We investigated the ROS generated by PpIX upon blue light excitation. The characteristic ESR spectral pattern has three intense lines from the 2,2,6,6-tetramethyl-4-hydroxyl-piperidinyloxy (4-OH-TEMPO) radical, indicating that 1O2 generation was observed when using blue light to excite PpIX in the presence of 2,2,6,6-tetramethyl-4-piperidinol (4-OH-TEMP) (Fig. 2A). Even at high concentrations of PpIX, this generation of 1O2 was hardly observed without blue light irradiation. The spin concentration was about 5 µM with the 100J blue light irradiation of 100 µM PpIX (Fig. 2B). The spin concentration of products from 100 µM PpIX excited with 100J blue light was inhibited by the addition of L-histidine (Fig. 2C).Figure 2Singlet oxygen generation induced by PpIX upon blue light irradiation. (A) Typical in vitro ESR spectrum induced by PpIX upon blue light irradiation. (B) The concentration of generated singlet oxygen. (C) Generation of singlet oxygen with and without L-histidine at 100J of irradiation. The data are expressed as means ± SD (n = 3). Significant differences are expressed with * for p < 0.01 and † for p < 0.0001.