New approach on Tyrosine photosensitized oxidation mechanism

NEYRA RECKY, JAEL R.; SERRANO, MARIANA P; THOMAS, ANDRÉS HÉCTOR; DANTOLA, MARIA LAURA; LORENTE, CAROLINA

Viña del Mar

Congreso; 14th Encuentro Latinoamericano de Fotoquímica y Fotobiología, XIV ELAFOT; 2019

Universidad Andrés Bello, Pontificia Universidad Católica de Chile, Universidad de Santiago de Chile, Universidad de Chile

Pterins are a family of heterocyclic compounds widespread in biological systems playing differentfunctions. Under UV-A radiation (320?400 nm), these molecules can act as photosensitizersthrough both type I and type II mechanisms.1,2 Pterin (Ptr), the parent unsubstituted compound ofoxidized pterins, is able to oxidized tyrosine (Tyr)3 under UV-A radiation. The mechanism isinitiated by an electron transfer reaction from Tyr to Ptr triplet excited state (3Ptr*) to yield the Ptrradical anion (Ptr??) and tyrosine radical cation(Tyr?+) (reaction 1). The Tyr?+ may deprotonateto generate tyrosyl radical (Tyr(-H)?) (reaction2). In the presence of O2, electron transfer fromPtr?? to O2 regenerates Ptr producing O2??(reaction 3). The O2?? may disproportionate toform H2O2 (reaction 4). Finally, a group ofprocesses represented by reaction 5 led to Tyroxidation. Alternatively, radical dimerizationmight contribute to Tyr consumption (reaction6). Tyrosine dimers (Tyr2), 3,4-dihydroxyphenylalanine (DOPA) anddopachrome were identified as photoproducts. The mechanism of Tyr2 generation is well known.Nevertheless, in literature, there?s no consensus on the pathways that originate DOPA anddopachrome.The main goal of this work is to elucidate the pathways involved in the photogeneration of DOPAand dopachrome, by evaluating the role of reactive oxygen species (ROS) and Tyr(-H)?. Acidaqueous solutions (pH 6.0) containing Ptr (60µM) and Tyr (100µM) were exposed to UV-Aradiation (365 nm) for different periods of time and in different experimental conditions: (i) in thepresence of superoxide dismutase (SOD) (0 ? 50 U/mL), to evaluate the role of O2??; (ii) in D2O toevaluate the role of 1O2; and (iii) in presence of resveratrol (RSV), a natural antioxidant inhibitor oftype I photooxidation mechanism, to evaluate the role of (Tyr(-H)?).4The results obtained using different experimental conditions suggest that the main productspreviously reported (Tyr2, DOPA and dopachrome) are generated from Tyr(-H)?. The experimentsperfomed in the presence of SOD, confirmed that DOPA is a product of the reaction between Tyr(-H)? and O2?-. On the other hand, the contribution of 1O2 to the formation of these oxidation productsmay be discarded.References1. Lorente C., et. al.,Acc. Chem. Res., 2006, 39, 395.2. Dántola M. L., et. al.,Pteridines, 2017, 28(3-4), 105.3. Castaño C., et. al.,Photochem. Photobiol, 2013, 89, 1448.4. Neyra Recky J. R., et. al., Phys. Chem. Chem. Phys., 2019, 21, 16190.