Sonochemistry of cytochrome c. Evidence for superoxide formation by ultrasound in argon-saturated aqueous solution

Abstract

Formation of superoxide anion radicals (O 2 ·−) induced by 50 kHz ultrasound in argon (Ar)-saturated aqueous solution was studied. Although EPR-spin trapping study with 5,5-dimethyl-1-pyrroline N-oxide (DMPO;100 mM) revealed the formation of DMPO-adducts of hydroxyl radicals ( ·OH) and hydroperoxy radicals (HO 2 ·; acid form of O2 ·−) in O 2-saturated solution after sonication, no evidence of HO 2 · was found in Ar-saturated solution. When ferricytochrome c (cytochrome c) in Ar-saturated aqueous solution was sonicated, the reduced form of cytochrome c was observed and 80% of its formation was inhibited by the addition of superoxide dismutase (SOD). Sodium formate enhanced the production of the reduced form of cytochrome c. The % inhibition by SOD for the reduction increased in the order of Xe > Ar > He in accord with the higher temperatures of the cavitation bubbles. These results indicate that the O 2 ·− is formed by directly by the sonolysis of water in the absence of O 2 when the temperature of cavitation bubble collapse is sufficiently high.