A non-singlet oxygen mediated reaction photoinduced by phenalenone, a universal reference for singlet oxygen sensitization

CAROLINA LORENTE; EMMANUEL ARZOUMANIAN; CAROLINA CASTAÑO; ESTHER OLIVEROS; ANDRÉS H. THOMAS

Maresias, São Paulo

Congreso; XII Encuentro Latinoamericano de Fotoquímica y Fotobiología (XII Elafot); 2015

ELAFOT

Althoughphotosensitized oxidations may involve the generation of radicals (type Imechanism) and/or the production of singlet molecular oxygen (1O2)(type II mechanism), it is generally accepted that the photosensitization ofproteins occurs mainly through the reactions of 1O2 withtryptophan (Trp) and other amino acids. In this work, we have investigated themechanism of the oxidation of Trp photosensitized by phenalenone, one of thewell-known type II photosensitizers, widely used as a 1O2 referencesensitizer. From the results obtained, it could be established that 1O2played a minor role and that the predominant mechanism of thephotosensitization of Trp by PN involved an electron transfer process (type Imechanism).Themechanism proposed is summarized as follows. After UV-A excitation of PN andformation of its triplet excited state (3PN*), threereaction pathways compete for the deactivation of the latter: intersystemcrossing to singlet ground state, energy transfer to O2 leading tothe regeneration of Ptr and the production of 1O2, andelectron transfer between Trp and 3PN* yielding thecorresponding pair of radical ions (PN?‒ and Trp?+). Inthe following step, the electron transfer from PN?‒ to O2regenerates PN and forms O2?‒. Finally, the reaction ofTrp?+/Trp(-H)?with O2 leads to the formation ofproducts. In the absence of O2 recombination of PN?‒ andTrp?+ recovers the reactants and, therefore no consumption of theamino acid is observed under anaerobic conditions.            Thesefindings call into question many reported mechanisms and even the role of 1O2in processes involved in the photodynamic effects of a variety of photosensitizers in living systems.