Showing papers on "Homochirality published in 1995"

Asymmetric autocatalysis and amplification of enantiomeric excess of a chiral molecule

Abstract: THE homochirality of natural amino acids and sugars remains a puzzle for theories of the chemical origin of life1–18. In 1953 Frank7 proposed a reaction scheme by which a combination of autocatalysis and inhibition in a system of replicating chiral molecules can allow small random fluctuations in an initially racemic mixture to tip the balance to yield almost exclusively one enantiomer. Here we show experimentally that autocatalysis in a chemical reaction can indeed enhance a small initial enantiomeric excess of a chiral molecule. When a 5-pyrimidyl alkanol with a small (2%) enantiomeric excess is treated with diisopropylzinc and pyrimidine-5-car-boxaldehyde, it undergoes an autocatalytic reaction to generate more of the alkanol. Because the reaction involves a chiral catalyst generated from the initial alkanol, and because the catalytic step is enantioselective, the enantiomeric excess of the product is enhanced. This process provides a mechanism by which a small initial imbalance in chirality can become overwhelming.

Chirality and life.

Abstract: 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 non-viable, 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 primative Earth by direct accretion of the interstellar dust or through impacts of comets and asteroids.

Asymmetric catalytic reduction of carbonyl compounds using C2 symmetric diamines as chiral ligands

Abstract: The catalytic asymmetric reduction of prochiral ketones by hydride transfer using various C2 symmetric chiral diamines as ligands and rhodium complexes is studied. Kinetic studies show an increase of the enantiomeric excess with the conversion.

Origin of the homochirality of biomolecules

Abstract: Molecules built up from a given set of atoms may differ in their three-dimensional structure. They may have one or more asymmetric centres that serve as reference points for the steric distribution of the atoms. Carbon atoms, common to all biomolecules, are often such centres. For example, the Cα atom between the carboxyl and amino groups in amino acids is an asymmetric centre: looking ON ward (i.e. from the carbOxyl to the amiNo group, with the Cα oriented so that it is above the carboxyl and amino groups) the radical characterizing the amino acid may be to the right (D-molecules) or to the left (L-molecules). Nineteen of the 20 amino acids occurring in proteins have such a structure (the exception is glycine, where the radical is a hydrogen atom). These pairs of molecules cannot be brought into coincidence with their own mirror image, as is the situation with our hands. The phenomenon has therefore been named handedness, or chirality, from the Greek word cheir, meaning hand. The two forms of the chiral molecules are called enantiomers or antipodes. They differ in rotating the plane of the polarized light either to the right or to the left. The sense of rotation depends on the wavelength of the measuring light, but at a given wavelength it is always opposite for a pair of enantiomers. Chirality may also occur when achiral molecules form chiral substances during crystallization (for example, quartz forms D-quartz or Lquartz). A detailed theoretical treatment of molecular chirality is given by Barron (1991).

Perturbation of the Racemic Equilibrium between D3 Lanthanide Complexes through the Addition of Sugars

Abstract: The perturbation of the racemic equilibrium of the D3 complexes of lanthanide (III) ions with 2,6pyridinedicarboxylate by a number of sugars is studied by circularly polarized luminescence spectroscopy. Various spectroscopic probes show that the addition of the chiral sugar has no observable effect on the structure of the lanthanide complex. The dependence of the enantiomeric excess on the concentration of added sugar is shown to be linear for all of the sugars studied. This dependence is in agreement with an equilibrium model in which the optically-active lanthanide complex forms weakly bound diastereomeric outer-sphere associated species with the various sugars. No structural correlation is found between the configuration and conformation of the sugars studied and the sign of the enantiomeric excess.