The secret of the left and right-handed crystals: conference with Cristobal Viedma
The second conference in the cycle of seminars titled, "Chemistry for chemists", celebrated last December 1st, was headed by Cristóbal Viedma, geologist at the Universidad Complutense de Madrid. In his conference, "When left and right-handed crystals cannot coexist; Implications about the origins of life and the pharmaceutical industry", Professor Viedma analyzed homochirality present in amino acids and sugars generated by biological systems, one of the mysteries of life that Viedma may have solved with his hypotheses published in the journals Physical Review Letters and in the Journal of the American Chemical Society. The conference set forth a passionate trail for the research that yielded these surprising results through a clear exposition, fresh discourse, and a dose of good humour.
In the words of J. Michael McBride (Nature 2008, 452, 161,) the findings of Viedma can be considered "the first original method for isolating single handed crystals from a mixture of mirror-image forms since Pasteur used tweezers to effect such a separation in 1848". The editor of Physical Review Letters informed Viedma that his article had become the most cited work by physicists in the months after its publication. It is not unusual that the Complutense professor was proud of such positive feedback, above all because his research was based on "cheap science, a very simple experiment, for less than two euros, that could have been done in the 19th century".
Two molecules are chiral when the structure of one is the mirror-image of the other. They have the same molecular formula, but the spatial distribution of their atoms differs just as the left hand is different from the right, and resultantly, certain physical and chemical properties may differ from one to the other. The amino acids that generate life are only type-L, and never type-D, and the crystals that form also maintain the same chirality. But, when synthesizing the compounds in a laboratory, a racemic mixture is always observable (50% L and 50% D), given that both compounds are thermodynamically equivalent. This is what Stanley Miller procured in 1953 by simulating the pre-biotic conditions that generated life, resulting in racemic mixtures of alanine and glutamic and aspartic acids.
But as Cristóbal Viedma pointed out, "life breaks the rules", and cases such as climbing plants (in which certain species only spiral in one predetermined direction), or snail shells, or helical bacteria and viruses, make clear that "only life is capable of generating homochirality". While explaining that in living systems, sugars are type-D and amino acids are type-L, he remembered a joke that he had once made in front of the media teasing that those who work are leftists and those who organize are from the right. Although this allusion brought smiles to the faces of the audience, the professor explained that the original reaction of the press was the opposite, which made him reflect on the delicacy of "joking about politics in Spain".
In any case, the importance of controlling homochirality in the pharmaceutical industry evoked the tragic memory of a series of thalidomide intoxications; the drug, which was used in the 1960's to reduce nausea during the initial months of pregnancy, contained an isomer that has now been shown to cause fetal deformation.
There used to exist methods for separating one enantiomer from the other, but they were seldom effective. The importance of the Viedma's work lays in that it allows for the obtainment of 100% of one of the two types of crystallized isomers, without racemic mixtures, while at the same time explaining the existence of homochirality in nature. Much to his surprise, Profesor Viedma discovered that by using the chiral resolution concept (which induces the crystallization of both types of isomers by using seeds as precipitation nuclei), 100% of the crystals of a unique isomer were formed over the course of a few days from a racemic mixture of crystals, generated in a solution of achiralic molecules of sodium chlorate. Of the ten systems that he used in the experiment, there was a racemic mixture in all but one. After analyzing all of the possible variables and conducting countless replications, Viedma realized that the only element present in the system where the mixture had not been produced was a very concrete model of the magnetic stir bar.
The teflon-coated bar of the system that produced 100% one of the two possible enantiomers had a pivot ring that acted as a millstone. Thus, the crystals of a unique type were ground, favouring their redissolution in achiralic molecules. This process enhanced the growth of the crystals of the other enantiomer in a cycle that in the long run favoured one of the two types of crystals. After the publication of these results in 2005 in Physical Review Letters, Viedma repeated the experiment using a conglomerate racemate solution of chiralic molecules of aspartic acid. He obtained the same results: Upon the formation a racemic mixture of the crystals, the unique enantiomer was produced, which meant that all of the crystals were converted into a single crystal type. The only possible explanation could be that the resulting crystal was generated with its corresponding enantiomer thanks to the slow isomer racemisation. This assumption awakened suspicions from some chemists because Viedma was not a trained chemist but rather a geologist.
In spite of his critics, his work was published in 2008 in the Journal of the American Chemical Society, though it has yet to be discovered why the system favours one enantiomer. But Cristóbal Viedma's research offers a plausible hypothesis about the prebiotic conditions for the origin of life. Furthermore his method of obtaining the deracemisation of enantiomers has generated a Dutch patent for a machine that produces the grinding of the system which, in minutes, results in the complete conversion of one of the isomers, highly impacting the pharmaceutical industry.
At the end of the conference, many of the attending chemists were surprised by the fact that a geologist had conducted such notable research in their own field, provoking suspicions among some, but rousing the admiration of all.
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For more information, consult this video about the instantaneous transformation of trigonal crystals of potassium nitrate when entering in contact with the rhombic phase of the same compound.
1.- Cristóbal Viedma, José E. Ortiz, Trinidad de Torres, Toshiko Izumi and Donna G. Blackmond. "Evolution of Solid Phase Homochirality for a Proteinogenic Amino Acid", J. Am. Chem. Soc., 2008, 130 (46), pp 15274-15275.
2.- Cristóbal Viedma. "Chiral Symmetry Breaking during Crystallization: complete chiral purity induced by nonlinear autocatalysis and recycling", Physical Review Letters, 2005, 94, 065504.
