The contribution of hydrogen peroxide to the radiosensitizing effect of gold nanoparticles

Plasmid DNA in aerated aqueous solution is used as a probe to determine whose of the reactive oxygen species (ROS) generated after absorption of ultra-soft X-rays (USX) take part in biomolecule damage in the presence and in absence of Gold Nano-Particles (GNP) and specific scavengers. Citrate-coated GNPs with core sizes of 6, 10 and 25 nm are synthesized and characterized, especially in terms of plasmon band shift, zeta-potential and hydrodynamic radii (respectively 9, 21 and 30 nm). We confirm the radiosensitizing effect of GNP and show that the SSB number per plasmid increases when, for a same mass of gold element, the core size of the gold nano particles decreases. Hydroxyl radicals ((OH)-O-center dot) are scavenged using the positively-charged 2-amino-2-hydroxymethyl-1,3-propanediol (TRIS) and the neutral dimethyl sulfoxide (DMSO) molecules. Due to both negatively charged environments of DNA and GNP, at identical scavenging capacity, TRIS is more effective at quenching (OH)-O-center dot than DMSO. The strong radiosensitizing effect of hydroxyl radicals is confirmed. Methanoate anions are then used to transform (OH)-O-center dot into hydrogen peroxide; the latter being known to be non-aggressive regarding DNA in the absence of easily oxidable metallic ions (Fenton reactions). Surprisingly, in the presence of GNP, high DNA damage yields are observed even though hydrogen peroxide might not be hold as responsible. Conversely, the radiosensitizing effect of GNP is not observed anymore when H2O2 is scavenged using pyruvate ions. We demonstrate that hydrogen peroxide constitutes quite unexpectedly a hidden stock of (OH)-O-center dot which are activated at the surface of the GNP by decomposition of H2O, molecules.