Tungsten particle-induced nicking of supercoiled plasmid DNA.

Small particles of metallic tungsten, known also as tungsten microprojectiles, are routinely used for biotechnological purposes. In such applications, tungsten was observed to affect the integrity of plasmid DNA. Here we present evidence that interaction between tungsten particles and intact circular plasmids pU19, pUC119, and ColE1 may result in generation of a limited number of single-strand DNA breaks. As a consequence, supercoiled DNA is converted into its open circular form and no fragmentation products can be detected. The rate of the tungsten-mediated reaction depends on pH but is not influenced by ascorbate, Tris, or EDTA. No DNA nicking can be observed when the tungsten particles are replaced by substances that can be leached out from these particles with water or incubation buffers. Likewise, commercial sodium tungstate, tungsten (VI) oxide, and tungsten (VI) chloride and products of its decomposition remain DNA undamaged. Native plasmid DNA molecules, upon adsorption on the surface of tungsten microparticles, may undergo some nicking without a need for participation of external catalysts.