**4.2 Combination of components with type II as the main reaction (riboflavin and tryptophan)**

The other type of photosensitization occurs between riboflavin and tryptophan. Since tryptophan has a maximum absorption wavelength in the UVB region (especially at 280 nm), exposure to UVB is known to produce tryptophan oxides, such as FICZ and kynurenine derivatives. These tryptophan oxides have absorption wavelengths in the UVA region, and it has been reported that UVA exposure produces superoxide, H2O2, and other types of ROS [57–59]. In our previous study, since tryptophan does not have a UV absorption region, UV exposure to tryptophan alone did not cause the oxidative degradation of tryptophan and did not produce kynurenine. However, exposure to UVA in the presence of riboflavin decreased the 280 nm absorption by tryptophan and increased the 360 nm absorption by kynurenine. Those results indicate that the oxidative degradation of tryptophan, which does not occur in aqueous solution with tryptophan alone, may be initiated by the photosensitization of riboflavin. This phenomenon

was markedly suppressed by the addition of NaN3, which suggests that the oxidative degradation of tryptophan may be promoted by a singlet oxygen generated at an earlier time by the photosensitization of riboflavin [52]. Therefore, the singlet oxygen quencher is considered to be effective for these types of reactions.

As a point to be noted, it has been reported that HEPES and phenol red can enhance the cytotoxicity and the production of ROS by photosensitization reaction under the coexistence condition with riboflavin [60, 61]. When considering a photosensitizing reaction with multiple components, it is necessary to consider the possibility that a component other than the object to be evaluated may become noise.
