Photoeradication of bacteria with porphycenes: Substituent effects on the efficiency

Considering the world-wide problem of growing antibiotic resistance of bacteria, photodynamic inactivation (PDI) has a potential to become the treatment approach against some infectious diseases. In our study, four differently substituted porphycenes were compared in terms of their bactericidal activity against E. faecalis. All tested compounds had a similar photophysical characteristics, i.e., there were no significant differences in the location of absorption bands or molar absorption coefficients. Also, singlet oxygen generation quantum yields were very similar. Surprisingly, differently substituted porphycenes caused very diverse PDI effects. Special attention was drawn to the tert-butyl moieties. Our studies demonstrated that the presence of these substituents lowers the bactericidal potential significantly and can completely block the activity when more than one moiety is introduced to the molecule. The porphycenes lacking tert-butyl groups exhibited much higher PDI potential and we assign this effect to different interactions of the differently substituted porphycenes with the bacterial cells. Most likely, the presence of tert-butyls impairs cell penetration by the photosensitizer. These results remind that the favorable photophysical characteristics do not ensure that the compound considered as a potential PDI agent can reach the microbial cells.