Intrinsic Rashba coupling due to hydrogen bonding in DNA and Oligopeptides*

Recently experiments have shown very significant spin activity in biological molecules such
as DNA, proteins, oligopeptides, and aminoacids. Such molecules have in common their
chiral structure, time-reversal symmetry, and the absence of magnetic exchange interactions.
The spin activity is then assumed to be due to either the intrinsic spin-orbit (SO) interaction
or SO coupled to the presence of strong local sources of electric fields. Here
we derive an analytical tight-binding Hamiltonian model for oligopeptides that contemplates
both intrinsic SO and Rashba interaction induced by hydrogen bonding. We use lowest order
perturbation theory band-folding scheme and derive the reciprocal space intrinsic and Rashba
type Hamiltonian terms to evaluate the spin activity of the oligopeptide and its dependence on
molecule uniaxial deformations. SO strengths in the tens of meV are found and explicit spin active
deformation potentials. We find a rich interplay between responses to deformations
both to enhance and diminish SO strength that allows for experimental testing of the orbital model.
Hydrogen bonding as the source of spin activity further enhances, coupled to chirality, the
ubiquity of spin effects in biological molecular structures.