  Coherent control of OH-free radicals interacting with the spin-triplet state
of a DNA molecule is investigated. A model Hamiltonian for molecular spin
singlet-triplet resonance is developed. We illustrate that the spin-triplet
state in DNA molecules can be efficiently populated, as the spin-injection rate
can be tuned to be orders of magnitudes greater than the decay rate due to
small spin-orbit coupling in organic molecules. Owing to the nano-second
life-time of OH free radicals, a non-equilibrium free energy barrier induced by
the injected spin triplet state that lasts approximately longer than one-micro
second in room temperature can efficiently block the initial Hydrogen
abstraction and DNA damage. For a direct demonstration of the spin-blockade
effect, a molecular simulation based on an {\em ab-initio} Car-Parrinello
molecular dynamics is deployed.
