Singlet 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.