Emission properties of dihydropterins in aqueous solutions

MARIANA P. SERRANO; MARIANA VIGNONI; M. LAURA DÁNTOLA; CAROLINA LORENTE; ANDRÉS H. THOMAS

Mendoza

Congreso; XXI Inter-American Photochemical Society Conference; 2011

Pterins, heterocyclic compounds derived from 2-aminopteridin-4-(3H)-one or pterin [1], are present in biological systems in multiple forms. They can exist in different oxidation states: oxidized (or aromatic) pterins and reduced pterins. Within the latter group, 7,8-dihydropterins and 5,6,7,8-tetrahydropterins are the most important derivatives due to their biological activity, e.g., dihydroneopterin (H2Nep) is secreted during the oxidative burst of stimulated macrophages [2], dihydrobiopterin (H2Bip) and tetrahydrobiopterin are involved in the metabolism of aminoacids [3]. In addition, H2Bip, biopterin (Bip) and other pterin derivatives accumulate in the skin of patients affected by vitiligo, a depigmentation disorder, where the protection against UV radiation fails due to the lack of melanin [4-5]. Upon UV-A excitation, unconjugated oxidized pterins present emission bands with maximum around 450 nm, relatively high fluorescence quantum yields (ФF > 0.10) and fluorescence lifetimes (tF) in the range 2-14 ns. Dihydropterins are more complicated to work with because they are unstable in air-equilibrated solutions, undergoing oxidation with a rate depending on the chemical nature of the substituent [6]. In addition the emission of dihydropterins is very weak and, and the solutions prepared from chemicals of the highest purity commercially available are contaminated with small concentrations of oxidized pterins. For that reason, given that the absorption spectra of dihydropterins and oxidized pterins are in general more or less superimposed, the emission of the latter is a main interference in the analysis of the fluorescence of the former. The aim of this work was to investigate the emission properties of dihydropterins in their neutral forms, in aqueous solutions upon UV-A irradiation. We have chosen for this study 6 pterin derivatives: 6-formyldihydropterin (H2Fop), sepiapterin (Sep), dihydrobiopterin (H2Bip), dihydroneopterin (H2Nep), 6-hydroxymethyldihydropterin (H2Hmp) and 6-methyldihydropterin (H2Mep). In particular, we have investigated the emission spectra, determined the FF and tF values, and discussed about the effects of the substituents on the emission properties.     References:   [1] D. J. Brown, The Chemistry of Heterocyclic Compounds, John Wiley & Sons, New York, 1988, 24, 1-42. [2] C. Huber, D. Fuchs and A. Hausen, Journal of Immunology, 1983, 130, 1047-1050. [3] C. A. Nichol, G. K. Smith and D. S. Duch, Annu. Rev. Biochem., 1985, 54, 729-764. [4] K. U. Schallreuter, J. M. Wood, M. R. Pittelkow, M. Gütlich, K. R. Lemke, W. Rödl, N. N. Swanson, K. Hitzemann and I. Ziegler, Science, 1994, 263, 1444-1446. [5] H. Rokos, W. D. Beazley and K. U. Schallreuter, Biochem. Biophys. Res. Commun., 2002, 292, 805-811. [6] M. L. Dántola, M. Vignoni, A. L. Capparelli, C. Lorente and A. H. Thomas, Helv. Chim. Acta, 2008, 91, 411-425.