The emergence of antibiotic resistance amongst pathogenic bacteria has led to a major research effort to find alternative antibacterial therapies. In this way, bacterial photodynamic inactivation (PDI) represents an interesting alternative to inactive microorganisms. Two oxidative mechanisms are considered to be principally implicated in the photodamage of cells. In the type I photochemical reaction, the photosensitizer interacts with a biomolecule to produce free radicals, while in the type II mechanism, singlet molecular oxygen, O2(1Dg), is produced as the main species responsible for cell inactivation. Both mechanisms can occur simultaneously and the ratio between two processes depends mainly of the sensitizer, substrate and the nature of the medium.
In the present work, we have investigated the O2(1Dg) production of porphyrin derivatives in solution bearing a photooxidizable substrate and in vitro using cellular suspension of Escherichia coli.
These compounds produce singlet molecular oxygen, O2(1Dg), with quantum yields of ~0.41-0.53 in N,N-dimethylformamide. Also, L-tryptophan decomposition photosensitized by these porphyrins was found in methanol. In vitro, direct observation of O2(1Dg) phosphorescence at 1270 nm was analyzed using E. coli cells in D2O. The results show a very strong O2(1Dg) quenching sensitized by cationic porphyrins, which are highly binding to E. coli cells. This behavior can be interpreted considering that the O2(1Dg) generated inside the cells fast reacts with the biomolecules of the cellular microenvironment.
