Photochemistry of dihydropterins

Dihydroneopterin (H2Nep), like neopterin (Nep), is synthesized mainly in macrophages when the cellular immune system is activated. Dihydrobiopterin (H2Bip) and its oxidized analogue, biopterin (Bip), accumulate in the skin of patients suffering from vitiligo, a chronic depigmentation disorder in which the protection against UV radiation fails.

Under anaerobic conditions, UV-A excitation of H2Bip in aqueous solution leads to the formation of dimers. In the presence of air, initially the dimers are again the main photoproducts. However, a small proportion of the reactant is converted into Bip. As the reaction proceeds and a certain amount of Bip accumulates in the solution, a new pathway appears: an autocatalytic photochemical reaction in which Bip photoinduces the oxidation of H2Bip to Bip, and H2O2 is formed. Excitation of H2Nep leads also to the formation of isomeric dimers, but, in contrast to H2Bip, no oxidation of H2Nep was observed in the presence of O2.

6-Formyl-7,8-dihydropterin (H2Fop), is a product of oxidation of several natural occurring pterin derivatives, and its absorption spectrum presents as special feature an intense band in the visible region. In air-equilibrated aqueous solution H2Fop, exposed to visible light, is oxidized to 6-formylpterin (Fop), and H2O2 released. When the photolysis took place in the absence of O2, a red compound is generated, which is rapidly oxidized to Fop on admission of O2.

Related publications
"Chemical and photochemical reactivity of 6-hydroxymethyl-7,8-dihydropterin in aqueous solutions"
A. H. Thomas, E. Oliveros, A. M. Braun, M. S. Espinosa, C. Lorente, M. L. Dántola
Journal of Physical Organic Chemistry, 26, 2–8 (2013).

"Characterization and reactivity of photodimers of dihydroneopterin and dihydrobiopterin"
M. Vignoni, C. Lorente, F. M. Cabrerizo, R. Erra-Balsells, E. Oliveros, A. H. Thomas
Photochemical and Photobiological Sciences, 11, 979–987 (2012).

"Chemical and photochemical reactivity of 6-hydroxymethyl-7,8-dihydropterin in aqueous solutions"
A. H. Thomas, E. Oliveros, A. M. Braun, M. S. Espinosa, C. Lorente, M. L. Dántola, J. Phys. Org. Chem., in press.

"Characterization and reactivity of photodimers of dihydroneopterin and dihydrobiopterin"
M. Vignoni, C. Lorente, F. M. Cabrerizo, R. Erra-Balsells, E. Oliveros, A. H. Thomas, Photochem. Photobiol. Sci., 11, 979–987 (2012).

"Photodimerization of 7,8-dihydroneopterin in aqueous solution under UV-A irradiation"
M. Vignoni, M. P. Serrano, E. Oliveros, A. H. Thomas, Photochem. Photobiol. 87, 51, 2011.

"Visible-light photochemistry of 6-formyl-7,8-dihydropterin in aqueous solution"
M. L. Dántola, A. H. Thomas, E. Oliveros, C. Lorente, J. Photochem. Photobiol. A: Chem 209, 104, 2010.

"Photochemistry of dihydrobiopterin in aqueous solution"
M. Vignoni, F. M. Cabrerizo, C. Lorente, C. Claparols, E. Oliveros, A. H. Thomas, Org. Biomol. Chem. 8, 800, 2010.
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