Applications
based on near-infrared femtosecond laser-induced plasma
in biological materials involve numerous ionization events that inevitably
mediate physicochemical effects. Here, the physical chemistry underlying
the action of such plasma is characterized in a system of biological
interest. We have implemented wavefront shaping techniques to control
the generation of laser-induced low electron density plasma channels
in DNA aqueous solutions, which minimize the unwanted thermo-mechanical
effects associated with plasma of higher density. The number of DNA
base modifications per unit of absolute energy deposited by such cold
plasma is compared to those induced by either ultraviolet or standard
ionizing radiation (γ-rays). Analyses of various photoinduced,
oxidative, and reductive DNA base products show that the effects of
laser-induced cold plasma are mainly mediated by reactive radical
species produced upon the ionization of water, rather than by the
direct interaction of the strong laser field with DNA. In the plasma
environment, reactions among densely produced primary radicals result
in a dramatic decrease in the yields of DNA damages relative to sparse
ionizing radiation. This intense radical production also drives the
local depletion of oxygen.