60Co-Gamma radiolysis of cysteine in aerated solutions at pH 5.5-6.0

In the γ-radiolysis of cysteine (CySH) in aeratedsolutions in the pH range of 5.5 to 6.0, the G-values of the radiolytic products, cystine (CyS-SCy), cysteine sulphinic acid (CySOzH), serine (CyOH), alanine (CyH), lanthionine (CySCy), hydrogen peroxide and C(-CySH) have been determined. The results demonstrate good material balance for the radiolytic amino products. The G(CyOH) = 1.9, C(CyH) = 0.3 and G(CySCy) = 0.2, (all possessing a Cy unit) at 10-2 M cysteine concentration in aerated solutions suggest that most of the primary hydrated electrons (e-aq) react with cysteine to give Cy radicals which give rise to CyO2 radial in presence of oxygen. CyO2 further reacts with cysteine to give serine and lanthionine. Electron scavengers like NO3 - and acetone have been found to reduce the yields of serine, alanine and lanthionine. These competition experiments further strengthen the conclusion that CyO2 radical is the precursor for serine and lanthionine. Based upon the mechanism suggested in the paper, a rate expression for G(-CySH) has been evaluated. The results of the dose rate experiments suggest a radical-radical chain termination step.