The Antimicrobial Activity of Mononuclear Ruthenium(II) Complexes Containing the dppz Ligand

The cis-α isomer of [Ru(bb₇ )(dppz)]²⁺ (dppz=dipyrido[3,2-a:2',3'-c]phenazine; bb₇ =bis[4(4'-methyl-2,2'-bipyridyl)]-1,7-alkane) has been synthesised. The minimum inhibitory concentrations and the minimum bactericidal concentrations of [Ru(bb₇ )(dppz)]²⁺ and its parent complex [Ru(phen)₂ (dppz)]²⁺ (phen=1,10-phenanthroline) were determined against a range of bacteria. The results showed that both ruthenium complexes exhibited good activity against Gram-positive bacteria, but [Ru(bb₇ )(dppz)]²⁺ showed at least eightfold better activity against the Gram-negative bacteria than [Ru(phen)₂ (dppz)]²⁺ . Luminescence assays demonstrated that [Ru(bb₇ )(dppz)]²⁺ accumulated in a Gram-negative bacterium to the same degree as in a Gram-positive species, and assays with liposomes showed that [Ru(bb₇ )(dppz)]²⁺ interacted more strongly with membranes than the parent [Ru(phen)₂ (dppz)]²⁺ complex. The DNA binding affinity for [Ru(bb₇ )(dppz)]²⁺ was determined to be 6.7 × 10⁶ m^-1 . Although more toxic to eukaryotic cells than [Ru(phen)₂ (dppz)]²⁺ , [Ru(bb₇ )(dppz)]²⁺ exhibited greater activity against bacteria than eukaryotic cells.