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Alanine Enantiomers in the Murchison Meteorite

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Alanine Enantiomers in the Murchison Meteorite scientific correspondence water movement by leaving the hoop sta- 8. Carvell, G. E. & Simons, D. J. J. Neurosci. 10, 2638–2648 (1990). step before gas-chromatographic analysis in tion during the period of stimulus presenta- 9. Dehnhardt, G. & Dücker, G. Anim. Learn. Behav. 24, 366–374 order to avoid resolution problems. (1996). tion, after which it received a fish reward. 10.Oliver, G. W. Behaviour 67, 97–114 (1978). We find that several meteoritic amino As soon as the seal was blindfolded, it 11.Harris, G. G. & van Bergeijk, W. A. J. Acoust. Soc. Am. 34, acids are racemic, including alanine. An brought its whiskers into the most forward 1831–1841 (1962). l-alanine excess was seen only in an extract 12.Reep, R. L. et al. Mar. Mamm. Sci. 14, 257–273 (1998). position. Although the whiskers of seals can 13.Bleckmann, H. et al. J. Comp. Physiol. A 168, 749–757 (1991). of a sample that included exterior frag- be easily moved by muscular action, as has ments. Figure 1 shows the chromatographic been described for manatees12, our test ani- resolution of alanine from this extract, in mal did not actively move the protracted the same chromatographic phase (Chirasil- 1 whiskers during a trial. Alanine enantiomers in L-valine) as that used by Engel and Macko8. The harbour seal responded to extreme- There are five amino acids that elute over a ly weak hydrodynamic stimuli. However, it the Murchison meteorite period of about a minute, from just before never responded when wearing a muzzle of L-alanine to sarcosine. Although only one 1 wire mesh, which let the water movement Engel and Macko have reported that ala- of these amino acids overlaps L-alanine in through but impeded whisker movements. nine indigenous to the Murchison mete- our chromatogram, in the separation The shapes of the threshold curves orite has an l-enantiomer excess of about shown in ref. 1 the chromatogram seems to (Fig. 1b) — plotted as particle displacement 33%. Furthermore, they reported very simi- be compressed and baseline separation is 11 15 (mm), velocity (mm s ) or acceleration lar d nitrogen values for L-alanine and not quite achieved between L-alanine and 12 (mm s ) against frequency (Hz) of the D-alanine; these values are quite high in sarcosine. Thus it seems probable that more stimulus — are similar to those of behav- comparison with those of terrestrial amino than one of these five amino acids eluted or ioural and physiological threshold curves acids. On this basis, they argued that conta- overlapped with L-alanine, contributing to for other aquatic animals (see ref. 3 and ref- mination of the meteorite by terrestrial the appearance of an L-alanine excess. It is erences therein). The nearly constant accel- L-alanine can be ruled out because, if it con- unlikely that these amino acids were absent eration threshold values in the range tributed to the L-enantiomer excess, the in Engel and Macko’s sample as different 15 10–50 Hz indicate that at lower frequencies lower N content of terrestrial L-alanine Murchison stones are qualitatively similar the seal responded to the acceleration com- would significantly lower the d15N value with respect to amino-acid content4. 15 ponent of the hydrodynamic stimulus. At that they observed for L-alanine. We do not know the d N values of the higher frequencies, water displacement In agreement with earlier analyses2, we amino acids that we suspect may be inter- seems to be the relevant stimulus parame- find the alanine from the interior of fering but, if their d15N values were close to ter. A similar change in operation mode is Murchison to be racemic and believe that that of indigenous L-alanine, their presence achieved by the fish lateral line3. incomplete chromatographic resolution of could go undetected isotopically. On the 15 With regard to absolute thresholds, the L-alanine and other meteoritic amino acids other hand, if their d N values were greater sensitivity of the seal’s whiskers to hydro- may have affected both the enantiomer than that of L-alanine, their presence could 15 dynamic stimuli compares well with that ratio and the δ N determinations of Engel mask the presence of terrestrial L-alanine. described for hydrodynamic receptor sys- and Macko. Given Murchison amino-acid values of up tems of, for example, the decapod crus- The Murchison meteorite contains a to +184‰, it is clear that their chromato- tacean Cherax destructor or the marine complex suite of amino acids. Whereas ter- graphic overlap, even if they were present in teleost Xiphister atropurpureus, whereas the restrial samples are dominated by the 20 relatively small amounts, would increase 15 lateral line system of some species of fish protein amino acids, over 70 amino acids the N content of the L-alanine peak and may be more sensitive by up to one or two have been positively identified in this mete- compensate, wholly or partially, for the 15 orders of magnitude (see ref. 3 and refer- orite, many of which appear to be uniquely lower N content of terrestrial L-alanine ences therein). The seal could detect water extraterrestrial. Others are present that have that might have been present. velocity at speeds as low as 245 mm s11, not been positively identified. This com- In conclusion, Engel and Macko’s argu- which in turn is several orders of magnitude plexity challenges methods of amino-acid ment that their chromatographic peak for below the water-particle velocities mea- analysis. For example, in our study of enan- L-alanine is composed almost entirely of 3 sured in the wake of a swimming fish of tiomeric ratios of meteoritic amino acids meteoritic L-alanine depends on the peak 13 about 22 cm in body length . we found that we needed a fractionation containing only L-alanine. Their argument Our results show that the whiskers of for a large excess of the L-enantiomer in the harbour seals form a hydrodynamic recep- 1.2E7 3 alanine indigenous to their Murchison tor system with a spectral sensitivity that is 1.0E7 sample would be greatly strengthened well tuned to the frequency range of fish- 8.0E6 8 if they were to provide evidence that 13 1 undance L generated water movements . 6.0E6 -alanine and the closely eluting amino Guido Dehnhardt, Björn Mauck, Ab acids that we have identified were 4.0E6 Horst Bleckmann 6 completely resolved in their gas chromato- 2.0E6 2 5 7 University of Bonn, Institute of Zoology, 4 graphy/isotope-ratio mass spectrometry 0 Poppelsdorfer Schloss, D-53115 Bonn, Germany 9.0 9.5 10.0 10.5 analyses. Such analyses of appropriate e-mail: [email protected] Time (min) amino-acid standards might also help Figure 1 Total ion chromatogram (Chirasil- L-Val, 1. Newby, T. C. et al. J. Mamm. 51, 152 (1970). to establish whether or not incomplete 2. Watkins, W. A. & Wartzok, D. Mar. Mamm. Sci. 1, 219–260 50 m20.25 mm, Alltech) of amino acids (N-trifluo- resolution affected their result. (1985). roacetyl isopropyl esters) from a desalted Murchi- S. Pizzarello, J. R. Cronin 3. Bleckmann, H. Reception of Hydrodynamic Stimuli in Aquatic son water extract. The dotted line is a single ion Department of Chemistry and Biochemistry, and Semiaquatic Animals (Gustav Fischer, Stuttgart, Jena, New York, 1994). (m/z 180) trace. Amino acids were identified from Arizona State University, Tempe, 4. Dehnhardt, G. & Kaminski, A. J. Exp. Biol. 198, 2317–2323 mass spectra. 1, D-alanine; 2, 2-amino-2-ethyl Arizona 85287-1604, USA (1995). butanoic acid; 3, L-alanine; 4, methyl proline isomer 5. Hyvärinen, H. J. Zool. 218, 663–678 (1989). 1. Engel, M. H & Macko, S. A. Nature 389, 265–268 (1997). or unsaturated acyclic C6 amino acid; 5, DL-2-methyl 6. Dykes, R. W. J. Neurophysiol. 38, 650–662 (1975). 2. Kvenvolden, K. et al. Nature 228, 923–926 (1970). 7. Mills, F. H. J. & Renouf, D. J. Exp. Mar. Biol. Ecol. 100, 3–9 norvaline; 6, DL-N-methyl alanine; 7, unsaturated C4 3. Cronin, J. R. & Pizzarello, S. Science 275, 951–955 (1997). (1986). amino acid; 8, sarcosine. 4. Cronin, J. R. & Pizzarello, S. Adv. Space Res. 3, 5–18 (1983). 236 NATURE | VOL 394 | 16 JULY 1998 Nature © Macmillan Publishers Ltd 1998.
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