\subsection{Aqueous solution}

Bip and H$_{2}$Bip were purchased from Schircks Laboratories (Jona, Switzerland) and used without further purification. The pH of the aqueous solutions was adjusted at 5.5 and 7.0 for Bip and H$_{2}$Bip, respectively, by adding drops of HCl or NaOH solutions from a micropipette. The concentrations of the acid and base used for this purpose ranged from 1 to 2M. Subsequently, the solutions were diluted in bi-distilled water (at the corresponding pH value), until reaching two concentration conditions. One of them, approaches 10$\mu$m, value chosen to simulate physiological media concentrations and the other one approaches a high concentration ($\approx100\mu m$). Absorbance spectra ($A_{\lambda}$) were recorded on a Shimadzu UV-1800 spectrophotometer. The aqueous solutions were handled using quartz cells of 0.5 cm optical path length and each spectrum was measured previously for each serial exposure. 