Antimicrobial photodynamic inactivation with decacationic functionalized fullerenes: Oxygen-independent photokilling in presence of azide and new mechanistic insights

Functionalized fullerenes are gaining wide interest for mediating photodynamic therapy (PDT) of diseases such as cancers and infections. We recently reported the synthesis of two new decacationic fullerene monoadducts: C₆₀[>M(C₃N₆⁺C₃)₂]-(I^-)₁₀(LC14) and its derivative with a light-harvesting antenna conjugated as a C₆₀[>CPAF-(MN₆⁺C₃)₂]-(I^-)₁₀ nanostructure (LC15). We studied the ability of these compounds to mediate PDT of human cancer cells in vitro when excited by UVA light or by white light. Here we report the synthesis of a new fullerene derivative C₆₀[>M(C₃N₆⁺C₃)₂][>M(C₃N₆C₃)₂]-(I^-)₁₀ (LC16 derived from LC14), as a malonate bisadduct containing a covalently bound decatertiary amine arm. We investigated the relative abilities of the three compounds to generate singlet oxygen (¹O₂), hydroxyl radicals (HO·), and hydrogen peroxide (H₂O₂) after excitation by UVA or by white light. We used three different classes of pathogenic microbial cells (Gram-positive bacterium, methicillin-resistant Staphylococcus aureus (MRSA), Gram-negative bacterium Escherichia coli, and fungal yeast Candida albicans). LC15 was the most powerful broad spectrum antimicrobial fullerenyl photosensitizer (FPS) followed by LC16, and LC14 was least powerful. Killing depended on both fullerene monoadduct concentration and light fluence. UVA was five times more effective than white light for killing, but not for generation of ROS and relative absorption was greater in white spectral region. Bacterial killing was not much inhibited by addition of azide anions and in some cases was potentiated. In the absence of oxygen, microbial photokilling was highly potentiated (up to 5 logs) by the addition of azide anions. We conclude that molecular functional addends that encourage a type I electron-transfer mechanism increase the ability of photoactivated fullerene monoadducts to kill microbial cells. Oxygen-independent photokilling is possible with fullerene monoadducts in the presence of azide anions, probably mediated by azidyl radicals. UVA excitation may kill bacteria partly by an electron-transfer mechanism directly into bacteria as well as by ROS.