Essais de réactions dissymétriques sur quartz optiquement actif

Chirality in Organic and Mineral Systems: A Review of Reactivity and Alteration Processes Relevant to Prebiotic Chemistry and Life Detection Missions

Chirality is a central feature in the evolution of biological systems, but the reason for biology’s strong preference for specific chiralities of amino acids, sugars, and other molecules remains a controversial and unanswered question in origins of life research. Biological polymers tend toward homochiral systems, which favor the incorporation of a single enantiomer (molecules with a specific chiral configuration) over the other. There have been numerous investigations into the processes that preferentially enrich one enantiomer to understand the evolution of an early, racemic, prebiotic organic world. Chirality can also be a property of minerals; their interaction with chiral organics is important for assessing how post-depositional alteration processes could affect the stereochemical configuration of simple and complex organic molecules. In this paper, we review the properties of organic compounds and minerals as well as the physical, chemical, and geological processes that affect organic and mineral chirality during the preservation and detection of organic compounds. We provide perspectives and discussions on the reactions and analytical techniques that can be performed in the laboratory, and comment on the state of knowledge of flight-capable technologies in current and future planetary missions, with a focus on organics analysis and life detection.

Asymmetric adsorption of alanine by quartz

Radioactive D- and L-alanine hydrochloride in 10(-5) molar dimethylformamide solution was adsorbed by d- and 1-quartz to the extent of 20 to 30 percent, as shown by radioactivity loss. d-Quartz preferentially adsorbs D-alanine and 1-quartz adsorbs L-alanine. The extent of asymmetric preferential adsorption is about 1.0 to 1.8 percent, at the 99.9 percent confidence level.

Circular dichroism tensor of a triarylmethyl propeller in sodium chlorate crystals

In 1919, Perucca reported anomalous optical rotatory dispersion from chiral NaClO(3) crystals that were colored by having been grown from a solution containing an equilibrium racemic mixture of a triarylmethane dye (Perucca, E. Nuovo Cimento 1919, 18, 112-154). Perucca's chiroptical observations are apparently consistent with a resolution of the propeller-shaped dye molecules by NaClO(3) crystals. This implies that Perucca achieved the first enantioselective adsorption of a racemic mixture on an inorganic crystal, providing evidence of the resolution of a triarylmethyl propeller compound lacking bulky ortho substituents. Following the earlier report, NaClO(3) crystals dyed with aniline blue are described herein. The rich linear optical properties of (001), (110), and (111) sections of these mixed crystals are described via their absorbance spectra in polarized light as well as images related to linear dichroism, linear birefringence, circular dichroism, and anomalous circular extinction. The linear dichroism fixes the transition electric dipole moments in the aromatic plane with respect to the growth faces of the NaClO(3) cubes. Likewise, circular dichroism measurements of four orientations of aniline blue in NaClO(3) fix a bisignate tensor with respect to the crystal growth faces. Electronic transition moments and circular dichroism tensors were computed ab initio for aniline blue. These calculations, in conjunction with the crystal-optical properties, establish a consistent mixed-crystal model. The nature of the circular extinction depends upon the crystallographic direction along which the crystals are examined. Along 100, the crystals evidence circular dichroism. Along 110, the crystals evidence mainly anomalous circular extinction. These two properties, while measured by the differential transmission of left and right circularly polarized light, are easily distinguished in their transformation properties with respect to reorientations of the sample plates. Circular dichroism is symmetric with respect to the wave vector, whereas anomalous circular extinction is antisymmetric. Analysis of Perucca's raw data reveals that he was observing a convolution of linear and circular optical properties. The relatively large circular dichroism should in principle establish the absolute configuration of the propeller-shaped molecules associated with d- or l-NaClO(3) crystals. However, this determination was not as straightforward as it appeared at the outset. In the solid state, unlike in solution, a strong chiroptical response is not in and of itself evidence of enantiomeric resolution. It is shown how it is possible to have a poor resolution-even an equal population of P and M propellers-within a given chiral NaClO(3) crystal and still have a large circular dichroism.

Robert Boyle's chiral crystal chemistry: Computational re-evaluation of enantioselective adsorption on quartz

While searching for early examples of interactions of organic chromophores with minerals in the context of a systematic study of the process of dyeing crystals, we came across Robert Boyle's description of an experiment that may have been evidence of the enantioselective adsorption of a natural product, carminic acid (7-beta-D-glucopyranosyl-9,10-dihydro-3,5,6,8-tetrahydroxy-1-methyl-9,10-dioxo-2-anthracenecarboxylic acid), to the chiral surfaces of alpha-quartz, three centuries before such interactions became the subject of active chemical investigations. In order to determine whether Boyle did indeed observe enantioselective adsorption--albeit unbeknownst to him--we attempted to dye quartz with carminic acid according to his recipe. Quartz adsorbs carminic acid only because on heating it develops a network of microfissures that adsorb dye. This process depends on capillarity, not on specific non-covalent interactions; there is no evidence of enantioselectivity adsorption to heated crystals or enantioselective epitaxy to unheated crystals. These failures changed the focus of our inquiry: Why have almost all attempts to demonstrate the enantioselective adsorption of additives to quartz crystal surfaces been generally confounding and equivocal? In order to answer this question, we complement our experimental historical re-investigation with contemporary computational techniques for modeling crystal surface structure and the adsorption of additives. Minimizations of the energies associated with the adsorption of carminic acid to relaxed, hydrated d- and l-quartz {10(-)0} surfaces are analyzed in light of quartz's abysmal record as an enantioselective stationary phase.

Chiral selection on inorganic crystalline surfaces

From synthetic drugs to biodegradable plastics to the origin of life, the chiral selection of molecules presents both daunting challenges and significant opportunities in materials science. Among the most promising, yet little explored, avenues for chiral molecular discrimination is adsorption on chiral crystalline surfaces - periodic environments that can select, concentrate and possibly even organize molecules into polymers and other macromolecular structures. Here we review experimental and theoretical approaches to chiral selection on inorganic crystalline surfaces - research that is poised to open this new frontier in understanding and exploiting surface-molecule interactions.

Asymmetric autocatalysis and the origin of chiral homogeneity in organic compounds

The discovery and development of asymmetric autocatalysis, in which the structures of the chiral catalyst and the chiral product are the same, are described. Chiral 5-pyrimidyl, 3-quinolyl, and 5-carbamoyl-3-pyridyl alkanols act as highly enantioselective asymmetric autocatalysts in the enantioselective addition of diisopropylzinc to the corresponding aldehydes, such as pyrimidine-5-carbaldehyde. 2-Alkynyl-5-pyrimidyl alkanol with an enantiomeric excess (ee) of >99.5% automultiplies practically perfectly as an asymmetric autocatalyst in a yield of >99% and >99.5% ee. Asymmetric autocatalysis with an amplification of ee has thus been realized. Consecutive asymmetric autocatalysis starting with chiral 2-alkynylpyrimidyl alkanol of only 0.6% ee amplifies its ee significantly, and yields itself as the product with >99.5% ee. The reaction of pyrimidine-5-carbaldehyde and diisopropylzinc in the presence of chiral initiators with low ee's, such as secondary alcohol, amine, carboxylic acid, mono-substituted [2.2]paracyclophane, and chiral primary alcohols due to deuterium substitution, regulates the absolute configuration of the resulting pyrimidyl alkanols, and the ee of the resulting pyrimidyl alkanol is much higher than that of the chiral initiator. Leucine and [6]helicene with very low ee's, which are known to be induced by circularly polarized light (CPL), also serve as chiral initiators to produce pyrimidyl alkanol with higher ee's. Overall, the process represents the first correlation between the chirality of CPL and an organic compound with very high ee. Chiral inorganic crystals, such as quartz and sodium chlorate, act as chiral inducers in the asymmetric autocatalysis of pyrimidyl alkanol. The process correlates for the first time ever the chirality of inorganic crystals with an organic compound with very high ee.

Asymmetric autocatalysis and its application to chiral discrimination

Chiral pyrimidyl, quinolyl, and pyridyl alkanols act as asymmetric autocatalysts with significant amplification of enantiomeric excess (ee) in the enantioselective addition of diisopropylzinc to pyrimidine-5-, quinoline-3-, and pyridine-3-carbaldehydes, respectively. 2-Alkynyl-5-pyrimidyl alkanol with as low as 0.6% ee automultiplies during the consecutive asymmetric autocatalysis with increasing ee to as high as >99.5%. Asymmetric autocatalysis is applied to chiral discrimination of organic compounds. In the presence of methyl mandelate or 2-butanol with very low ee's (0.05-0.1%) as chiral initiators, the reaction between pyrimidine-5-carbaldehyde and diisopropylzinc affords pyrimidyl alkanol with higher ee's with the correlated absolute configurations to those of the chiral initiators. Chirality of amino acids (such as leucine) and helicenes with very low ee's are also discriminated by asymmetric autocatalysis, affording pyrimidyl alkanol with very high ee's. Asymmetric autocatalysis also discriminates the chirality of primary alcohols-alpha-d, monosubstituted [2.2]paracyclophanes and octahedral cobalt complex with achiral ligands of which the chirality is due to the topology of coordination of the achiral ligand. Even the chirality of inorganic crystals such as quartz and sodium chlorate is discriminated by asymmetric autocatalysis of pyrimidyl alkanol. Thus, asymmetric autocatalysis provides a unique method for the discrimination of chiral compounds and crystals.

Chirality and life

The crucial role of homochirality and chiral homogeneity in the self-replication of contemporary biopolymers is emphasized, and the experimentally demonstrated advantages of these chirality attributes in simpler polymeric systems are summarized. The implausibility of life without chirality and hence of a biogenic scenario for the origin of chiral molecules is stressed, and chance and determinate abiotic mechanisms for the origin of chirality are reviewed briefly in the context of their potential viability on the primitive Earth. It is concluded that all such mechanisms would be nonviable, and that the turbulent prebiotic environment would require an ongoing extraterrestrial source for the accumulation of chiral molecules on the primitive Earth. A scenario is described wherein the circularly polarized ultraviolet synchrotron radiation from the neutron star remnants of supernovae engenders asymmetric photolysis of the racemic constituents in the organic mantles on interstellar dust grains, whereupon these chiral constituents are transported repetitively to the primitive Earth by direct accretion of the interstellar dust or through impacts of comets and asteroids.

Experiments on the abiotic amplification of optical activity

Our earlier experiments are briefly reviewed, involving the abiotic generation of optical activity by exposure of DL-amino acids to various "chiral" physical forces. The enantiomeric enrichments so obtained were low, however, and additional experiments were undertaken with the objective of abiotically enhancing such small enantiomeric excesses. D not equal to L Mixtures of leucine N-carboxy anhydride gave enantiomerically enriched polymers on partial polymerization, while valine NCA mixtures behaved oppositely. Leucine polymers were also found to hydrolyze stereoselectively, providing for additional enantiomeric enhancement. A repetitive sequence of partial polymerization-hydrolysis steps is suggested as a possible mechanism for the abiotic genesis of optically enriched polypeptides on the primitive Earth.

The asymmetric adsorption of alpha-aminopropionitrile on D-quartz

alpha-Aminopropionitrile was adsorbed on the powdered D-quartz, recovered as N-trifluoro-acetylalanyl-(+)-secondarybutyl ester, and analyzed by means of gas chromatography. The asymmetric result supporting the preferential adsorption of L-antipode was obtained and its significance in chemical evolution has been discussed.

Asymmetric adsorption by quartz: a model for the prebiotic origin of optical activity

One mechanism previously proposed for the abiotic accumulation of molecules of one chirality in nature is asymmetric adsorption on the chiral surfaces of optically active quartz crystals. Earlier literature in this field is reviewed, with the conclusion that previous investigations of this phenomenon, using optical rotation criteria, have afforded ambiguous results. We now have studied the adsorption of radioactive D- and L-alanine on powdered d- and l-quartz, using change in radioactivity level as a criterion for both gross and differential adsorption. d-Quartz preferentially adsorbed D-alanine from anhydrous dimethyl-formamide solution, and l-quartz L-alanine. The differential adsorption varied between 1.0 and 1.8%. The implications of these observations are discussed from the viewpoint of early chemical evolution and the origin of optically active organic compounds in nature.