Oxo-sulfur derivatives of these compounds were also examined for their ability to prevent DNA damage with copper or iron and H2O2. Methylcysteine sulfoxide and methionine sulfoxide inhibit copper-mediated DNA damage with IC50 values in the low micromolar range (8–18 μM). In contrast, oxo-sulfur compounds also show much lower antioxidant activity with iron; methylcysteine sulfoxide and methyl methanethiosulfonate inhibited little (~20%) iron-mediated DNA damage at high concentrations (1000–5000 μM). The primary antioxidant activity for these oxo-sulfur compounds was attributed to metal coordination; however, a secondary ROS scavenging mechanism was also identified [13].