Meteorites, extraterrestrial chirality

Direct Examination of Extraterrestrial Chirality in Meteorites Using Asymmetric Autocatalysis... 

Highly sensitive chiral discrimination of amino acids with low ee was described. Amino acids with low ee act as a chiral initiator of asymmetric autocatalysis. In the presence of amino acids with low ee, pyrimidine-5-carbaldehyde was treated with z-P Zn to produce chiral pyrimidyl alkanol with the absolute configuration correlated with that of the amino acid by the consecutive asymmetric autocatalysis with amplification of ee. In addition, direct examination of extraterrestrial chirality was performed using meteorites by applying the asymmetric autocatalysis as the chiral sensor. The results indicated the presence of some chiral factor in the meteorites other than known organic compounds such as amino acids. 

The question also arises as to where the chiral molecules came from. Were the L-amino acids or the D-sugars selected on the primeval Earth, or are exuaterresuial sources responsible for the homochirality This second possibility is dealt with by hypotheses on the effect of circularly polarised light, of extraterrestrial origin, on chiral molecules in the molecular clouds from which the solar system was formed. One such hypothesis was proposed by Rubenstein et al. (1983) and developed further by others, particularly A. W. Bonner (Bonner and Rubenstein, 1987) both scientists worked at Stanford University. The authors believe that the actual radiation source was synchrotron radiation from supernovae. The excess of one enantiomeric form generated by this irradiation process would have needed to be transported to Earth by comets and meteorites, probably during the bombardment phase around 4.2-3.8 billion years ago. 

Chiral separation of FITC-labeled amino acid enantiomers was performed on a glass chip using fluorescent detection. Analysis time ranged from 75 s for the most basic amino acids to 160 s for the most acidic ones. y-CD was used as the chiral selector [627]. Chiral separation of amino acids in extraterrestrial samples or meteorites were also performed [610,628],... 

The FITC labeling method was also applied to chiral separations of amino acids on a microchip to determine the enantiomeric ratios of amino acids found on a meteorite [27], Since biotic amino acids are normally single enantiomers, chiral separations of amino acids are not truly clinical in nature, but illustrate the potential for chiral separations of small molecules of clinical interest. Ma-thies and co-workers used this technique to search for evidence of life in extraterrestrial environments. Enantiomeric forms of Val, Ala, Glu, and Asp could be discriminated by addition of a-, (3-, or y-cyclodextrin (CD) to the run buffer. Improved resolution with faster separations was found with respect to conventional CE. This method has been modified, by addition of SDS to the buffer, to perform cyclodextrin-modified micellar electrokinetic chromatography (CD-MEKC) [28]. Increasing the SDS concentration decreased the magnitude of elec-troosmotic flow (EOF), increasing the effective migration distance, and therefore the resolution on the microchips. 

Furthermore, does an intrinsic reason exist in our world to exclusively use the L-amino acids instead of the D-series It is remarkable that the amino acids isolated from extraterrestrial organic matter from meteorites that are at least somewhat carbonaceous are slightly enriched in the enantiomers of the same L-series as those present in the amino acids of living organisms (Pizzarello, 2004). This would become a serious question only once we have discovered a few other life-forms in the universe and found them all to be based on L-amino acids. I know that others have a different point of view and consider the origin of chirality and of homochirality to be an important problem. I think it is not, in contrast with some related but simpler questions, such as that of the propagation of homochirality. 

These four nonnatural amino acids are of extraterrestrial origin, since isotopic distributions of carbon and nitrogen are different from those of terrestrial amino acids. In addition, pristine samples of a meteorite found in South Pole regions collected under controlled conditions showed similar ee of a-methylated amino acids. The natural amino acids in the meteorites are racemic due to the ionization of their a-proton. In contrast, a-alkylated amino acids cannot completely racemize since they do not have a proton but a methyl group at the a-position. Terrestrial chirality likely originated from distinct meteorites since the a-methyl-amino acids of the Murchison meteorite occur in enantiomeric excesses. It is unknown whether the meteorites contained most of the standard amino acids during about 4.5 billion years the earth and the sun travelled through space. 

Enantiomeric excesses (ee) in chiral molecules in natural products are usually associated with biochemical processes. Initially, all chiral amino acids in meteorites were found to be racemic, a finding that had helped to establish their abiotic (and therefore extraterrestrial) origin (5). More recently, a sub-group of a-amino acids, the a-dialkyl-a-amino acids, which are unknown in the terrestrial biosphere, was found to possess L-ee of up to 15 % in the Murchison and Murray (CM2) meteorites, while all the a-H-amino acids were present as racemic mixtures 24,25). 

The connection between the asymmetry of the physics of the universe and the production of chiral molecules may well exist, but the mechanisms for it remain tenuous. Another theory suggests that initial symmetry breaking in molecular terms may have been random, rather than deterministic. By chance, an aggregate of molecules had a small preponderance of one enantiomer over the other, and this was then amplified over time. Up to 60 amino acids have been isolated from carbonaceous meteorites, and in all of these, the S-amino acids are in slight preponderance (begging another question as to how this comes about). Were these amino acids, of extraterrestrial origin, the start of chirality... 

Last, but not least, chirality studies are also of vital interest for astrochem-istry and astrobiology. In these fields, scientists are interested in tracing evidence of possible extraterrestrial life in the meteorites and other material samples of... 

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