\section{Conclusions}

In the present work, we have investigated the photochemistry of dihydrobiopterin (H$_{2}$Bip) and biopterin (Bip) in aqueous solution upon solar irradiance and outdoor conditions. Sun exposure of H$_{2}$Bip leads to the formation of dimers and to its oxidation to Bip, which, in turn, is photooxidized into 6-formylpterin (Fop). Further excitation induces the oxidation of Fop to 6-carboxypterin, which is much more photostable than Bip and Fop and then it is accumulated in the solution. This reaction scheme is in agreement with that previously proposed for the photolysis of H$_{2}$Bip and Bip carried out using artificial UV radiation sources \citep{Vignoni2009,Vignoni2010}. The quantum yields of reactant disappearance ($\Phi_{R}$) were determined to be 0.05$\pm$0.01 and 0.037$\pm$0.004 for H$_{2}$Bip and Bip, respectively. The reactions were carried out under different conditions and the results reveal that the intensity of the sun is enough to efficiently oxidize H$_{2}$Bip into Bip and Bip into its oxidized derivatives within a few minutes. In particular, we have demonstrated that these photochemical reactions take place when the solar irradiance is small in comparison to that corresponding to the noon in clear sky day, e.g. during the afternoon and in cloudy days. 

Taking into account the solar radiation reaching the skin cells at different depths and the localization of pterins, the results presented in this work suggest that sunlight might be responsible for the generation of oxidized pterins, which are the photochemically active derivatives, under outdoor conditions. In particular, the energy of the sun under different environmental conditions is enough to oxidize the biologically occurring H$_{2}$Bip to Bip, which is photochemically active and, in turn, generates other oxidized derivatives and reactive oxygen species. 
