The role of gold nanoparticles (AuNPs) in the degradation of tyrosine intermediates formed during the radiation-induced (OH)-O-center dot 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 x 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 (OH)-O-center dot and N-center dot(3) with tyrosine. The formation of tyrosyl radical in the presence of AuNPs is found to be a major pathway through the decay of tyrosine-(OH)-O-center dot adducts via the water elimination reaction, which is found to be 3X 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-(OH)-O-center dot adducts leading to reduced DOPA formation is proposed. The proposed mechanism is also complemented by the electronic spectra and energetics of the reaction of (OH)-O-center dot with tyrosine using density functional theory calculations. Significantly, the H-shift reaction of ortho-tyrosine-(OH)-O-center dot 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.