Mechanically modulated spin-orbit couplings in 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 a 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 allow for experimental testing of the orbital model. Qualitative consistency with recent experiments shows the role of hydrogen bonding in spin activity. Hydrogen bonding as the source of spin activity further enhances, coupled to chirality, the ubiquity of spin effects that may be pervasive and functional in biological molecular structures.