We report experimental results that show a large and symmetric population of D and L crystals moving into complete chiral purity, with one of the enantiomers completely disappearing. The results indicate (i) a new symmetry breaking process incompatible with the hypothesis of an initial single chiral phase or "mother crystal," (ii) that total symmetry breaking and complete chiral purity can be achieved from a system that initially includes both enantiomers, and (iii) that this is achieved through a nonlinear autocatalytic-recycling process.

The inexorable evolution of solid-phase single chirality is demonstrated for the first time for a proteinogenic amino acid. Enantioenrichment is observed both under attrition-enhanced conditions and without the aid of particle grinding. Differences in the form of the conversion profiles for the process under the two sets of conditions provide suggestions concerning the mechanism of the transformation.

Insights into the mechanism of attrition-enhanced deracemization and resolution of solid enantiomorphic chiral compounds are obtained by crystal size and solubility measurements and by isotopic labeling experiments. Together these results help to deconvolute the various chemical and physical rate processes contributing to the phenomenon. Crystal size measurements highlight a distinct correlation between the stochastic, transient growth of crystals and the emergence of a single solid enantiomorph under attrition conditions. The rapid mass transfer of molecules between the solution and solid phases under attrition is demonstrated, and the concept of a crystal-size-induced solubility driving force is exploited to overcome the stochastic nature of the crystal growth and dissolution processes. Extension to non-racemizing conditions provides a novel methodology for chiral resolution. Implications both for practical chiral separations and for the origin of biological homochirality are discussed.

Chiral symmetry breaking occurs when a physical or chemical process spontaneously generates a large excess of one of the two enantiomers--left-handed (L) or right-handed (D)--with no preference as to which of the two enantiomers is produced. From the viewpoint of energy, these two enantiomers can exist with an equal probability, and inorganic processes that involve chiral products commonly yield a racemic mixture of both. The fact that biologically relevant molecules exist only as one of the two enantiomers is a fascinating example of complete symmetry breaking in chirality and has long intrigued the science community. The origin of this selective chirality has remained a fundamental enigma with regard to the origin of life since the time of Pasteur, some 140 years ago. Here, it is shown that two populations of chiral crystals of left and right hand cannot coexist in solution: one of the chiral populations disappears in an irreversible autocatalytic process that nurtures the other one. Final and complete chiral purity seems to be an inexorable fate in the course of the common process of growth-dissolution. This unexpected chiral symmetry breaking can be explained by the feedback between the thermodynamic control of dissolution and the kinetics of the growth process near equilibrium. This "thermodynamic-kinetic feedback near equilibrium" is established as a mechanism to achieve complete chiral purity in solid state from a previously solid racemic medium. The way in which this mechanism could operate in solutions of chiral biomolecules is described. Finally, based on this mechanism, experiments designed to search for chiral purity in a new way are proposed: chiral purity of amino acids or biopolymers is predicted in solid phase from a previously solid racemic medium. This process may have played a key role in the origin of biochirality.

A single-chirality solid phase can be obtained in boiling solutions containing a racemic mixture of left-and right-handed enantiomorphous crystals due to dissolution-crystallization cycles induced by a temperature gradient. This phenomenon provides further insights into asymmetric amplification mechanisms under presumably prebiotic conditions.Mirror-image symmetry breaking, as evidenced by the occurrence of only L-amino acids and D-sugars, constitutes an essential feature of living organisms. Although the appearance of single chirality can be understood by means of different biotic and abiotic hypotheses, 1 there has been a certain consensus on the plausibility of an autocatalytic cycle that exhibits self-recognition and mutual inhibition between enantiomers.2 Unfortunately, some elegant asymmetric reactions developed in the laboratory would hardly be compatible with prebiotic scenarios and primitive metabolic pathways. 3Phase transitions provide an alternative path to enantioenrichment as compounds accumulated in a given phase may be sorted out by natural agents and such equilibria are governed by thermodynamic and kinetic effects. The simplest model in this context is most likely the crystallization of sodium chlorate (NaClO 3 ). Like natural quartz, achiral molecules of NaClO 3 are capable of forming a supramolecular arrangement of either leftor right-handed helicity that leads to a chiral solid (chiral space group P2 1 3). 4 While static solutions of NaClO 3 give rise to statistically equal distributions of D-and L-crystals, other perturbations alter significantly this distribution.5 As demonstrated by Kondepudi and associates stirring yields mostly enantiomorphous crystals of single handedness. 6 In this case secondary nuclei grow from a mother crystallite, which may be either D-or L-, thereby leading to homochiral crystallization in a random manner. 7 Other influences on mirror symmetry breaking include b-radiation, fluid flow effects, or spontaneous resolution in gel media. 8 The effect of chiral cosolutes, especially sugars, has however been questioned. El-Hachemi et al. demonstrated recently that the effect of stirring in the Kondepudi experiment can be bypassed on inducing crystallization of NaClO 3 in boiling supersaturated solutions, which also leads to optically active crystals of arbitrary chirality. 10Nucleation was guided by withdrawing water through the distilling head from the reflux system. This strategy is also related to an aerosol-liquid cycle of a supersaturated NaClO 3 solution induced by an ultrasonic generator.8d More recently, Alexander and associates equally showed enantiomorphous segregation of either D-or L-crystals from molten NaClO 3 with stirring. 12 A continuous process of dissolution-recrystallization takes place coupled with crystal ripening that results in the emergence of a single chiral phase by the conversion of one solid enantiomorph into the other. This protocol has also been successfully applied to several organic molecules that undergo racemization in solution faster ...