Photooxidation of Pterin in Aqueous Solutions: Biological and Biomedical Implications by Franco M. Cabrerizo a ), MarÌa L. Da ¬ntola a ), Andre ¬ s H. Thomas a ), Carolina Lorente b ), Andre ¬ M. Braun c ), Esther Oliveros* c ),and Alberto L. Capparelli* a ) a )Instituto de Investigaciones FisicoquÌmicas Teo ¬ricas y Aplicadas (INIFTA), Departamento de QuÌmica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, CONICET, CIC Casilla de Correo 16, Sucursal 4, 1900-La Plata, Argentina (phone/fax:  54-221-4242333; e-mail: alcappa@quimica.unlp.edu.ar) b )CHIDECAR - CONICET, Departamento de QuÌmica Orga ¬nica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabello ¬n 2, 3p, Ciudad Universitaria, 1428-Buenos Aires, Argentina c ) Lehrstuhl f¸r Umweltmesstechnik, Engler-Bunte Institut, Universit‰t Karlsruhe, D-76128 Karlsruhe (phone:  49-721-6082557; fax:  49-721-6086240; e-mail: Esther.Oliveros@ciw.uni-karlsruhe.de) Studies of the photochemical reactivity of pterin ( 2-aminopteridin-4(3H)-one;PT)inacidic(pH5.0± 6.0) and alkaline (pH 10.2±10.8) aqueous solutions have been performed. The photochemical reactions were followed by UV/VIS spectrophotometry, thin layer chromatography (TLC), high-performance liquid chromatography (HPLC), and an enzymatic method for H 2 O 2 determination. PT is not light-sensitive in the absence of molecular oxygen, but it undergoes photooxidation in the presence of O 2 , yielding several nonpteridinic products. The quantum yields for PT disappearance were found to be 8.2 ( 0.6)  10 4 and 1.2 ( 0.2)  10 3 in acidic and alkaline media, respectively. H 2 O 2 was detected and quantified in irradiated solutions of PT; and its importance from a biomedical point of view is discussed. The rate constant of the chemical reaction between singlet oxygen ( 1 O 2 ) and PT was determined to be 2.5 ( 0.2)  10 5 l mol 1 s 1 in alkaline medium, and the role of 1 O 2 in the photooxidation of pterin was evaluated. Introduction. ± Pterins ( 2-aminopteridin-4(3H)-ones; PTs) are a family of heterocycliccompoundsthatoccurinawiderangeoflivingsystemsandparticipatein relevant biological functions. Folic acid, a conjugated PT, acts as a coenzyme in reactions related to the synthesis of purine and pyrimidine bases [1]. Tetrahydrobio- pterinactsasacoenzymeinmetabolichydroxylationreactionsofsomeaminoacids[2] and is also relevant for the metabolism of nitric oxide [3]. Some PT derivatives (e.g. , xantopterin or leucopterin) are present in butterflies as natural pigments [4]. ThelightsensitivityofPTsisknownsinceseveraldecades[5].Theparticipationof PT derivatives in different photobiological processes has been recently suggested or demonstrated, and interest in the photochemistry of this group of compounds has subsequently increased. The folic acid derivative -5,10-methenyltetrahydrofolate× is presentasthelight-harvestingantennainDNAphotolyases[6],whichareinvolvedin DNA repair after UV irradiation. PTs are also interesting as possible photosensitizers in photochemical processes that take place in organisms exposed to UV-A radiation. Theyaregoodsingletoxygen( 1 O 2 )sensitizersinaqueoussolutions[7]andparticipate as sensitizers in photochemical reactions that induce DNA damage [8]. Biomedical studies have suggested that PTs are involved in the pathogenesis of vitiligo, a depigmentationdisorder[9],andthatdifferentPTderivatives(e.g. ,6-and7-biopterin) CHEMISTRY & BIODIVERSITY ± Vol.1 (2004) 1800 ¹ 2004 Verlag Helvetica Chimica Acta AG, Z¸rich