Photoinduced antibacterial activity of two dicationic 5,15-diarylporphyrins.

Antimicrobial photodynamic treatment combines the use of photosensitizers (PSs) and visible light to kill bacterial cells. Cationic porphyrins are PSs largely used against bacteria and, among them, those featuring one positive charge on each of the 5,10,15,20-tetraaryl substituent (tetracationic) are the most used. The aim of this study was to synthesize two dicationic 5,15-di(N-alkyl-4-pyridyl)porphyrins, bearing methyl (PS 3) and benzyl (PS 4) N-alkylating groups, and to compare the efficiency in antibacterial photodynamic treatment, upon irradiation with a halogen-tungsten white lamp. The killing efficiency of the PS 4 was constantly found higher than that of the PS 3 against both pure and mixed cultures of laboratory model microorganisms as well as against wild wastewater microflora. The two PSs are comparable as regards singlet oxygen generation, but show a different repartition coefficient; the more lipophilic benzylated PS 4 shows a better interaction with the bacterial cells than the methylated one (PS 3). The data support the hypothesis that an efficient PS-cell binding is required to obtain significant effects. A correlation among cell binding, photoinactivation and PS lipophilicity is suggested.