Comparison of two functionalized fullerenes for antimicrobial photodynamic inactivation: Potentiation by potassium iodide and photochemical mechanisms

A new fullerene (BB4-PPBA) functionalized with a tertiary amine and carboxylic acid was prepared and compared with BB4 (cationic quaternary group) for antimicrobial photodynamic inactivation (aPDI). BB4 was highly active against Gram-positive methicillin resistant Staphylococcus aureus (MRSA) and BB4-PPBA was moderately active when activated by blue light. Neither compound showed much activity against Gram-negative Escherichia coli or fungus Candida albicans. Therefore, we examined potentiation by addition of potassium iodide. Both compounds were highly potentiated by KI (1-6 extra logs of killing). BB4-PPBA was potentiated more than BB4 against MRSA and E. coli, while for C. albicans the reverse was the case. Addition of azide potentiated aPDI mediated by BB4 against MRSA, but abolished the potentiation caused by KI with both compounds. The killing ability after light decayed after 24 h in the case of BB4, implying a contribution from hypoiodite as well as free iodine. Tyrosine was readily iodinated with BB4-PPBA plus KI, but less so with BB4. We conclude that the photochemical mechanisms of these two fullerenes are different. BB4-PPBA is more Type 2 (singlet oxygen) while BB4 is more Type 1 (electron transfer). There is also a possibility of direct bacterial killing by electron transfer, but this will require more study to prove.