Reaction of protonated tyrosine with the lowest electronically excited singlet state of molecular oxygen, O-1(2) (a(1)Delta(g)), is reported over the center-of-mass collision energy (E-col) range from 0.1 to 3.0 eV, using an electrospray-ionization, guided-ion-beam scattering instrument, in conjunction with ab initio electronic structure calculations and direct dynamics trajectory simulations. Only one product channel is observed, corresponding to generation of hydrogen peroxide via transfer of two hydrogen atoms from protonated tyrosine. Despite being exoergic, the reaction is in competition with physical quenching of O-1(2) and is very inefficient. At low E-col, the reaction may be mediated by intermediate complexes and shows strong inhibition by collision energy. At high E-col, the reaction efficiency drops to similar to 1% and starts to have contribution from a direct mechanism. Quasi-classical trajectory simulations were performed to probe the mechanism at high collision energies. Analysis of trajectories shows that, at E-col of 3.0 eV, a small fraction of hydrogen peroxide (25%) is produced via a direct, concerted mechanism where two hydrogen atoms are transferred simultaneously, but most hydrogen peroxide (75%) is formed by dissociation of hydroperoxide intermediates. According to ab initio calculations and trajectory simulations, collisions also lead to formation of various endoperoxides, and dissociation of endoperoxides may play a role in physical quenching of O-1(2). The apparatus and experimental techniques are described in detail.