Quinine and Quinidine Derivatives as Photosensitizers for Photodynamic Inactivation of Bacterial Pathogens

This study reports the synthesis and characterization of 5-azoniabenzo[no]tetraphene derivatives of Cinchona alkaloids with potential applications in antimicrobial photodynamic therapy (aPDT). These compounds exhibit absorption maxima at 430–441 nm, as well as in the UV-A (340–342 nm) and UV–C (270–275 nm) regions, with fluorescence emission peaks ranging from 519 to 534 nm. At nontoxic concentrations of 2 μg mL^–1 and upon irradiation with blue light (418 nm), these compounds demonstrated potent bactericidal activity depending on the light dose. Staphylococcus aureus was eradicated after 5 min of irradiation (50 J cm^–2) with the isomer of 7R,2’S configuration (QN-1) as a photosensitizer. Gram-negative bacteria exhibited reduced sensitivity, with Acinetobacter baumannii requiring an energy dose of 150 J cm^–2 for effective killing, whereas Proteus mirabilis showed no significant reduction after exposure to 200 J cm^–2. The differences in sensitivity are attributed to the structure of the cell envelopes, which influence the uptake of the photosensitizer: the compounds accumulated intracellularly in Gram-positive bacteria but remained extracellular in Gram-negative rods. It is suggested that the aPDT mechanism involves a combination of oxidative pathways for QN-1 and QD-1 as photosensitizers and a Type II mechanism when QN-2 was applied.