RESUMO
The role of gold nanoparticles (AuNPs) in the degradation of tyrosine intermediates formed during the radiation-induced â¢OH reaction with tyrosine at pH 6.5 is investigated by measuring the radiolytic yields, G, of tyrosine (-Tyr), dityrosine (DT), and 3,4 dihydroxyphenylalanine (DOPA). The G(DT) is doubled, whereas G(-Tyr) calculated is halved in the presence of 6.0 × 10-10 mol dm-3 AuNPs. Pulse radiolysis studies are carried out to elucidate the mechanism and nature of the transient formed in the reaction of â¢OH and â¢N3 with tyrosine. The formation of tyrosyl radical in the presence of AuNPs is found to be a major pathway through the decay of tyrosine-â¢OH adducts via the water elimination reaction, which is found to be 3× faster in the presence of AuNPs. Quantum chemical calculations on the system showed favorable formation of the tyrosine-AuNP complex. A new plausible mechanism of tyrosine-AuNP complex acting as a Lewis type catalyst in the decay of tyrosine-â¢OH adducts leading to reduced DOPA formation is proposed. The proposed mechanism is also complemented by the electronic spectra and energetics of the reaction of â¢OH with tyrosine using density functional theory calculations. Significantly, the H-shift reaction of ortho-tyrosine-â¢OH adducts is also found to be energetically viable. The investigation provides a new physical insight into the effect of AuNPs on the decay of free-radical transient species and demonstrates the potential of radiation chemical techniques and quantum chemical calculations as a tool for understanding the impact of metal nanoparticles in free-radical oxidation of amino acids, which is important in the use of metal nanoparticles for biomedical applications.