Water-soluble phosphorus porphyrins with high activity for visible light-assisted inactivation of Saccharomyces cerevisiae

In an effort to develop water-soluble singlet-oxygen (1O2) photosensitizer, we synthesized a series of phosphorus porphyrin complexes (1, (RO)2P(tpp)+Cl−, tpp = tetraphenylporphyrinato ligand, R = –(CH2CH2O)m–(CH2)n–H, m = 0–3, n = 1, 2, 4, 6) and evaluated their performance characteristics. Water-solubilities of 1 (m = 1, 2 and n = 1, 2, 4, 6) were determined to be >1.0 mM, which was higher than the parent (HO)2P(tpp)+Cl−. The quantum yields for the generation of 1O2 in water based on the amount of 11* were determined to be 0.73, 0.69, and 0.62 for 1d (m = 2, n = 4), 1e (m = 2, n = 2), and 1f (m = 2, n = 1), respectively. Photo-inactivation by 1 was performed with yeast (Saccharomyces cerevisiae), and was evaluated using a inactivation factor AF = ([1]M × T1/2)−1, where [1]M is the minimum effective concentration of 1 and T1/2 is a half-life of the yeast initially used. In the case of 1b (m = 1, n = 6) and 1c (m = 2, n = 6), [1]M was determined to be 5 nM in the photo-sterilization. Bio-affinity was evaluated by the adsorbed concentration of 1 ([P]ad) into the yeast, which was measured by absorption spectrophotometry using a confocal laser-scanning microscope. These findings revealed that a large amount of 1 was adsorbed into the yeast. The estimated amount of 1b obtained from 10 μM aqueous solution came to 171 mM (17,100 fold) in terms of concentration. As an increase in carbon number (n) of the alkyl group on axial ligands, AF and [P]ad increased. It is strongly suggested that the photo-inactivation took place with the assistance of the attack of 1O2 inside the cells.