\subsection{General}

H$_{2}$Bip, Bip and other pterins were purchased from Schircks Laboratories (Switzerland) and used without further purification. Other chemicals were from Sigma Chemical Co. The pH of the aqueous solutions was adjusted by adding drops of HCl or NaOH solutions from a micropipette. The concentrations of the acid and
base used for this purpose ranged from 0.1 to 2 M. The ionic strength was approximately $10^{-3}$ M in all the experiments. As a control, the electronic absorption spectrum of each solution was recorded on a Shimadzu UV-1800 spectrophotometer.