The UV radiation-driven reduction of pterins in aqueous solution

Abstract

This paper demonstrates the capability of pterins, a strongly suspected photoreceptor in UV-governed physiological and developmental processes, to undergo redox transitions when excited by UV radiation. Aqueous solutions of 2-amino-4-hydroxy-6,7-dimethyl-pteridine (DMP) and 2-amino-4-hydroxy-6-tetrahydroxy-butyl-(D-arabo)-pteridine (TOP) were anaerobically irradiated with the total spectrum from a high pressure Hg lamp or in the range 290–390 nm cut off by absorption filters. Changes in the electronic spectra of the pterins were indicative of their reduction after irradiation in the presence of Na 2-EDTA. Kinetic analysis, together with a direct demonstration of the photoreduction of DMPH 2, revealed a stepwise transformation of DMP to the final product DMP → k 1 DMPH 2 → k 2 DMPH 4 The rate of DMP conversion into DMPH 2 was the same for both neutral and anionic molecules, whereas for the conversion from DMPH 2 to DMPH 4, anionic molecules were four times more active than neutral molecules. The structure of the residue at position 6 of the pteridine heterocycle influenced the photoreaction: the rate constant for TOP reduction exceeded by 2.5 times the k 1 value for DMP. Since the concentration of KI required to quench the fluorescence from DMP and TOP excited singlet states was found to be at least one order of magnitude higher than that required to inhibit their photoreduction, the involvement in the reaction of the triplet state of the pterins rather than the excited singlet state was presumed.