Photochemical generation of superoxide radicals in aqueous solutions

Superoxide (O/sub 2//sup -/) is shown to be readily generated in aqueous solutions with vacuum-uv lamps of simple design. The lamps are microwave-powered rare gas lamps emitting in the wavelength region from 165 to 185 nm. An internal Suprasil coil contains the aqueous solution. Photolysis of an oxygen-saturated formate solution yields O/sub 2//sup -/ at an initial rate of 5 to 20 ..mu..M/s depending on lamp design. The O/sub 2//sup -/ is identified by its ESR and ultraviolet absorption spectrum and also by its rate of decay in acid and base. The yield of O/sub 2//sup -/ is dependent on photolysis time, microwave power, and oxygen concentration, but is independent of formate concentration. Hydrogen peroxide is formed as a secondary product. A mechanism is proposed and shown to account for the results by comparison to computer calculations. Superoxide can also be formed in good yield by photolysis of hydrogen peroxide solutions. The photolysis of air-saturated water at pH 2 results in hydrogen sesquioxide. These lamps provide a convenient source of superoxide radicals which will facilitate studies of the reactions of this biologically important species. 6 figures, 2 tables.