Novel ruthenium(II) polypyridyl complexes conjugated with bis-dipicolylamine as antibacterial photosensitisers

The inexorable and rapid rise of antimicrobial resistance in various Gram-positive and Gram-negative bacteria strains is severely compromising our global healthcare system. Facing a diminishing quantity of effective antibiotics against the threat, alternative therapeutics and drugs such as photodynamic therapy and antibacterial photosensitisers are vital in treating bacterial infections. Multiple reports of ruthenium(II) polypyridyl complexes possessing antibacterial activities make them attractive candidates as potential antibacterial photosensitisers. The present work reports the synthesis of a series of novel ruthenium(II) polypyridyl complexes conjugated with bis-dipicolylamine derivatives and their assessments as membrane-targeting antibacterial photosensitisers. Of the four conjugated complexes synthesised, conjugated complexes 4a and 4b were found to be bactericidal against methicillin-susceptible Staphylococcus aureus (ATCC 35923) and bacteriostatic against methicillin-resistant Staphylococcus aureus (MRSA) (ATCC 43300) strains with a minimum inhibition concentration of 64 µM under irradiated conditions. Conjugated complexes 4a, 4b and 4d were active against Gram-negative bacteria, Escherichia coli (ATCC BAA-196) at 128 µM in both irradiated and non-irradiated conditions. The conjugated complexes 4a and 4b were observed to have some bacterial interaction upon excitation at 559 nm wavelength viewed under fluorescence microscope at ×600 nm magnification. The findings demonstrated the capability of ruthenium(II) polypyridyl complexes conjugated with bis-dipicolylamine as possible theranostics agents due to their fluorescence properties and their antibacterial activities through photosensitisation.