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Lipid peroxidation (LP) is involved in many physiological and pathological events [1, 2]. The main compounds of biological membranes, phospholipids containing polyunsaturated fatty acids (PUFA), are predominantly susceptible to the process of LP that includes as a first step, a hydrogen atom abstraction. Liposomes can be used to learn more about issues that cannot be studied in biological membranes. On the other hand, pterins, heterocyclic compounds derived from pterin (Ptr), can be found in biological systems in many forms and playing different roles (pigments or enzymatic cofactors for numerous redox and one-carbon transfer reactions). They act as photosensitizers under UVA radiation on biomolecules such as nucleotides, plasmid DNA, amino acids, proteins [3, 4]. In addition, it has been found that pterins have phototoxic effect on cell culture [5]. Sonicated liposomes of soybean phosphatidylcholine (SoyPC) were prepared in buffer Tris containing pterin. The location of pterin in the liposomes was investigated by size exclusion chromatography and absorption/fluorescence spectra. Later on, liposomes were submitted to LP, under air atmosphere and UVA irradiation, with Ptr as a photosensitizer. Conjugated dienes and trienes were determined by absorption at 234 and 270 nm, respectively. Mass spectrometry was used to identify peroxidation products. As a conclusion, steady UVA irradiation of solutions containing Ptr and SoyPC led to peroxidation of PUFA, whereas the Ptr concentration remained unchanged. Ptr showed an enhanced effect on peroxidation rate and steady state concentration of oxidation products. This model constitutes a valuable system to study LP intermediaries and products in an aqueous environment and to determine the effect of Ptr as photoinducers. [1] Chem. Phys. Lipids, 2009, 157, 1 [2] Front Physiol. 2015, 5, 520 [3] J. Am. Chem. Soc., 2008, 130, 3001 [4] Org. Biomol. Chem., 2014, 12, 3877 [5] Photochem. Photobiol., 2011, 87, 862