Propagation of Biochirality: Crossovers and Nonclassical Crystallization Kinetics of Aspartic Acid in Water TU LEE,* YU KUN LIN, YA CHUNG TSAI, AND HUNG LIN LEE Department of Chemical and Materials Engineering, National Central University, Jhong-Li City, Taiwan, R.O.C. ABSTRACT All experimental procedures discussed could be treated as a screening tool for probing the existence of molecular association among the chiral molecules and the solvent sys- tem. The molecular association phases of a racemic conglomerate solution (CS) and a racemic compound solution (RCS), and the templating effect of aspartic acid solid surface were observed to minimize the chance of redissolving racemic conglomerate and racemic compound aspartic acid in water and reforming an RCS in crossovers experiments. Only 1 %wt% of l-aspartic acid was adequate enough to induce a transformation from a racemic compound aspartic acid to a ra- cemic conglomerate aspartic acid. This would make the propagation of biochirality more feasible and sound. However, tetrapeptide, (l-aspartic acid) 4 , failed to induce enantioseparation as tem- plates purely by crystallization. Nonclassical crystallization theory was needed to take into ac- count the existence of a CS. Fundamental parameters of the crystallization kinetics such as the induction time, interfacial energy, Gibbs energetic barrier, nucleation rate, and critical size of sta- ble nuclei of: (i) racemic compound aspartic acid, (ii) racemic compound aspartic acid seeded with 1 %wt% l-aspartic acid, (iii) racemic conglomerate aspartic acid, and (iv) l-aspartic acid were evaluated and compared with different initial supersaturation ratios. Morphological studies of crystals grown from the crystallization kinetics were also carried out. Chirality 25:768–779, 2013. © 2013 Wiley Periodicals, Inc. KEY WORDS: racemic conglomerate solution; racemic compound solution; tetrapeptide; aspartic acid; crossovers; enantioseparation; templating effect INTRODUCTION Statistical analyses demonstrated only about 5 to 10% of the racemates form conglomerates, 1 and aspartic acid falls into the remaining 90% category of a racemic forming system as evidenced by (i) the seeding experiment, 2 and (ii) the solubility test. 2 Several interesting cases such as (R,S)-2-chloromandelic acid, free base of venlafaxine, and disulfide-based iodoplumbate, whose solid state transition depending on the crystallization conditions can take place either from a racemic compound to a racemic conglomerate or vice versa. 3–5 Intriguingly, when aspartic acid was crystallized from solu- tions inside porous media, racemic conglomerate crystals of d- and l-aspartic acid were always produced. 6 Recently, we have also discovered the unusual molecular association of aspartic acid enantiomers in water forming a “racemic conglomerate solution” (CS). CS might have offered an opportunity for converting the thermodynamically stable racemic compound aspartic acid 7 into the metastable racemic conglomerate in water by either rapid acid–base reactions or antisolvent crystallization with cooling, without being concerned about its back conversion later to a racemic com- pound for quite some time! 2 As a result, symmetry breaking and chiral enrichment of aspartic acid by preferential crystal- lization should have been very common and easy to occur near the sandy seashores and the hot volcanic areas on the primitive earth 2 when the process was further coupled with homochirogenesis of aspartic acid at 90°C 8 and chiral transmission. 9 However, the birth of the first generation of left-handed aspartic acid have invoked a few questions with regard to the propagation of biochirality—the birth of the second and other generations of left-handed aspartic acid. For example, could racemic conglomerate and racemic compound aspartic acid be redissolved in water and reformed into a racemic compound solution even if the two forms have previously been separated by chance? 10 How could a regional enantioseparation of aspartic acid have propagated to a global event? Was crystallization or polymerization of aspartic acid responsible for propagation of biochirality? 11,12 Therefore, the aim of this article is to address those ques- tions by looking at the consequences of the crossovers among racemic conglomerate aspartic acid, enantiomeric aspartic acid (i.e., l-aspartic acid), tetrapeptide (i.e., (l-asp) 4 ), and racemic compound aspartic acid in water. The crystalliza- tion kinetics of: (i) racemic compound aspartic acid, (ii) race- mic compound aspartic acid seeded with 1 %wt% of l-aspartic acid, (iii) racemic conglomerate aspartic acid, (iv) l-aspartic acid compound, and (v) racemic compound aspartic acid seeded with 1 %wt% of (l-asp) 4 , were monitored and determined by electrical conductance. To avoid the temperature effect on conductivity measurements, antisolvent acetone was added into all water-based systems. Additional Supporting Information may be found in the online version of this article. Contract grant sponsor: National Science Council of Taiwan, R. O. C; Contract grant number: NSC-98-2113-M-008-006. *Correspondence to: Tu Lee, Department of Chemical and Materials Engi- neering, National Central University, 300 Jhong-Da Road, Jhong-Li City 320, Taiwan, R.O.C. E-mail: tulee@cc.ncu.edu.tw Received for publication 7 August 2012; Accepted 23 May 2013 DOI: 10.1002/chir.22212 Published online 19 July 2013 in Wiley Online Library (wileyonlinelibrary.com). © 2013 Wiley Periodicals, Inc. CHIRALITY 25:768–779 (2013)