Amino acids extraterrestrial

Analysis of carbon compounds—even amino acids—from extraterrestrial sources might provide deeper insights into this mystery. John Cronin and Sandra Pizzarello have examined the enantiomeric distribution of unusual amino acids obtained from the Murchison meteorite, which struck the earth on September 28, 1969, near Murchison, Australia. (By selecting unusual amino... 

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 analysis of extraterrestrial matter is concentrated on the detection of nucleic acid and protein building blocks, i.e., N-heterocycles and amino acids. The search for such compounds began immediately after the fall of the Murchison meteorite. Twenty-two amino acids were detected in it as early as 1974 eight of them pro-teinogenic, ten which hardly ever occurred in biological material, and four which were unknown in the biosphere. Up to now, about 70 amino acids have been identified (Cronin, 1998), the most common being glycine and a-aminoisobutyric acid. The latter is a branched-chain amino acid with the smallest possible number of carbon atoms. The most frequently found amino acids occur in concentrations of... 

New analyses of material from the interior of the Orgeuil and Ivona meteorites show the presence of P-alanine, glycine and y-amino-n-butyric acid as the main components (0.6-2.0ppm) traces of other amino acids were also detected. The amino acids were present as racemic mixtures, i.e., d/l = 1, so that an extraterrestrial origin can be assumed (Ehrenfreund et al., 2001). 

The number of scientific articles published on meteorites has increased dramatically in the last few years few of these, however, concern themselves with small meteorites, the size of which lies between that of the normal meteorites (from centimetres to metres in size) and that of interplanetary dust particles. In the course of an Antarctic expedition, scientists (mainly from French institutions) collected micrometeorites from 100 tons of Antarctic blue ice (Maurette et al 1991). These micrometeorites were only 100 400 pm in size five samples, each consisting of 30-35 particles, were studied to determine the amount of the extraterrestrial amino acids a-aminoisobutyric acid (AIBS) and isovaline—both of which are extremely rare on Earth—which they contained. The analysis was carried out using a well-tested and extremely sensitive HPLC system at the Scripps Institute, La Jolla. Although the micrometeorites came from an extremely clean environment, the samples must have been contaminated, as they all showed traces of L-amino acids. Only one sample showed a significantly higher concentration of AIBS (about 280 ppm). The AIBS/isovaline ratio in the samples also lay considerably above that previously found in CM-chondrites. 

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. 

Knenvolden, K., Lawless, J. G., Pering, K., et al. (1970). Evidence for extraterrestrial amino acids and hydrocarbons in the Murchison meteorite. Nature, 228, 923-6. 

Zhao, M. and Bada, J. L. (1989). Extraterrestrial amino acids in cretaceous/tertiary boundary sediments at Stevns Klint, Denmark. Nature, 339, 463-5. 

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. 

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. 

Some N15 enrichment in individual Murchison amino acids (versus terrestrial counterparts) suggests an extraterrestrial origin for an L-enantiomer excess-Engel, M. H., and Macko S. A. Nature, 1997, 389,265. 

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. 

The question of the (nearly) exclusive use of l amino acids and d sugars in living organisms is one of the unsolved enigmas of the chemical sciences. As there are nonracemic amino acids found in meteorites [143,144] and as cpl occurs in outer space [145] and can be produced under natural terrestrial environmental conditions, cpl photolysis is discussed as a source of homochirality (up to scenarios like the synchronization of terrestrial cpl day cycles with the periodicity of tides [146]). The question of amino acid homochirality being due to asymmetric photolysis in space is discussed by Cerf and Jorisson [147], who summarize, In conclusion, we hope to have convinced the reader that the role of extraterrestrial asymmetric photolysis in the origin of the homochirality of natural amino acids on Earth, if at all involved, is far more complicated than is usually apprehended in the astronomical literature. The same authors more recently gave a critical review of the ample literature on asymmetric photochemistry as a possible source of homochirality [148]. 

The configuration of natural amino acids has led to studies on the possibility that homochirality emerged at a prebiotic stage, which may be supported by the presence of amino acids as non-racemic mixtures in meteorites (32). This enantiomeric excess may have resulted from the exposition of extraterrestrial matter to circularly polarized light (42). Whatever the origin of this enantiomeric excess, it may have initiated stereoselective processes through different catalytic pathways (7). Symmetry breaking may also have resulted from reactivity in connection with other processes such as crystallization or interfacial chemistry (43, 44) and polymerization of amino acids (45). 

Ehrenfreund P., Glavin D. P., Botta O., Cooper G., and Bada J. L. (2001) Extraterrestrial amino acids in Orgueil and Ivuna tracing the parent body of Cl type carbonaceous chondrites. Proc. Natl. Acad. Set 98, 2138-2141. 

Engel M. H. and Macko S. A. (1997) Isotopic evidence for extraterrestrial non-racemic amino acids in the Murchison meteorite. Nature 389, 265-261. 

Lawless J. G., Kvenvolden K. A., Peterson E., Ponnamperuma C., and Jarosewich E. (1972) Evidence for amino-acids of extraterrestrial origin in the Orgueil meteorite. Nature 236, 66-67. 

Amino acids, the units from which proteins are made, have been reported present in meteorites, but in such tiny amounts that the speculation has been made that what appears to be the pitter-patter of heavenly feet is probably instead the print of an earthly thumb. Part of the evidence that the ammo acids found in a meteorite by Cyril Pon-namperuma (of NASA) are really extraterrestrial in origin is that they are optically inactive—not optically active as earthly contaminants from biological sources would be. 

Blood, urine, cerebrospinal, and other physiological fluids contain a great number of posttranslationally modified amino acids (approximately 170 have been studied to date) and in a wider range of concentrations than protein hydrolysates [6], Additionally, plant sources produce about 500 nonprotein amino acids and, in geological samples, highly unusual amino acids may indicate extraterrestrial origin [7, 8],... 

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