Mirror Symmetry Breaking in Conglomerate Crystals: A Temperature Gradient Model En Route to Homochirality

Abstract

Deracemization, conversion of a racemate into a single enantiomer without separation of reactants or intermediates, is surely the most straightforward way to achieve the homochiral bias shown by living organisms or required in chiral drugs. It is now well established that solid-state racemates composed of homochiral unitsconglomeratescan be efficiently deracemized by physical methods that involve either attrition or thermal cycles. The latter has been less exploited, and an accurate interpretation along with the actual influence of the experimental conditions remains an open question. Here, we establish a comprehensible temperature gradient model based on a closed system with two different temperature domains interconnected through a bidirectional flow. The model allows for an equilibrium solution as it is based on a micro-reversible Becker-Döring-like system. Our results can be interpreted in terms of growth−dissolution cycles of sub-critical clusters under racemizing conditions. Like grinding, the non-equilibrium thermal gradient gives rise to a nonlinear bifurcation phenomenon, allowing complete deracemization of conglomerate-forming crystals. Asymmetric amplification results from continuous ratchet-type non-equilibrium growth−dissolution cycles. Notably, a synergistic effect is observed when grinding and thermal gradient are coupled, thus enhancing both deracemization and amplification.