Enantioselective Radical Reactions Can Be Induced by Electron Spin Polarization: A Quantum Mechanism for Nature's Emergent Homochirality?

Biomolecules that constitute life on Earth are chiral, but the precise mechanism by which homochirality emerged remains a mystery. In this work, it is demonstrated that reactions of radical pairs, where one of the radical electron spins is polarized, can be enantioselective. This phenomenon arises from transient coherent quantum dynamics of the radical pair electron spins, which is known to occur even in warm and noisy condensed phase environments, where energetic perturbations much smaller than thermal energy can have strong effects on reactivity. A quantitative theory is presented based on the molecular theory of chirality-induced spin selectivity (CISS), where electron exchange interactions and chirality-dependent spin-orbit coupling effects control enantioselectivity. This theory provides useful bounds on the maximum enantiomeric excess for these reactions, which are found to be consistent with previous experiments. The enantioseletive radical pair mechanism presented here provides an alternative mechanistic basis to a recent proposal that spin-polarized photoelectrons from magnetite provided the initial chiral symmetry breaking necessary for the inception of homochirality in Nature and suggests a new strategy for asymmetric synthesis using spin-polarized electrons.