  Electron transfer (ET) in biological molecules such as peptides and proteins
consists of electrons moving between well defined localized states (donors to
acceptors) through a tunneling process. Here we present an analytical model for
ET by tunneling in DNA, in the presence of Spin-Orbit (SO) interaction, to
produce a strong spin asymmetry with the intrinsic atomic SO strength in meV
range. We obtain a Hamiltonian consistent with charge transport through $\pi$
orbitals on the DNA bases and derive the behavior of ET as a function of the
injection state momentum, the spin-orbit coupling and barrier length and
strength. A highly consistent scenario arises where two concomitant mechanisms
for spin selection arises; spin interference and differential spin amplitude
decay. High spin filtering can take place at the cost of reduced amplitude
transmission assuming realistic values for the spin-orbit coupling. The spin
filtering scenario is completed by addressing the spin dependent torque under
the barrier, with a consistent conserved definition for the spin current.
