Chirality-Induced Spin Selectivity in Biology:The Role of Spin-Polarized Electron Current in Biological Electron Transport & Redox Enzymatic Activity

Quantum mechanics is the fundamental theory that describes the behaviour of nanoscale systems. In the past three decades, we have seen a growing number of manifestations of quantum mechanical effects in a variety of biomolecular systems connected to essential biological functions. In photosynthesis, long-lived vibrational coherences have been detected for light-harvesting photosynthetic complexes.

Such unexpected long-living quantum coherence at room temperature confers remarkable energy transfer efficiencies to natural photosynthetic complexes. In avian navigation, photo-generation of a radical pair inside a cryptochrome protein is exploited as a mechanism of magnetoreception. The time the radical pair stays in a singlet (inactive) or a triplet (active) spin state is influenced by the direction of the weak terrestrial magnetic field.

This is exploited as a precise compass in many bird species and plants. The above mentioned radical pair mechanism offers also scientific foundations to explain the production and control of Reactive Oxygen Species (ROS), with essential roles in cell signalling and homeostasis.

The above cases provide a quick survey of examples in the emerging field of Quantum Biology, which extends to other very different areas such as olfaction, cognition, DNA oxidative damage, etc. The most striking point in all the above examples is not just the manifestation of quantum mechanics itself, which arises when studying the biological system down to the molecular level, but in how nature has evolved to control such molecular-scale processes to fulfil vital functions.

Understanding how nature orchestrate quantum mechanical processes to its advantage is of much relevance to the current quantum technology era.

The above examples have taught us about the unexpected important role of the electronic spin in biological processes. This project proposes to study a new quantum biological effect based on the electron spin whose impact extends to all chiral redox biomolecular systems. The key underlying mechanism is based on the Chirality-Induced Spin Selectivity (CISS), which refers the inherent ability of chiral molecular structures to select one particular component of electronic spin, thereby leading to spin polarization.

When an electric current flows through a chiral molecular system, the transient electrons experience a degree of spin polarization similar to what occurs in a standard magnetic device under applied magnetic fields. The CISS effect presents two essential ingredients relevant to biology; (1) it occurs at room temperature and (2) operates in the absence of external magnetic fields.

The translation of the CISS effect into biology means a chiral peptide scaffold surrounding a redox co-factor acts as a "smart matrix" which magnetically prepares the spin of the crossing electrons going into the redox centre.

Our working team brings a unique theoretical-experimental approach combining advanced single-peptide/protein electrical characterization and the latest developments in the theory of spin-polarized electron transport. Using this synergistic approach, this consortium has already demonstrated exceedingly large electron spin polarization (>60%) in an individual 3 nanometres long alpha-helical peptide sequence trapped in a controlled nanoscale gap immersed in a physiological medium.

This project builds upon these outstanding results to (1) generate an atomistic picture of the CISS effect in a helical peptide, (2) quantify the impact of (1) in the charge transport across a redox protein, and (3) study the CISS effect on a redox enzymatic reaction. We anticipate our results will open a new area in quantum biology bringing fundamental knowledge to biological redox chemistry relevant to disease mechanisms and bioenergy.

This knowledge transcends the biological arena bringing revolutionary solutions to the design of new materials and applications in quantum information.