\subsection{The effect of several solar exposure conditions}

The intensity of the solar radiation depends on many factors, such as the time of the day, the date and the presence of clouds. Therefore, to investigate the effect of the mentioned factors on the photodegradation of pterin derivatives, aqueous solutions of Bip and H$_{2}$Bip were exposed to solar radiation under different conditions.

In a first study, in order to characterize the consumption rate as a function of local time, a set of samples were exposed for different times (10, 20, 40 and 60 s) to sun's rays for a clear sky day. This process was repeated every hour for a part of the day, centered at solar noon. For each time, $\left(d\left[R\right]/dt\right)_{0}$ was obtained from the HPLC analysis and $q^{a,V}_{n,p}$ was calculated combining the $E^{S}_{\lambda}$ value registered with the spectrometer and the corresponding absorption spectrum of the sample (as shown in Figure \ref{overlap}), as explained in Subsection \ref{quantumyield}. $\left(d\left[R\right]/dt\right)_{0}$ was proportional to $q^{a,V}_{n,p}$ with a maximum at noon (Figure \ref{bobesponja}a). For each photolysis the corresponding $\Phi_{R}$ value was calculated and no variation of this parameter was registered during the day (Figure \ref{bobesponja}b). The mean quantum yield along the day was $\Phi_{R}$=0.037$\pm$0.003 consistent with the values obtained in Subsection \ref{quantumyield}. Moreover, the mean hourly value of temperature was increased towards the afternoon from 24.8${^\circ}$C (at 11:00) to 29.5${^\circ}$C (at 17:00). It is worth mentioning that, although there was a variation of temperature during the day, $\Phi_{R}$ did not change significantly.

On the other hand, the current investigation involved sampling and analyzing for four months to evaluate monthly changes in the consumption rates. The relative concentration of Bip ([Bip]/[Bip]$_{0}$) as a function of irradiation time is plotted in Figure \ref{monthly} for different experiments carried out in clear sky days at noon. As expected taking into account the increase in the values of $E^{S}_{UV}$, the $\left(d\left[Bip\right]/dt\right)_{0}$ increased from August to December.

Up to now, all the results presented correspond to experiments carried out in cloud-free weather conditions. Clouds are one the most important parameters to influence the solar radiation. For the purpose of assessing the energy required to trigger the studied reactions, a new set of experiments, similar to those described for clear sky conditions, were performed in cloudy days. Nevertheless, the variability in form and height of cloud is very large and can hardly be determined. A cloud coverage is a challenge to perform an exposure of serial samples. It would be very difficult to proceed ensuring a relatively constant intensity (as interposing a 'uniform filter') in a long time.

Therefore, the exposures were carried out in short times at noon in a cloudy day, in which the cloud coverage was a homogeneous layer distributed over La Plata city and its surroundings, about 100 km away (minimizing atmospheric fluctuations). Measurements of $E^{S}_{n,p,\lambda}$ were performed on November 11$^{th}$, 2013 as detailed in Figure \ref{cloudsi}. Additionally, we added for comparative purposes a measure recorded in a clear sky day (on November 6$^{th}$,2013). The values of $E^{S}_{UV}$ for the compared cloudy and clear sky days were 24.22 and 45.94 $W\cdot m^{{–}2}$ respectively. It is clear that the solar energy of the cloudy day is significantly lower than that corresponding to the clear sky day, but it is not negligible. In fact, $E^{S}_{UV}$ was enough to cause the oxidation of half of the initial Bip in 2 minutes (inset Figure \ref{cloudsi}), which means that even in cloudy weather conditions the photolysis of pterins was significant.

To consider the feasibility of these reactions in the skin, the solar radiation reaching the cells at different depths and the localization of pterins should be taken into account. Earlier studies have identified an accumulation of H$_{2}$O$_{2}$ and pterins oxidation products at the epidermis level, in patients affected by vitiligo \citep{Schallreuter2001, Rokos2002}. The evidence that significant quantities of UV light transverse the stratum corneum and reaching the epidermis have been provided by in vivo experiments \citep{PHILP1989, Everett_1966, Diffey_1980}. Moreover, is well known that in the epidermis, the major absorber of radiation is the melanin \cite{AndersonParrish1981} and if absent, the protection against UV radiation fails. Therefore, this rough analysis suggests that pterin photochemistry can take place in the affected tissues upon solar exposure.
