In the Laboratory
Chiral Crystallization of Ethylenediamine Sulfate
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Lawrence Koby, Jyothi B. Ningappa, Maria Dakessian, and Louis A. Cuccia* Department of Chemistry and Biochemistry, Concordia University, Montréal, Québec, Canada, H4B 1R6; *
[email protected] Scientists are continuously searching for the origin of homochirality (1, 2). The intriguing topic of molecular asymmetry has captured the interest of many scientists since the second half of the 19th century (2, 3). In 1811 and 1812, François Arago and Jean Baptiste Biot performed experiments with chiral quartz crystals that led to the discovery of optical rotation (4). Having viewed colorless quartz crystals between two polarized sheets Arago and Biot noticed that as one of the polarized sheets was rotated, different colors could be seen within the crystals. Following these observations it was still unclear as to what property of optically-active materials allowed them to rotate polarized light. It was the work of the French chemist Louis Pasteur that addressed this uncertainty (5, 6). In 1848 Pasteur was the first to resolve a racemic mixture by manually separating the two enantiomorphous crystal forms of sodium ammonium tartrate under a microscope. Pasteur studied different organic crystals and their relation to optical activity of the organic substances in solution. Having carefully studied the left- and right-handed crystals of sodium ammonium tartrate he came to the conclusion that molecular dissymmetry is the property that allows solids, liquids, and gases to rotate polarized light (7–10). Readers interested in repeating Pasteur’s historic and remarkable experiment (11) can refer to experimental protocols reported by Kauffman et al. (12) and Tobe (13). By looking around us it is difficult not to notice that chirality is part of our everyday lives. For example at the macroscopic level, one of the most obvious examples is our hands, which are non-superimposable mirror images of each other. Chirality is also evident at the molecular level. Some examples include nucleic acids (DNA and RNA), and α-amino acids that can exist in either the L form or the D form (4). It is the B form of the DNA double helix that is found in living organisms, which is a right-handed helix. Furthermore, naturally occurring amino acids are almost invariably of the L form (14). Interestingly enough, chiral materials can be made from achiral building blocks (15). A macroscopic example of this is a spiral staircase. Although the steps that make up the staircase are not chiral themselves, the staircase has chirality because it can either be a right-handed or a left-handed spiral (9). The realization of the significance of chirality in our world makes one wonder why nature seems to favor only one enantiomer of complex molecules (16). Although many theories on the origins of biological homochirality have been studied, a precise explanation has yet to be found (17). A popular theory speculates that optically-active crystals such as quartz could have been the chiral influence that led to the origin of optically-active molecules that were present in the prebiotic world. Speculations such as these have drawn a great deal of attention towards optically-active crystals (15, 18). Molecules can crystallize in one of 230 space groups. Sixty-five of these space groups are chiral and enantiomerically pure chiral molecules necessarily crystallize in one of these www.JCE.DivCHED.org
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65 space groups (19). “The reason is that one relies on the property of molecules keeping their own identity in the crystal structure” (20). Solutions or melts of chiral molecules, containing only one enantiomorph, must crystallize in a chiral manner. Racemic mixtures can form either: (i) racemic heterochiral crystals (racemic compound) or (ii) an equal number of dextrorotatory and levorotatory homochiral crystals (conglomerate) (21). Although achiral molecules normally crystallize in achiral space groups, in some cases they can form chiral crystals. If a crystal that is made up of achiral molecules crystallizes into one of the 65 chiral space groups then the crystal structure is chiral regardless of the symmetry of the individual molecules (19). The chiral crystallization of sodium chlorate and sodium bromate has been known for over 100 years (22). It was shown by Kipping and Pope that when crystallized from an aqueous solution, statistically equal numbers of dextrorotatory and levorotatory sodium chlorate crystals are obtained, and so an overall state of chiral symmetry is reached (16). However, to obtain homochiral crystals, the symmetry of the system must be broken or a resolution step is required. One method of chiral symmetry breaking was reported by Kondepudi et al. (23). They showed that if an aqueous solution of sodium chlorate was stirred during crystallization, almost all of the sodium chlorate crystals in a particular crystallization had the same chirality. It is believed that the crystals randomly start nucleating, but the first crystal that is struck by the stirrer clones new nuclei of the same chirality (16). However, this “chiral autocatalytic secondary nucleation” does not predetermine the final chirality of the crystals (i.e., there is an equal probability of obtaining levorotatory or dextrorotatory crystals) (24). Although stirring leads to more of one enantiomorph than the other, from a large number of stirred crystallizations, approximately half should give mainly one enantiomorph and half mainly the other. Consequently, this mechanism does not lead to any net optical activity. Although this chiral symmetry breaking does not provide a direct solution to the origin of homochirality in nature, it has nevertheless built onto the path that could eventually lead to an answer (23). In this article we present optimal conditions for the crystallization of ethylenediamine sulfate into large chiral crystals that are ideal for polarimetry studies and observation using Polaroid sheets. This experiment is most suitable for an undergraduate organic or physical organic chemistry course at the mid- to upper-levels or any course that includes stereochemistry in the curriculum. A crystallographic description of ethylenediamine sulfate was reported by Groth in 1910, and it was noted that the crystals had strong optical rotatory power (25). In 1952, Okaya and Nitta reported a novel determination of the crystal structure of tetragonal ethylenediamine sulfate, (H2NCH2CH2NH2)H2SO4 (26). Ethylenediamine sulfate is an example of an achiral salt that crystallizes in a chiral space group, P 41 (27). As expected, in a typical crystallization of ethylenediamine sulfate an equiva-
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In the Laboratory
to seven days crystals were attained when the initial concentration of 300 mL of ethylenediamine sulfate in water is 0.44 g兾mL. This solution was left undisturbed in a crystallizing dish (12-cm × 6-cm) at room temperature to allow for slow evaporation of the aqueous solution. After approximately five to seven days, once some of the solution had evaporated, single crystals were collected with tweezers and wiped dry with Kimwipes. The crystals obtained ranged in thickness from 0.35 mm to 5.35 mm, with an average thickness of 2.06 mm. The flat, colorless, relatively-large crystals were ideal for polarimetry studies and observation using Polaroid sheets. Of the many crystals that were grown, 100 crystals were randomly sampled and their optical rotation determined for the D line of sodium (Autopol II Automatic Polarimeter, Rudolf Research, Fairfield, NJ). Hazards
Figure 1. Helical three-dimensional crystal structure of ethylenediamine sulfate. Ethylenediammonium adopts a gauche conformation enforced by hydrogen bonding with sulfate anions (27).
Ethylenediamine sulfate is harmful if absorbed through skin. Gloves and safety glasses should be used when preparing the solution for the crystallization experiment. Gloves and safety glasses should also be used when collecting and handling the crystals for measurements. Results and Discussion
Figure 2. Two mirror-image conformations of gauche ethylenediammonium.
lent number of dextrorotatory and levorotatory crystals are obtained (i.e., a racemic mixture). The crystal structure of ethylenediamine sulfate formed by the ethylenediammonium and sulfate groups is helical (Figure 1). According to Sakurai, “a striking feature of this crystal structure is that the conformation of the ethylenediammonium group is not anti but gauche with the symmetry C2”, and as a result, “two mirrorimage conformations are possible, and only one kind of the two forms exists in one of the enantiomorphous forms of this crystal” (28) (Figure 2). Experiment The optimal conditions for the growth of ethylenediamine sulfate into large single crystals require crystallization in an undisturbed evaporation dish. Within a period of five
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As expected, of the 100 crystals, approximately half, 47, showed a (+)-rotation while approximately half, 53, showed a (−)-rotation (29). The absolute value of the average optical rotation of the 100 crystals was found to be 15兾mm ± 2 for the D line of sodium. This is in agreement with the value of 15.5兾mm reported by Groth in 1910 (25). The optical rotation of the crystals was also observed using polarizing filters. The relative direction of rotation was determined by simply observing the change in optical rotatory dispersion of the crystals placed between two Polaroid sheets as one of the sheets was rotated clockwise or counterclockwise. Four crystals of similar thickness, two dextrorotatory crystals (+) and two levorotatory crystals (−), viewed with a polarization of +45 and −45 are shown in Figure 3. As the light passes through the crystals, the direction of polarization is rotated clockwise by the (+) crystals, and counterclockwise by the (−) crystals. The appearance of colors in the crystals is because of different angles of rotation for each wavelength of light (5). Having analyzed many dextrorotatory and levorotatory crystals with polarizing filters we found that the color of dextrorotatory crystals transform from colorless to violet to amber when the polarizing filter is rotated in the clockwise direction. Whereas the color of levorotatory crystals transform from colorless to amber to violet when the polarizing filter is rotated in the clockwise direction. The transformation of colors for a levorotatory crystal as the polarizing filter is rotated in the clockwise direction is shown in Figure 4. However, we were unable to distinguish between dextrorotatory and levorotatory crystals by simply comparing the crystal shapes as Pasteur did in his seminal experiment probably because of the high symmetry of the P 41 space group of ethylenediamine sulfate.
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Supplemental Material
Instructions for the students and notes for the instructor are available in this issue of JCE Online. Literature Cited
Figure 3. The optical rotatory dispersion of two dextrorotatory (upper) and two levorotatory (lower) crystals of ethylenediamine sulfate viewed with a polarization of (a) +45° and (b) 45°.
Figure 4. Plane-polarized light passing through an ethylenediamine sulfate crystal. Different colors appear by rotating the second polarizing filter (angles indicated). (Crystal thickness: 1.91 mm; optical rotation: −29.5°; optical rotation/mm: −15.4°/mm).
Conclusion The results that we have obtained from our experiments lead us to believe that the optimized conditions for the crystallization of achiral ethylenediamine sulfate provide an ideal undergraduate experiment. This experiment clearly demonstrates the chiral crystallization of an achiral molecule. Furthermore, since the crystals are easily grown and are ideal for polarimetry studies the students have an opportunity to become familiar with polarizing filters and how they can be used to distinguish between dextrorotatory and levorotatory crystals. The better we are able to understand chiral crystallization, the further along we will be in understanding the origin of biological homochirality (19). Acknowledgments This manuscript is dedicated to Jean-Marie Lehn, an inspiring mentor. We thank David Harpp and Subramanian S. Iyer for providing valuable suggestions and for proofreading the manuscript.
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1. Avetisov, V.; Goldanski, V. Proc. Natl. Acad. Sci. 1996, 93, 11435–11442. 2. Cintas, P. Angew. Chem., Int. Ed. Engl. 2002, 41, 1139–1145. 3. Prelog, V. Science 1976, 193, 17–24. 4. Suh, I.; Park, K. H.; Jensen, W. P.; Lewis, D. E. J. Chem. Educ. 1997, 74, 800–805. 5. Applequist, J. Am. Scientist 1987, 75, 59–68. 6. Hargittai, M.; Hargittai, I. Mendeleev Commun. 2003, 91– 92. 7. Pasteur, L. Ann. Chim. Phys. 1848, 24, 442–459. 8. Kauffman, G. B.; Myers, R. D. Chem. Educ. 1998, 3, 1–18. 9. Pasteur, L. Researches on the Molecular Asymmetry of Natural Organic Products, Alembic Club Reprints, No. 14; William F. Clay: Edinburgh, 1897. Translated from: Leçons de Chimie Professés en 1860; Société Chimique de Paris: Paris, 1861. 10. Kostyanovsky, R. G. Mendeleev Commun. 2003, 85–90. 11. Freemantle, M. Chem. Eng. News 2003, 81 (34), 27–30. 12. Kauffman, G. B.; Myers, R. D. J. Chem. Educ. 1975, 52, 777– 781. 13. Tobe, Y. Mendeleev Commun. 2003, 93–94. 14. Vogl, O.; Qin, M.; Bartus, J. J. Macromol. Sci. Pure Appl. Chem. 1994, A31, 1501–1517. 15. Pagni, R. M.; Compton, R. N. Cryst. Grow. Design 2002, 2, 249–253. 16. McBride, J. M.; Carter, R. L. Angew. Chem., Int. Ed. Engl. 1991, 30, 293–295. 17. Avalos, M.; Babiano, R.; Cintas, P.; Jiménez, J. L.; Palacios, J. C. Chem. Comm. 2000, 887–892. 18. Klabunovskii, E. I. Astrobiology 2001, 1, 127–131. 19. Caswell, L.; Garcia-Garibay, M. A.; Scheffer, J. R.; Trotter, J. J. Chem. Educ. 1993, 70, 785–787. 20. Flack, H. D.; Bernardinelli, G. Cryst. Eng. 2003, 6, 213–223. 21. Flack, H. D. Helv. Chim. Acta 2003, 86, 905–921. 22. Kipping, F. S.; Pope, W. J. J. Chem. Soc. Trans. 1898, 73, 606– 617. 23. Kondepudi, D. K.; Kaufman, R. J.; Singh, N. Science 1990, 250, 975–976. 24. Asakura, K.; Kobayashi, K.; Mizusawa, Y.; Ozawa, T.; Miura, T.; Tanaka, A.; Kushibe, Y.; Osanai, S. Recent Res. Devel. Pure & Applied Chem. 1997, 1, 123–132. 25. Groth, P. Chemische Krystallographie Teil 3; Engelman: Leipzig, 1910. 26. Okaya, Y.; Nitta, I. Acta Cryst. 1952, 5, 564–570. 27. Jayaraman, K.; Choudhury, A.; Rao, C. N. R. Solid State Sciences 2002, 4, 413–422. 28. Sakurai, K. J. Phys. Soc. Jap. 1961, 16, 1205–1213. 29. Siegel, J. S. Chirality 1998, 10, 24–27.
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