Quinolinium-substituted porphyrins as promising antimicrobial photosensitizers

Characterized by a remarkable chemical versatility and outstanding photophysical properties, porphyrins stand out as one of the most promising classes of photosensitizers for antimicrobial photodynamic therapy (aPDT). In this work, we described the synthesis, as well as the chemical and photophysical characterization of quinolinium-substituted porphyrins, and their corresponding Zn(II) complexes. The efficacy of these porphyrins in the photoinactivation of bacteria was evaluated, both alone and combined with the co-adjuvant potassium iodide, against methicillin-resistant Staphylococcus aureus (MRSA, Gram-positive) and Escherichia coli (Gram-negative) strains.

Overall, the results demonstrated the high potential of both cationic porphyrins to effectively photoinactivate bacterial strains. Cationic derivative 2, at a concentration of 1.0 μM, achieved reductions exceeding > 99.99999 % (>7.0 log₁₀ CFU mL⁻¹) for S. aureus after 10 min of white light irradiation (25 mW cm⁻²) and > 99.9 % (3.52 log₁₀ RLU) for E. coli after 60 min aPDT treatment. Similarly, the Zn(II) counterpart 2a also showed excellent efficiency, achieving ∼99.999 % reduction (4.95 log₁₀ CFU mL⁻¹) against MRSA after 60 min of aPDT at 1.0 μM, and a 3.52 log₁₀ RLU reduction for E. coli after 30 min of PDT at 5.0 μM. The action mechanism of both cationic quinolinium-substituted porphyrins demonstrated to be dependent on the singlet oxygen production, consistent with a Type II photodynamic pathway. The use of potassium iodide (KI) as co-adjuvant led to the production of reactive iodine species (RIS), namely iodine, and increased the photodynamic effect.