Circulation of electrons is essential in electronics and also for living organisms. While in our computers, we use semiconductor made mainly of silicon crystal, Nature has found a more effective way: proteins. Protein structures facilitate long-range electron-transfer. Scientists have shown that structural features of proteins have elements that facilitate electronic conductivity.
This phenomenon is largely due to the chiral-induced spin selectivity (CISS). It causes in particular the reduction of the elastic backscattering in electron-transfer through chiral molecules. In fact, electron transmission shows that ordered films of chiral organic molecules act as electron spin filters. The CISS effect gives us important insight for spin-selective processes in biology and allows the use of chiral molecules in spintronics applications.
The electron-transfer process allows for the transfer of energy and information from one place to another in the living organism. Therefore, an efficient electron-transfer is essential for life.
This chiral effect is notably present in DNA molecules or in the photosynthesis process. In the first case, spin selectivity is observed and could play an essential role at the quantum level to code information. The involvement of electrons in information transmission and the CISS effect within the chiral DNA polymer has been confirmed by previous studies: i.e. DNA Acts Like a Wire to Conduct Electron Signals Between Proteins for Repair and Replication.
In photosynthesis, the electron’s high conductivity is a key component inside the photosynthetic protein–pigment complexes. The conductivity of the photosynthetic apparatus is due to its geometric and atomic structure. The main function of the light-harvesting complexes is to gather light energy and to transfer this energy to the reaction centers for the photo-induced redox processes in which a high and efficient electron transfer is essential.
The photosynthetic machinery produces high energy electrons that should quickly react with other atoms within the complex. However, because of the unique molecular structure and quantum properties of biomolecules within the living system, the high energy electrons are transferred with 100% efficiency within the photosynthetic core. This is the opposite result of what was expected from physicists, who generally regards the cell as a disorganized, chaotic environment that should be wholly inhospitable to maintaining quantum states. However, because of the unique structure of biomolecules, producing properties like the CISS effect, the cellular machinery is observed to be able to transmit high energy electrons with 100% efficiency—a level of efficiency usually only seen in superconductors.
Due to the CISS effect, electron-transfer in chiral molecules is spin dependent. The phenomenon is particularly visible in the spin-selective transmission of electrons through self-assembled monolayers of double-stranded DNA. In this case, spin polarization is higher than any other known spin filters and the spin-polarized photoelectrons are observed even when the photoelectrons are generated with unpolarized light.
In a recent article, a numerical model was proposed to understand the CISS effect. Xu Yang from University of Groningen presented an electron transmission model that evaluates the role of the CISS effect in two-terminal and multiterminal linear-regime electron transport experiments. It revealed that for the CISS effect, the chirality-dependent spin transmission is accompanied by a spin-flip electron reflection process.
This would be a great contribution to society, as it may enable a whole new approach to the future of electronics.
By Dr. Olivier Alirol, RSF Research Scientist
Read more at:
ACS Publication: Proteins as Solid-State Electronic Conductors
Physicial Chemistry Letters: Chiral-Induced Spin Selectivity Effect