Optimal axial alkylpyridinium-bonded tricationic P-porphyrin in photodynamic inactivation of Escherichia coli

Alkylpyridinium (Apy)-bonded porphyrins have received considerable attention as singlet-oxygen (O-1(2)) sensitizers for photodynamic inactivation (PDI). It is expected that the introduction of Apy makes porphyrins water-soluble and enhances the affinity of porphyrins to DNA. Here, we focused on Apy-bonded P-porphyrins that were prepared through the modification of axial ligands of meso-tetraphenylporphyrinatophosphorus by the Apy's group and linkers. These water-soluble porphyrins (1) were applied to sensitize the inactivation of Escherichia coli under visible-light irradiation, since there are only few O-1(2) sensitizers that can efficiently inactivate E. coli at low concentrations. The PDI activities were evaluated using the half-life (T-1/2 in min) of E. coli and the minimum effective concentrations ([P]) of the porphyrin sensitizers. It was found that the PDI activity towards E. coli depends on the alkyl chain length of Apy. The [P] value for E. coli was optimized to be 0.25 mu M of bis[5-(3-ethyl-1-pyridinio)-3-oxapentyloxo]tetraphenylporphyrinatophosphorus dibromide chloride (1b). Since the previous results on the optimized [P] value for S. cerevisiae was 50 nM for 1b, it was found that the [P] value for E. coli was larger than that for S. cerevisiae.