Mechanistic insight into the photodynamic effect mediated by porphyrin-fullerene C60 dyads in solution and in Staphylococcus aureus cells

The photodynamic action mechanism sensitized by a non-charged porphyrin-fullerene C-60 dyad (TCP-C-60) and its tetracationic analogue (TCP-C-60(4+)) was investigated in solution and in Staphylococcus aureus cells. The ability of both dyads to form a photoinduced charge-separated state was evidenced by the reduction of methyl viologen in N,N-dimethylformamide (DMF). Moreover, the formation of superoxide anion radicals induced by these dyads was detected by the reduction of nitro blue tetrazolium. Also, photosensitized decomposition of l-tryptophan (Trp) was investigated in the presence of reactive oxygen species (ROS) scavengers. The addition of -carotene and sodium azide had a slight effect on reaction rate. However, photooxidation of Trp mediated by TCP-C-60 was negligible in the presence of d-mannitol, while no protection was found using TCP-C-60(4+). In a polar medium, these dyads mainly act by a contribution of type I pathway with low generation of singlet molecular oxygen, O-2((1)(g)). In S. aureus cell suspensions, an aerobic atmosphere was required for the photokilling of this bacterium. The photocytotoxicity induced by TCP-C-60 was increased in D2O with respect to water, while a small effect was found using TCP-C-60(4+). Furthermore, photoinactivation of microbial cells was negligible in the presence of sodium azide. The addition of d-mannitol did not affect the photoinactivation induced by TCP-C-60. In contrast, S. aureus cells were protected by d-mannitol when TCP-C-60(4+) was used as a photosensitizer. Also, generation of O-2((1)(g)) in the S. aureus cells was higher for TCP-C-60 than TCP-C-60(4+). Therefore, TCP-C-60 appears to act in microbial cells mainly through the mediation of O-2((1)(g)). Although, a contribution of the type I mechanism was found for cell death induced by TCP-C-60(4+). Therefore, these dyads with high capacity to produce photoinduced charge-separated state represent interesting photosensitizers to inactivate microorganisms by type I or type II mechanisms. In particular, TCP-C-60 may be located in a non-polar microenvironment in the cells favoring a type II pathway, while a contribution of the type I mechanism was produced using the cationic TCP-C-60(4+).