The effect of oxygen on the radiolysis of tyrosine in aqueous solutions was investigated by using gamma and pulsed electron irradiation. Steady-state radiolysis was reexamined and extended to include the effect of pH and determination of hydrogen peroxide. The loss of tyrosine, G(-Tyr), during irradiation and yields of 3,4-dihydroxyphenylalanine, G(DOPA), and hydrogen peroxide, G(H2O2), are determined in the pH range from 1 to 9. In the whole pH range used G(-Tyr) equals G(DOPA), and a higher G(H2O2) than expected was observed. In slightly acid and neutral media, both G(-Tyr) and G(DOPA) equal the yield of hydroxyl radicals, GOH, formed in the radiolysis of water, while the excess of hydrogen peroxide equals 1/2 GOH. Hence it was concluded that all tyrosine OH-adducts react with oxygen yielding peroxy radicals. In acid and alkaline media all measured yields decrease. This is caused by formation of tyrosine phenoxyl radicals (TyrO), which react with superoxide anion (O2-) and hydroperoxy (HO2) radicals regenerating tyrosine. By using pulse radiolysis K(TyrO + O2) less than or equal to 2 X 10(5) mol-1 dm3 s-1 and k(TyrO + O2-) = (1.7 +/- 0.2) X 10(9) mol-1 dm3 s-1 were determined. On the basis of the results, a reaction mechanism is proposed.

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The absorption spectrum of electrons trapped in a glassy methanol matrix was investigated and its parameters were determined. The changes of these parameters on optical bleaching and their kinetics were studied ; it followed that thc broad spectrum is an envelope of partial spectra corresponding to different kinds of trapped electrons and to their various quantum transitions. The parameters of the spectra are influenced by the presence of some solutes.On optical bleaching, the electrons are excited to higher levels from which they collapse to a molecule giving the CH30ion and simultaneously a hydrogen atom. In accordance with this mechanism, the change in the yields of trapped electrons and total hydrogen caused by the presence of electron scavengers correspond to each other. The efficiencies of electron scavcngers at 77 K parallel roughly those in liquids at room temperature.The absorption of the irradiated glassy methanol in the u.-v. is evidently due to CH20H radicals as deduced from the kinetics of their disappearance and their reaction with tetranitromethane.

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