**4.1 Combination of components with type I as the main reaction (riboflavin and folic acid)**

One type of photosensitization occurs in aqueous solution containing riboflavin and folic acid. Folic acid coexisting riboflavin in aqueous solution easily undergoes oxidative degradation upon UVA exposure to produce a pterin derivative [54]. Pterin derivatives are reported to accumulate in vitiligo skin [55], and to induce UV stress in melanocytes. Pterin derivatives are known to produce superoxide, hydrogen peroxide, and other types of ROS through UVA-photosensitizing [56]. In our previous study, when aqueous solution containing both folic acid and riboflavin was exposed to UVA, blue fluorescence derived from pterin derivatives appeared earlier than in aqueous solution containing folic acid alone. Those results indicate that the oxidative degradation of folic acid proceeds only very slowly in HBSS containing folic acid alone, but occurs rapidly in the presence of the photosensitizer riboflavin. Since this reaction was not inhibited by NaN3, a singlet oxygen scavenger, it was thought that the oxidative degradation of folic acid was possibly promoted via photosensitization of type I generated by riboflavin photosensitization [52]. Therefore, it is considered that the superoxide quencher is effective for these types of reactions.
