DOI: 10.1021/cg1015274
Observation of Efficient Preferential Enrichment Phenomenon for a Cocrystal of (DL)-Phenylalanine and Fumaric Acid under Nonequilibrium Crystallization Conditions
2011, Vol. 11 607–615
Rajesh G. Gonnade, Sekai Iwama, Yuko Mori, Hiroki Takahashi, Hirohito Tsue, and Rui Tamura* Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan Received November 17, 2010; Revised Manuscript Received December 14, 2010
ABSTRACT: Described is a new strategy using cocrystallization, one of the crystal engineering techniques, to convert a racemic compound crystal into a racemic mixed crystal which can show preferential enrichment, a symmetry-breaking spontaneous enantiomeric resolution phenomenon observed upon recrystallization of a certain kind of racemic mixed crystal with a fairly ordered arrangement of the two enantiomers under nonequilibrium conditions. The 1:1 cocrystal (2) of (DL)-phenylalanine (Phe) and fumaric acid satisfied the requirements proposed for the occurrence of preferential enrichment, such as (i) a sufficient solubility difference (racemic crystal , enantiomeric crystal), (ii) the occurrence of polymorphic transition during crystallization, and (iii) the deposition of nonracemic mixed crystals with a unique crystal structure, and thus successfully showed preferential enrichment. In contrast, a racemic compound (DL)-Phe itself failed to show a polymorphic transition during crystallization and thereby preferential enrichment. These results indicate that (a) crystal engineering principles based on a high potential of two-component crystals to show a polymorphism and induce a polymorphic transition during crystallization can significantly contribute to the occurrence of preferential enrichment and (b) the proposed requirements described above for the occurrence of preferential enrichment are mandatory.
*To whom correspondence should be addressed. E-mail: tamurarui@ hes.mbox.media.kyoto-u.ac.jp.
be preferentially crystallized by seeding the supersaturated solution with a small amount of its enantiopure crystals (Figure 1b).24-27 Exploitation of economically and environmentally acceptable enantiomeric resolution methods have long been the subject of considerable interest in connection with the rationalization of the origin of biomolecular homochirality on Earth28 as well as the industrial and pharmaceutical needs of chiral organic substances.6,8,29 Since Pasteur’s first manual sorting of mirror-image D and L crystals (known as racemic conglomerate crystals) of a tartarate salt,24 a lot of modified methods have been devised until today for the resolution (preferential crystallization) of racemic conglomerate crystals composed of nonracemizable enantiomers25-27 and for the deracemization of ordinary or epitaxial racemic conglomerate crystals of racemizable molecules.30-38 However, racemates existing as a racemic conglomerate crystals occupy only less than 10% of the characterized crystalline racemates, and more than 90% of them are supposed to be racemic crystals39 which are further classified into (a) a racemic compound consisting of a regular packing of a pair of D and L enantiomers in the crystal or (b) a racemic mixed crystal composed of a random to highly ordered arrangement of equal amounts of two enantiomers in the defined positions.6-8,40 A notable difference between a racemic compound and a racemic mixed crystal is that racemic crystals are exclusively formed in the former type while nonracemic crystals are easily formed in the latter type. In this context, we speculated that a racemic compound with a high eutectic ee value may behave like a racemic mixed crystal under nonequilibrium crystallization conditions. Furthermore, if preferential enrichment, which was found to be observed for certain racemic mixed crystals exhibiting one of the two different types of polymorphisms (Figure 2a), is applicable to a racemic compound having a high eutectic ee
r 2011 American Chemical Society
Published on Web 01/05/2011
Introduction A concept of nonlinear complexity theory has been perceived to dominate various dynamic behaviors in both natural and social sciences. In the complexity system, symmetry is easily broken by a phase transition between two chaotic or metastable states.1-4 Accordingly, symmetry breaking as a novel chemical phenomenon is most likely to be observed if a phase transition is induced in the presence of appropriate detectable chemical probes such as molecular chirality. The process of crystallization can also be considered as an event of a complexity system in terms of kinetic behavior, far from thermal equilibrium, involving formation of metastable prenucleation molecular aggregates, nucleation and crystal growth, and polymorphic transition.5-8 In this context, crystallization of racemates of chiral compounds is anticipated to provide a chance of observing a chiral symmetry breaking.9-16 In 1996, we reported the first instance in which an ideal spontaneous enantiomeric resolution comprising alternating enrichment of two enantiomers largely in the mother liquor and slightly in the deposited crystals was feasible by repeating simple recrystallization of a certain kind of racemic mixed crystal (i.e., solid solution or pseudoracemate) with a fairly ordered arrangement of the two enantiomers without the aid of any external chiral element and racemization conditions, with respect to a series of chiral organic ammonium and sulfonium sulfonates (1) (Figure 1a).6-8,17-23 This unusual symmetry-breaking spontaneous enantiomeric resolution phenomenon, referred to as preferential enrichment,18 is completely opposed to the preferential crystallization of racemic conglomerate crystals composed of a mixture of homochiral R and S crystals, where one of the two enantiomers could
pubs.acs.org/crystal
608
Crystal Growth & Design, Vol. 11, No. 2, 2011
Gonnade et al.
traditionally classified as a racemic compound with high and moderate eutectic ee values, respectively.41 Both alanine and leucine exhibited polymorphic transition during crystallization in H2O-EtOH (v/v 1:1), with the former behaving like a racemic mixed crystal composed of a highly ordered arrangement of the two enantiomers and the latter existing as an achiral “lamellar twin” crystal of racemic compound type under nonequilibrium crystallization conditions using high concentrations in appropriate solvents, although the efficiency of resolution was very low due to the nature of the deposited crystals.41 Here we report a remarkable preferential enrichment phenomenon observed for the 1:1 cocrystal (2) of (DL)phenylalanine (Phe) and fumaric acid, which behaves like the desired racemic mixed crystal showing an appropriate polymorphism and allowing the deposition of nonracemic crystals of up to 5.7% ee under kinetic crystallization conditions, although a racemic compound (DL)-Phe itself could not show this phenomenon under the same conditions.
Experimental Section Figure 1. Principles of (a) preferential enrichment of a racemic mixed crystal in the case of substantial enrichment of R enantiomer in solution after first recrystallization of compounds 1 and (b) preferential crystallization of racemic conglomerate crystals.
Figure 2. Melting point diagrams with a polymorphism essential to induce preferential enrichment. (a) Two typical diagrams of a racemic mixed crystal capable of showing preferential enrichment with respect to compounds 1 (see refs 6-8). (b) A concept of transformation of a common racemic compound into another racemic compound with a high eutectic ee value and an appropriate polymorphism by crystal engineering. For clarity, only the temperatures of end of fusion are shown. Red and blue curves correspond to the metastable and stable polymorphs, respectively, near the racemic composition.
value and displaying an appropriate polymorphic transition during crystallization under nonequilibrium conditions (Figure 2b), it would provide a great impact on academic as well as industrial communities. Such indeed has been the case. Quite recently, we reported the preferential enrichment phenomenon observed for alanine and leucine, which are
General. Commercially available (DL)-Phe (>98%) and fumaric acid (>99%) were used without further purification. Melting points were measured by differential scanning calorimetry (DSC) at the scanning rate of 5 °C/min. The in situ FTIR spectra were recorded in solution and suspension by using attenuated total reflection (ATR) method on ReactIR 4000. HPLC analysis was carried out by using a chiral stationary phase column (Daicel Chiralpak CR(þ), 0.4 cmφ 15 cm), aqueous solution of HClO4 (pH = 1.5) as the mobile phase at the flow rate of 0.8 mL/min, and a UV-vis spectrometer (210 nm) as the detector. Powder X-ray diffraction patterns were recorded at a continuous scanning rate of 2° 2θ/min using Cu KR radiation (40 kV, 40 mA) with the intensity of the diffracted X-ray being collected at intervals of 0.02° 2θ. A graphite monochromator was used to remove Cu Kβ radiation. Preparation of Cocrystal 2. The DL-cocrystal 2 (5.621 g, ∼100%) was prepared by evaporation of an aqueous solution of (DL)-Phe (3.303 g, 20.0 mmol) and fumaric acid (2.321, 20.0 mmol) and characterized by DSC, powder X-ray diffraction (PXRD), and 1H NMR techniques. Similarly, the L-cocrystal 2 was prepared quantitatively and characterized. Preferential Enrichment Experiment of Cocrystal 2 in Water (Figure 5a). The D-rich cocrystals (1.089 g) of 0.4% ee were dissolved in water (10.0 mL) on heating. The resulting ca. 6-fold supersaturated solution was allowed to stand at 5 °C in a refrigerator for 60 h without stirring. The deposited cocrystals were separated from the mother liquor by filtration. From the D-rich solution, 0.082 g (48.8% ee) of D-rich cocrystals were obtained after evaporation of the solvent. The deposited L-rich cocrystals (0.999 g, 2.8% ee) were subsequently recrystallized from water (9.2 mL) in a similar manner, leading to the deposition of D-rich cocrystals (0.886 g, 3.7% ee) and an enrichment of the L-enantiomer in the mother liquor, from which L-rich cocrystals (0.101 g, 73.5% ee) were obtained after evaporation of the solvent. The recrystallization procedure was repeated five times.
Results and Discussion Requirements for Preferential Enrichment. The mechanism of preferential enrichment and the requirements for its occurrence, which we have unveiled with respect to compounds 1, are as follows (Figure 3):6-8,19 (1) Solubility Difference: The
Article
Figure 3. Mechanism of preferential enrichment in the case of crystallization from the supersaturated solution of slightly R-rich compounds 1. Homochiral 1D molecular association (a) in solution and (b) in the metastable crystal, (c) polymorphic transition, followed by (d) crystal disintegration and selective redissolution of the excess R enantiomer into solution, and (e) deposition of nonracemic mixed crystals enriched with the opposite S enantiomer.
solubility of the enantiomerically pure sample should be much higher than that of the corresponding racemic sample; this implies the high possibility of preferential formation of homochiral one-dimensional (1D) R and S chains in solution (Figure 3a). (2) Desired Crystal Structure & Polymorphic Transition: A solid-to-solid polymorphic transition of the initially formed metastable mixed crystals composed of the random alignment of the homochiral 1D R and S chains into the stable mixed crystals mainly comprising heterochiral RS dimer chains should occur during crystallization (Figure 3b, c). (3) Partial Crystal Disintegration: After this polymorphic transition, the transformed crystals become fragile due to the resulting incomplete hydrogen bonding sites in the crystal lattice. Consequently, partial crystal disintegration occurs in the transformed crystals to release the excess enantiomer selectively into solution until the deposited crystals were slightly enriched with the opposite enantiomer (Figure 3d). This process is closely associated with the higher solubility of the enantiomerically pure sample. (4) Deposition of Nonracemic
Crystal Growth & Design, Vol. 11, No. 2, 2011
609
Mixed Crystals: The resulting deposited nonracemic mixed crystals should not undergo further solvent-mediated polymorphic transition that may give exact racemic crystals of 0% ee and thereby decrease the ee value in solution, memorizing the event of chiral symmetry breaking (Figure 3e). Search for Cocrystals of Neutral Amino Acids and Fumaric Acid. The above requirement (2) suggests that the racemic compound crystal containing both homochiral 1D R and S chains and heterochiral RS dimer chain structures would be a good candidate to show preferential enrichment.6-8,19,41 Therefore, we have carried out an extensive search from the Cambridge Structural Database (CSD)42 with respect to the cocrystal of a racemic amino acid and an achiral carboxylic acid as the model compound which has the desired crystal structure, because a cocrystal has a high potential of showing a polymorphism and inducing polymorphic transition during crystallization.43 Consequently, we have found that the 1:1 cocrystal (2) of (DL)-Phe and fumaric acid satisfies all of the above requirements (high eutectic ee value, solubility difference, desired crystal structure and polymorphic transition, crystal disintegration, deposition of nonracemic mixed crystals) and indeed showed an efficient preferential enrichment phenomenon. We have also prepared the cocrystals of other neutral amino acids such as Val, Leu, Ile, Met, Ser, Tyr, and Ala with fumaric acid. Of these cocrystals, the L-cocrystal of Phe, Val, Ser, Tyr, or Ala and fumaric acid showed higher solubility than the corresponding DL-cocrystal in H2O at 25 °C (Table 1). However, only DL-cocrystal 2 exhibited a polymorphic transition during crystallization, which can be monitored by in situ ATR-IR spectroscopy as described in the latter section (Figure 7). In fact, the DL-cocrystal of Val, Leu, Ile, Met, Ser, Tyr, or Ala and fumaric acid failed to show preferential enrichment. Crystal Structures of DL- and D-Cocrystals 2. The crystal structure of DL-cocrystal 2 analyzed by us was identical to that retrieved from CSD44 and similar to that of D-cocrystal 2 from CSD45 (Figures 4 and S1, and Table S1), indicating a high possibility of existing as a racemic mixed crystal under nonequilibrium crystallization conditions. In both crystal structures of DL- and D-cocrystals 2, Phe molecules form an analogous homochiral 1D chain structure along the a-axis through an intermolecular N-H 3 3 3 O (1.80 A˚ and 1.88 A˚ for DL- and D-forms, respectively) hydrogen bond [Graph set: C(5)]46 between the ammonium hydrogen atom and the neighboring carboxylate oxygen atom (Figures 4a and S1a). In the DL-cocrystal 2, such adjacent antiparallel homochiral 1D D and L chains of Phe molecules constitute a heterochiral DL dimer chain structure along the a-axis by another intermolecular N-H 3 3 3 O (1.97 A˚) hydrogen bond [R44(10)]46 (Figure 4a). These heterochiral DL dimer chains interact with each other by weak hydrophobic forces between adjacent phenyl groups to form a 2D corrugated sheet structure on the ab plane. Furthermore, fumaric acid molecules serve as a bridge between the two neighboring parallel heterochiral 2D sheets by N-H 3 3 3 OdC (1.99 A˚) and O 3 3 3 H-O (1.69 A˚) hydrogen bonds along the c-axis (Figure 4b), creating a loosely packed crystal structure which can allow the partial crystal disintegration to occur in case irregular molecular alignment areas are formed in the crystal lattice under kinetic crystallization conditions, as discussed in the latter section (Figure 11). Preferential Enrichment. Efficient preferential enrichment was achieved when the cocrystal 2 was recrystallized from
610
Crystal Growth & Design, Vol. 11, No. 2, 2011
Gonnade et al.
Table 1. Solubility Difference between DL- and L-Cocrystals of Amino Acids and Fumaric Acida DL-cocrystal
(mg/mL) L-cocrystal (mg/mL) a
Phe (2)
Val
Leu
Ile
Met
Ser
Tyr
Ala
18.1b 50.1b
14.0c 46.1c
22.9c 6.3c
24.9b 21.2b
12.2b 4.8b
26.9b 30.7b
6.9b 10.0b
30.3b 36.5b
Measured in water at 25 °C. b A 1:1 cocrystal of amino acid and fumaric acid. c A 2:1 cocrystal of amino acid and fumaric acid.
Figure 4. Crystal structure of DL-cocrystal 2. (a) A view down the c-axis. Fumaric acid molecules were omitted for clarity. (b) A view down the a-axis. The carbon, oxygen, nitrogen, and hydrogen atoms are represented by gray, red, blue, and white circles, respectively.
H2O at 5 °C or from a mixed solvent of H2O and ethanol (v/v 1:1) at -16 °C (Figures 5a and S2). Although the original cocrystal 2 was prepared from (DL)-Phe and fumaric acid in H2O, the obtained cocrystals were no longer racemic but contained either enantiomer in small excess, indicating the occurrence of preferential enrichment during the cocrystallization procedure, as observed previously upon the final synthetic procedure of compounds 1.19 Then, repetition of recrystallization of the nonracemic cocrystals (0.4% ee, Drich) and each crop of resulting deposited cocrystals successively from the 6-fold supersaturated aqueous solution without stirring led regularly to not only a remarkable alternating enrichment of the two enantiomers up to 85% ee in the mother liquors but also a slight enrichment (35%) of both enantiomers after the last four consecutive recrystallizations. Furthermore, we found that the highest enantiomeric enrichment in the mother liquor was achieved when the 6-fold supersaturation limit was employed (Figure 5b). The time profile of the changes in the ee values in the deposited cocrystals as well as in the mother liquor during
crystallization was also monitored in another preferential enrichment experiment using L-rich cocrystal 2 of 2.3% ee (Figure 6). A slight enrichment of the opposite D enantiomer began in the deposited cocrystals immediately after the initial deposition of L-rich cocrystal of 1.0% ee and then the ee value in the deposited cocrystals became constant at around 3% after 8 h (Figure 6a). In concert with this behavior in the deposited cocrystals, enrichment of L-enantiomer occurred in the mother liquor and the ee value reached 60% after 8 h (Figure 6b), reflecting the gradual release of L enantiomer from the deposited cocrystals into solution. However, after 75 h the ee value in the mother liquor started to decrease and reached an equilibrium at around 40% ee (Figure 6c). This behavior can be explained in terms of the kinetic and thermodynamic events that take place during crystallization. It is quite natural to consider that the initial swift rise in the ee value in mother liquor could be attributed to the kinetic preferential enrichment process, followed by thermodynamical solid-solution equilibrium processes involving a partial solvent-mediated polymorphic transformation of the initially deposited D-rich cocrystals (>3.0% ee) into nearly racemic cocrystals eventually to result in the decrease in the overall ee value both in the deposited cocrystals (D-rich, 0.9% ee) and in the mother liquor (L-rich, 40% ee). Eutectic ee, Solubility Difference, and Polymorphic Transition. The eutectic ee of cocrystal 2 at 25 °C in H2O44 or
Article
Figure 5. Preferential enrichment of cocrystal 2. (a) Successive recrystallization. Conditions: aH2O (10.0 mL, 6-fold supersaturation) at 5 °C for 60 h. bH2O (9.2 mL) at 5 °C for 48 h. cH2O (8.7 mL) at 5 °C for 42 h. dH2O (7.8 mL) at 5 °C for 24 h. eH2O (6.9 mL) at 5 °C for 24 h. fRemoval of the solvent by evaporation. (b) Influence of the degree of supersaturation on the reached ee value in the mother liquor after recrystallization of D-rich cocrystal 2 of 3.7% ee from H2O at 5 °C for 48 h.
H2O-EtOH (v/v 1:1) (Figure S3, Supporting Information) was as high as 99% or 98%, respectively. The L-cocrystal 2 showed a higher solubility of 50.1 mg/mL in H2O or 55.6 mg/mL in H2O-EtOH (v/v 1:1) at 25 °C, compared to that of 18.1 mg/mL in H2O or 24.6 mg/mL in H2O-EtOH (v/v 1:1) for the nearly racemic sample (0.3% ee, L-rich) (Table 1). Furthermore, the latter nearly racemic cocrystals exhibited the occurrence of polymorphic transition during crystallization from both solvent systems, which was monitored by in situ attenuated total reflection infrared (ATRIR) spectroscopy6-8,17-23 focusing on the spectral region of 1800-1150 cm-1 including the asymmetrical and symmetrical N-H bending vibrations and asymmetrical and symmetrical COO- stretching vibrations.41 Namely, a distinct difference in the ATR-IR spectrum was observed between the supersaturated solution just before crystallization and the deposited cocrystals after crystallization (Figures 7a,b and S4). This is based on the observation that a molecular assembly structure in solution would be retained in the first-formed crystal by crystallization from the same solvent.47
Crystal Growth & Design, Vol. 11, No. 2, 2011
611
Figure 6. Time profile of the changes in the ee value during crystallization from the 6-fold supersaturated aqueous solution of L-rich cocrystal 2 of 2.3% ee at 5 °C. (a) In the deposited cocrystals and (b, c) in the mother liquor.
It is noteworthy that a racemic compound (DL)-Phe itself did not show a polymorphic transition during crystallization in H2O or H2O-EtOH and thereby failed to show preferential enrichment,41 despite the higher solubility of the L-Phe (29.6 mg/mL) compared to that of the (DL)-Phe (14.1 mg/mL) and the high eutectic ee (83%)48 of Phe in H2O at 25 °C. Deposition of Nonracemic Mixed Crystals. The powder X-ray diffraction pattern simulated from the X-ray crystallographic data of the single DL-cocrystal 2, which was distinctly different from that of the single D-cocrystal 2,45 was identical with the experimental X-ray diffraction patterns of the fine polycrystalline powder samples of the deposited D-rich cocrystal 2 of
