Enantioselective radical reactions can be induced by electron spin polarization: A quantum mechanism for Nature's emergent homochirality?

Abstract

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 polarised, 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 large 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-polarised photoelectrons from magnetite provided the initial chiral symmetry breaking necessary for the inception of homochirality in Nature, as well as suggests a new strategy for asymmetric synthesis using spin-polarised electrons.