Conglomerate Crystallization and Chiral Discrimination Phenomena In Vivo and In Vitro Ivan Bernal University of Houston, Houston, TX 77204-5641 Pasteur's famous experiment (1) provided the solution to the riddle of the nature of "normal" (RR) and "para" (RR + SS, e.g., racemic) tartaric acids. I t also provided the first documented evidence of the phenomenon now known as conglomerate crystallization, that is, the production of a mechanical mixture of chiral crystals from a saturated solution of a racemate. Pasteur's recognition of this previously unsuspected possibility provided the seminal idea on which modem stereochemistry has been built. Observations of Mixtures In 1897, Kipping and Pope (2) reported the results of a series of experiments intended to answer the question "Is there always a n equal amount of right- and left-handed material formed d u ~ % ~ each g conglomerated crystalliza1 . . . 5 . . . 10.. . 1 5 . . . 2 0 . . . 25.. . 3 0 . . . 35.. . . g o . . . . .46 tion experiment?" They selected NaC103 a s Exp. NO. test material because i t has several useful Graphical summation of Kipping and Pope's published data. properties. Like ouartz. the constituents of NaClO* are not chiFal. his, any potential bias due toeven enantiomorph was favored by margins as large as 72:28. the slightest imbalance of the two enantiomorphic constitMoreover, these solutions "sometimes yield four or five uents of a "racemic" solid sample is avoided. The use of consecutive crops of crystals in which one enantiomorachiral com~oundsthat c ~ s t a l l i z ein enantiomor~hic phous fonn predominated, but yet, on the average, equal space g ~ o u insures ~s that both enantiomeric crystailine numbers of crystals of each hand are obtained." 121 fonns will have an identical chance of forming. From a The Kipping and Pope experiment has recently been remore pragmatic standpoint, the material is transparent, peated by Mason (3)using sodium uranyl acetate, which is as is quartz. also cubic (space group PZ13). Mason examined the polarHowever, NaC103 is cubic (space groupP213), thus allowization of the luminescence of 3,596 crystals and obtained ing for rapid optical-rotation measurements that are eascomparable results. ier to perform and interpret. They are carried out using a polarizing microscope. I n just a few days, a carefully covBiomolecules that Conglomerate ered and thermostated water solution produced large The following is a list of the substances2known to play quantities of excellent crystals of similar size. an important role in living processes that undergo conThe results by Kipping and Pope are summarized by glomerate crystallization. their own published figures. Total No. of Crystals Examined Percentage of Dextro Crystals Found Weighted ~ e a nfor ' Dextro
3137 50.83 50.08(0.11)
A graphical presentation of their data is given in the figure. These results indicated several things. Within less than one standard deviation, the overall mass of accumulated material is a balanced 'mixture of right- and left-handed crystal. However, in some individual runs, one crystalline 'Weighted according to the number of crystals used in a given run. his is not an exhaustive list. Fulthermore. the field is not suffit omerale crystall zat on for many clent y explored (4)to rule o ~cong otner candoates when consloerat on 1s glven to s~~tabie crystallozaton cono vons and to the presence of sa IS, douo e sa Is, or s rnple derivatives.
468
Journal of Chemical Education
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As~aroaine. - . which canelomeratesas such (. 4.) Glutarnic acid, which conglomerates as the HCI salt and as the NHJ, M F , and Zn2+salts ( 4 ) Histidine, which eonglomerates as the HCI salt ( 4 ) Methionine, which conelomerates as such (6) Proline, which canglom&ates as the Li and Zn salts (4) Serine, which conglomerates as LiCI, LiI, and LiNO8 ad-
ducts ( 4 ) Threonine, which conglomerates as such ( 6 ) Valine, which conglomerates as the HCI salt ( 4 ) and as the amino acid itself (6)
According to ref 5, the percentage occurrence of each amino acid in protein is given below. ksparagine Glutamic acid Histidine
4.8% 4.8%
2.2%
Methionine Proline Serine Threonine Valine
1.6% 5.5% 7.8% 6.5% 6.9%
Consequences of the ~bservatibn There are two interesting consequences to the above observations that come immediately to mind. Unbalance in the Crystalline Mass
Each crystallization experiment of Kipping and Pope (2) was allowed to proeeed for many hours. Nevertheless, examination of the separated crystals revealed that they rarely constituted a 50-50 mixture of the two enantiomorph~.Instead, the crystalline mass that accumulated after a specific crystallization process usually was seriously unbalanced in one direction or the other. It-seems reasonable from the description above that when the experiment is carried out, its crystallization behavior will parallel the results of Kipping and Pope when an exactly balanced (racemate) solution of inherently chiral molecules is used. The obvious implication is that the mother liquor, which is above the unbalanced crystals, will be left concomitantly unbalanced with solute of the opposite chirality. Therefore, it is reasonable to expect this unbalance will last for long periods of time, certainly hours. Thus, if during that time some entity present in the fluid medium uses solute molecules. the chiral balance of the consuming entity in question &uld itself become severely unbalanced with resoect to the relevant enantiomor~h. The introduction oi'such a consuming entity renders the system no longer closed, and thus constituting a thermodynamically acceptable possibility for permanently uubalancing a racemic medium. Moreover, once such a chiral bias has been introduced in the consuming entity, the change could be permanent, thus rendering it selective. The nature of the consuming entity is irrelevant to this argument but could, for example, be a n oligopeptide undergoing polymerization in the racemic solution. In the
early stages, when the fluid is a balanced racemate, the would be random composition of the -mowing - .polVDeDtide .. . on the average, as expected. However, upon part~alcrystallkat~onof the medium, via a IGpping-Pope mechanrsm. .. the mother liquor would become an unbaianced nutrient for the growing oligopeptide and would remain so while unbalanced crystalline material remains undissolved. Required Testing
Experiments that are designed to test properties such as chiral induction or s~ecificitvof chemical reactions. in which a "racemic" sudstrate is used, must be subjected to one of two tests before further chemical work. Proof must be established that ihe "racemate" does not crystallize as a condomerate. This is readily achieved by measunnrr the sol;bility ofthe racemate and of the pure &antiomer.& by determining the space group of a crystal randomly selected from the mass of solid. Alternately, the solutions can be tested with a CD spectrophotometer throughout the visib l e w spectral range available. Without these tests, claims of induction or specificity could be subject to the nagging doubt that the "racemic" sample used in the emeriment mav be the product of a rec&tallization that &,duced a severely undalanced mcchanical mlxture of enanr~omorphouscwsrals. Without such assurance and, particularlyin the case of controversial results, it is possible the controversy may have arisen from using soluGons prepared from an unbalanced conglomerate mixture of crystals. Literature Cited 1. L.PaateurAnn. Chim Phys, 3rd &r. 1850,28,56. 2. Kipping,F.S.; Pop,W J J Ckem Soe., l)nns. l W , 71, 611. 3. Mamn, S. F. Pnvate mmuniestion, Mareh 20,1987.See d m , Mason, S. F.Nouv J. Chim. 1986.10.739. 4. Jacques,J.; Co1let.A.:W~len,S. H.Enodlomm, Rnopmofes ~ d R e s o l u t i o n sWiley; Intemdenee: New Ymk,1981,pp58-79. 5. Dayhoe M. 0.Allas orPmtein Seqmnee a d Stnrtun. National Biomedical Resesreh Foundation;Georgetam University Medical Center: Washington, D. C.: V d 5,p 355. 6. Albrecht, 0.:Schnakenberg, G. W:Dunn, M. S.; MeCullough, J. D. J. Phys Ckm. 1945.47.24.
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