Photodimerization of 7,8-Dihydroneopterin in Aqueous Solution Under UV-A Irradiation

7,8-Dihydroneopterin (H(2)Nep) is secreted during the oxidative burst of stimulated macrophages. The photochemistry of H(2)Nep was investigated in neutral aqueous solutions exposed to UV-A radiation (320-400 nm) at room temperature. The kinetics were followed by UV/Vis spectrophotometry and HPLC, whereas the photoproducts were analyzed by electrospray ionization mass spectrometry. Excitation of H(2)Nep leads to the formation of isomeric dimers with molecular masses equal to exactly twice the molecular mass of the reactant. The corresponding quantum yield of H(2)Nep consumption (Phi(-R) = (3.8 +/- 0.5) x 10(-2)) was independent of O-2 and reactant concentrations. Mechanistic implications are discussed.