Characterization and reactivity of photodimers of dihydroneopterin and dihydrobiopterin

Abstract

7,8-Dihydrobiopterin (H₂Bip) and 7,8-dihydroneopterin (H₂Nep) belong to a class of heterocyclic compounds present in a wide range of living systems. H₂Bip accumulates in the skin of patients suffering from vitiligo, whereas H₂Nep is secreted by human macrophages when the cellular immune system is activated. We have investigated the photochemical reactivity of both compounds upon UV-A irradiation (320–400 nm), the chemical structures of the products and their thermal stability. The study was performed in neutral aqueous solutions. The reactions were followed by UV/Visible spectrophotometry and HPLC and the products were analyzed by means of electrospray ionization mass spectrometry and ¹H-NMR. Excitation of H₂Bip and H₂Nep leads to the formation, in each case, of two main isomeric dimers. The latter compounds undergo a thermal process that may consist in a retro [2 + 2]-cycloaddition and hydrolysis to yield the reactant (H₂Bip or H₂Nep) and a product that has incorporated a molecule of H₂O.