Reflections on the origin of optical asymmetry on earth

Sr14Sn3As12 and Eu14Sn3As12: enantiomorph-like Zintl compounds

Two new chiral Zintl compounds, Sr14Sn3As12 and Eu14Sn3As12, were synthesized from tin-flux reactions, and the structures were determined by using single-crystal X-ray diffraction. Both compounds crystallize in the trigonal space group R3 (No. 146, Z = 3) with the anion structures containing various units: dumbbell-shaped [Sn2As6](12-) dimers, [SnAs3](7-) triangular pyramids, and isolated As(3-) anions. Very interestingly, these two compounds exhibit opposite chirality in the observed crystal structures, resembling enantiomorphs. Detailed structure analyses suggest possible steric effects among the anion clusters, and on the basis of the calculated electronic structures, substantial electron lone pairs exist on the anions of both compounds, which may provide a hint to understanding the origination of chirality in these intermetallic compounds.

The electroweak origin of biomolecular handedness

The parity-violating weak neutral current perturbation of the ground-state electronic energy has been calculated by ab initio methods for glycine over a range of chiral conformations, for L-alanine, L-α-aminopropionitrile, and for the peptide residue of polypeptides in the α-helix and the β-sheet conformation. It is found that L-alanine in its preferred conformation in aqueous solution and the L-peptides in the α-helix and the β-sheet conformation, have a lower ground-state energy than the corresponding D-enantiomers, because of the electroweak interaction. The enantiomer energy difference is small, of the order of 10 -14 J mol -1 , corresponding to an enantiomeric excess of 10 6 molecules of L-alanine or the L-peptide in one mole of the corresponding racemic mixture in thermodynamic equilibrium at ambient temperature. The significance of the energy difference between enantiomers arising from the electroweak interaction for the transition from racemic geochemistry to homochiral biochemistry in terrestrial evolution is discussed.

Homochirality as the signature of life: the SETH Cigar

A characteristic hallmark of life is its homochirality: all biomolecules are usually of one hand, e.g. on Earth life uses only L-amino acids for protein synthesis and not their D mirror images. It is therefore suggested that a search for extra-terrestrial life can be approached as a Search for Extra-Terrestrial Homochirality (SETH). A novel miniaturized space polarimeter, called the SETH Cigar, is described which could he used to detect optical rotation as the homochiral signature of life on other planets. Moving parts are avoided by replacing the normal rotating polarizer by multiple fixed polarizers at different angles as in the eye of the bee. It is believed that homochirality will be found in the subsurface layers on Mars as a relic of extinct life.

Search for chiral molecules and optical activity in extraterrestrial systems. Example of Titan

One of the main characteristics of terrestrial life is the role of optically active organic substances. Thus a search for chiral compounds and optical activity on an extraterrestrial body may give an indication of the presence of life, either fossilized or still in existence. If only abiotic conditions are prevailing the same search may still provide interesting information on the possible origins of homochiral families of biomolecules on Earth (e.g. the amino acids). In this respect, Saturn's satellite Titan is exemplary. A list of some of the most simple chiral derivatives devoid of oxygen atoms possibly present on Titan is presented. The interest of an investigation of optical activity is discussed taking into account some significant parameters. This raises numerous difficult technical problems which once solved may be helpful for further exploration of other planets.

Origins of biomolecular handedness

Classical mechanisms proposed for the transition from racemic geochemistry to homochiral biochemistry in terrestrial evolution generally ascribe to chance the particular handed choice of the L-amino acids and the D-sugars by self-replicating systems. The parity-violating weak neutral current interaction gives rise to an energy difference between a chiral molecule and its mirror-image isomer, resulting in a small stabilization of the L-amino acids and the L-peptides in the alpha-helix and the beta-sheet conformation relative to the corresponding enantiomer. The energy difference suffices to break the chiral symmetry of autocatalytic racemic reaction sequences in an open non-equilibrium system.

Dissident remarks on the origin of optical activity. An intrabiological explanation

The attempts to explain the origin of natural optical activity through external, extrabiological, agents, typically ionizing rays, i.e. as a purely incidental effect without deeper significance, have failed and should be abandoned. An alternative, intrabiological, Darwinian, explanation seeks to explain optical activity as a necessary phenomenon in early evolution. This explanation is based on the idea that organisms with the existing stereospecificity initially happened, by chance, to be more efficient than organisms with the opposite stereospecificity on independent grounds that have nothing to do with their stereospecificity. If cells based on L aminoacids are called L cells, it is to be assumed that cosmic bodies carrying L cells are about equally frequent as those carrying D cells.