclic ligand 1,10-phenanthroline (phen) results in the formation of metal complexes with the general formula [Mn(quinolonato)2(phen)]. The crystal structures of [KMn(oxo)3(MeOH)3] and [Mn(erx)2(phen)], exhibiting a 1D polymeric and a mononuclear structure, respectively, have been determined by X-ray crystallography. In these complexes, the deprotonated bidentate quinolonato ligands are coordinated to manganese(II) ion through the pyridone oxygen and a carboxylato oxygen. All complexes can act as potential antibacterial agents with [Mn(erx)2(phen)] exhibiting the most pronounced antimicrobial activity against five different microorganisms. Interaction of the complexes with calf-thymus DNA (CT DNA), studied by UV spectroscopy, has shown that they bind to CT DNA. Competitive study with ethidium bromide (EB) has shown that all complexes can displace the DNA-bound EB indicating their binding to DNA in strong competition with EB. Intercalative binding mode is proposed for the interaction of the complexes with CT DNA and has also been verified by DNA solution viscosity measurements and cyclic voltammetry. DNA electrophoretic mobility experiments suggest that [Mn(erx)2(phen)] binds strongly to supercoiled pDNA and to linearized pDNA possibly by an intercalative manner provoking double-stranded cleavage reflecting in a nuclease-like activity. The complexes exhibit good binding propensity to human or bovine serum albumin protein showing relatively high binding constant values. The binding constants of the complexes towards CT DNA and albumins have been compared to their corresponding zinc(II) and nickel(II) complexes.