Extraterrestrial Chirality

It is important to remember that both enantiomers are absorbing the radiation and being destroyed, one is just being photo-chemically destroyed more rapidly than the other. The longer the exposure to circularly polarized light, the more chiral the sample becomes, but, of course, too long an exposure will destroy all of the molecules. A rough estimate is that a chiral excess of 10% is probably the maximum one could expect from this process. 

Are we to assume, then, that the universe is providing a chiral environment to molecules present in outer space, generating the excess enantiomers that we need to impinge on earth and start the self-replicating chemistry leading to life From what we know of the universe and the physics of how circularly 

Although the most recent and presumably best analytical results from meteorites do indicate the presence of an excess of L-amino acids, the results obtained thus far are not conclusive for many scientists. Once a meteorite or dust particle enters our environment, it is exposed to a system in which L-amino acids dominate. The amount of amino acids found on meteorites is very small, and how can we be sure that that the system has not been contaminated Perhaps the answer to this question will have to wait until the year 2014. This is when the robotic lander Philae will detach itself from the orbiter of the Rosetta spacecraft and land on the comet 67P/Chur5nimov-Gerasimenko [8]. Some fairly sophisticated chemistry will be performed in order to separate and analyze the organic compounds that are discovered. One of these experiments will be to determine the chirality of the compounds that are detected. Maybe then we will know whether our region of space is producing an excess of L-amino acids, (see Sidebar 3.B). 

The Rosetta Mission. The Rosetta spacecraft with the lander Philae is the first spacecraft to include an experiment to detect chiral molecules outside our planet. The spacecraft was named in honor of the Rosetta stone, which allowed Egyptian hierogl5q hics to be translated, and resulted in 

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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. 

Hutt, L.D., Glavin, D.P., Bada, J.L., and Mathies, R.A., Microfabricated capillary electrophoresis amino acid chirality analyzer for extraterrestrial exploration, Anal. Chem. 71, 4000, 1999... 

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. 

The continuous availability of trillions of independent microreactors greatly multiplied the initial mixture of extraterrestrial organics and hydrothermal vent-produced chemicals into a rich variety of adsorbed and transformed materials, including lipids, amphiphiles, chiral metal complexes, amino add polymers, and nudeo-tide bases. Production and chiral amplification of polypeptides and other polymeric molecules would be induced by exposure of absorbed amino adds and organics to dehydration/rehydration cydes promoted by heat-flows beneath a sea-level hydro-thermal field or by sporadic subaerial exposure of near-shore vents and surfaces. In this environment the e.e. of chiral amino adds could have provided the ligands required for any metal centers capable of catalyzing enantiomeric dominance. The auto-amplification of a small e.e. of i-amino adds, whether extraterrestrially delivered or fluctuationally induced, thus becomes conceptually reasonable. 

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],... 

Skelley, A.M., Mathies, R.A., Chiral separation of fluorescamine-labeled amino acids using microfabricated capillary electrophoresis devices for extraterrestrial exploration. J. Chromatogr. A 2004, 1021, 191-199. 

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. 

All these uncertainties would be dispelled if clear evidence of extraterrestrial life could be obtained. Even then, kinship with earth life would have to be ruled out. Discovering life on Mars, for example, might not be decisive in itself. It is not considered impossible that Martian life could originate from earth, or terrestrial life from Mars, or both from some third site in the solar system. The two forms of life would have to differ in a significant way, for example in the chirality of some key constituent, for their independent origin to be incontrovertibly established. 

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). 

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