  {\bf Purpose}: Manipulation of the radio-sensitivity of the nucleotide-base
driven by the spin blockade mechanism of diffusive free radicals against
ionizing radiation.
  {\bf Materials and methods}: We theoretically propose a mechanism which uses
the simultaneous application of circularly polarized light and an external
magnetic field to control the polarization of the free radicals and create S=1
electron-hole spin excitations (excitons) on nucleotide-base. We deploy an
ab-initio molecular dynamics model to calculate the characteristic parameters
of the light needed for optical transitions.
  {\bf Results}: As a specific example, we present the numerical results
calculated for a Guanine, in the presence of an OH free radical. To increase
the radio-resistivity of this system, a blue light source for the optical
pumping and induction of excitons on guanine can be used.
  {\bf Conclusions}: The effect of spin-injection on the formation of a free
energy barrier in diffusion controlled chemical reaction pathways leads to the
control of radiation-induced base damage. The proposed method allows us to
manipulate and partially suppress the damage induced by ionizing radiation.
