Alkyl substituent effect on photosensitized inactivation of Escherichia coli by pyridinium-bonded P-porphyrins.

Activity of singlet oxygen sensitizers for photoinactivation of bacteria and photodynamic therapy of tumor cells has been evaluated using nonpathogenic model cells, such as Escherichia coli, Saccharomyces cerevisiae, and HeLa cells. Among them, E. coli, gram-negative bacterium, has complex membrane structures in the cell wall, resulting in an impermeable barrier to antimicrobial agents. Therefore, few singlet oxygen sensitizers have photoinactivation activities toward E. coli at low concentrations. Recently polycationic porphyrins have received much attention as a new type of singlet oxygen sensitizers because they have strong binding affinities for DNA and proteins. Here, we prepared 13 types of di- and tricationic P- and Sb-porphyrin sensitizers substituted with the N-alkylpyridinium (APy) group at the axial ligand or the meso position to examine their bactericidal activity toward E. coli under visible-light irradiation. Photobactericidal activities were evaluated using half-life (T1/2 in min) of E. coli and minimum effective concentrations of the porphyrin sensitizers. Di-cationic P-porphyrins containing the Apy group at meso position exhibited bactericidal activity under dark conditions. Tricationic porphyrins showed a higher bactericidal activity than monocationic porphyrins. It was found that the bactericidal activity depended on the alkyl chain length of APy. Tricationic porphyrin with N-heptylpyridinium in two axial ligands was the most reactive for the photoinactivation of E. coli.