Residual charge dependence of spin transport in chiral biomolecules

Abstract

Electron spin polarization in biomolecules has drawn significant attention as it embodies an unexplored mechanism for information propagation and electron transfer in biological systems. Despite extensive experimental and theoretical investigations of the CISS (Chirality-Induced Spin Selectivity), there are still many unanswered questions about how electrons are spin-polarized after passing through chiral molecules and, furthermore, how this process influences vital electron transfer reactions. Since peptides are excellent models to examine this aspect of the spin polarization phenomenon, calculations were carried out to compare the spin-dependent transport properties of a charge-neutral peptide composed of seven alanine residues (A7) to that containing a negatively charged aspartic acid residue (A6D), a positively charged lysine residue (A6K), and an aromatic tyrosine residue (A6Y). We focus our analysis on the spin polarization arising from both spinterface, that is, the interplay between the interfacial electric and magnetic dipole moments, and spin polarization induced by the CISS effect. We find that the asymmetry between the α and β spin transport channels is particularly pronounced in a peptide containing a tyrosine residue. Furthermore, the secondary structure of the peptide plays a key role in spin-dependent transport, with peptides possessing α-helical conformations exhibiting transmission higher than the corresponding extended structures. Tyrosine is a key molecular fragment in photosynthetic complexes and several other biological electron transfer systems. Our results indicate that the natural selection of tyrosine is linked to its versatile electronic structure that allows for a path to spin polarization, which in turn dramatically modifies the nature of electron transfer processes.