Evaluation of the photosensitizer activity of hydrophobic phosphorus(V)porphyrins using the absorption spectral change of 1-benzyl-1,4-dihydronicotinamide.

Under visible light irradiation, water-insoluble P(V)porphyrins oxidized 1-benzyl-1,4-dihydronicotinamide (BNADH), a model compound for nicotinamide adenine dinucleotide, and diminished the typical absorption of BNADH at around 340 nm. A singlet oxygen quencher, sodium azide, partially inhibited photosensitized BNADH oxidation. This BNADH oxidation photosensitized by P(V)porphyrins in the presence of sodium azide can be explained by electron transfer oxidation from BNADH to the photoexcited P(V)porphyrins. The quantum yields of BNADH oxidation via electron transfer by these P(V)porphyrins were larger than those of a singlet oxygen mechanism. Redox potential measurements supported the electron transfer mechanism from a thermodynamic point of view, and fluorescence lifetime measurement also suggests this mechanism. The process of this electron transfer oxidation involves the radical formation of BNADH and the further reaction of this radical to the oxidized form (cationic form of BNADH). Analysis of the quantum yields of BNADH photooxidation by P(V)porphyrins suggests that the photoinduced electron transfer from BNADH to photoexcited P(V)porphyrins triggers the radical chain reaction of BNADH oxidation. The electron transfer rate coefficient and this efficiency were increased with an increase in the Gibbs energy of electron transfer from tryptophan to photoexcited P(V)porphyrins (-ΔG). However, the BNADH oxidation quantum yield via electron transfer decreased with an increase in the -ΔG of electron transfer. These results suggest that reverse electron transfer inhibits the decomposition of BNAD radicals. This assay using BNADH can be used to evaluate the photosensitizer activity of water-insoluble compounds. These P(V)porphyrins may be used as photosensitizers for photodynamic therapy in a relatively hydrophobic environment in cancer tissues.