\section{Introduction} 
Many living organisms are naturally exposed to sunlight. Ultraviolet (UV) solar radiation effects have been widely studied mainly because of the profound influence on human health and the effectiveness to start photochemical processes. Any increase in the UV intensity is therefore expected to produce significant changes in biological systems, which have a spectral sensitivity in this interval. The UVA (320-400 nm) and UVB (280-320 nm) radiation can reach Earth’s surface, the latter range being strongly dependent on the ozone layer depletion in the stratosphere. Human skin has a significant damage risk under UV radiation. The consequence of the radiation below 320 nm can range from a slight erythema (sunburn) to skin cancer by a long-term exposure \citep{de_Gruijl_1999,COL:COL18}. 

Furthermore, UVA irradiation can induce modifications in DNA through photosensitized reactions \cite{Cadet2005}. The melanin of the epidermis, is the main protection against the harmful effects of UV solar radiation. The vitiligo is a chronic depigmentation disorder \cite{Glassman_2011}, characterized by loss of melanin, where the deactivation of enzymes necessary for melanin biosynthesis takes place, causing melanin's photo-protection failure.

Pterins are heterocyclic compounds present in living systems and, depending on the oxidation state, they can be classified as: oxidized (or aromatic) and reduced pterins (Figure \ref{scheme}). Dihydrobiopterin (H$_{2}$Bip) belongs to the latter group and is a naturally occurring pterin derivative involved in the metabolism of amino acids \cite{Nichol985}. Biopterin (Bip) and other oxidized pterins, which are not present in mammalians under physiological conditions, accumulate in the skin of patients suffering from vitiligo \cite{Schallreuter2001}. Among them, 6-carboxypterin (Cap), a product of Bip photolysis \citep{Vignoni2009,Lyudnikova2009}, has been isolated from the affected tissues \cite{Rokos2002}, suggesting that photooxidation of Bip takes place in vivo. Oxidized pterins are photochemically reactive upon UVA radiation exposure in aqueous solution and undergo photooxidation to produce several photoproducts and reactive oxygen species such as singlet molecular oxygen ($^{1}$O$_{2}$) and hydrogen peroxide (H$_{2}$O$_{2}$) \citep{Lorente2006, Neverov_1974}. In addition, it has been demonstrated that Bip and its photoproducts are able to photosensitize the oxidation of biomolecules \citep{Serrano2013,Serrano2012}. Therefore, the photochemistry of pterins is of vital importance for the knowledge of the mechanism of vitiligo disease.

The photochemistry of Bip and H$_{2}$Bip upon UVA exposure has been described in detail (Figure \ref{scheme}) \citep{Vignoni2010,Vignoni2009,Lyudnikova2009}. Briefly, UVA excitation of H$_{2}$Bip in the presence of O$_{2}$ leads to two photochemical pathways: the formation of isomeric dimers with molecular masses equal to exactly twice the molecular mass of the reactant and the oxidation to its aromatic analogue, Bip. This latter reaction has been considered as a potential source of Bip in the skin \cite{Vignoni2010}. This compound, under aerobic conditions, is photochemically converted into 6-formylpterin (Fop), which, in turn, undergoes photooxidation to Cap, which is much more photostable than Bip and Fop. However, those studies have been performed using quasi-monochromatic sources under controlled laboratory conditions. 

On the other hand, the effect of sunlight on Bip and H$_{2}$Bip has not been investigated. The aim of this work is to find out if the energy of the sun under different conditions is enough to cause significant chemical changes in Bip and H$_{2}$Bip and to compare those reactions triggered by sunlight to the photochemistry of these compounds already described in the literature. Therefore a study under outdoor conditions was performed to characterize the photochemical behavior of H$_{2}$Bip and Bip in a natural environmental context. In particular, we have identified the photoproducts, analyzed the kinetics and determined the quantum yields, simultaneously with the measurement of solar energy and the determination of atmospheric variables. The results are analyzed in the context of the general photochemical behavior of pterins in aqueous solution and the biological implications are discussed.