[extropy-chat] HISTORY: Solved & Unsolved Riddles

Fri Nov 14 09:38:09 UTC 2003

On Fri, Nov 14, 2003 at 03:08:23AM -0500, Eliezer S. Yudkowsky wrote:
> John K Clark wrote:
> 
> >One would have thought that the handedness of the molecules of life was 
> >just
> >a 50 50 luck of the draw, once one got in a slight excess a standard was 

An excess of one functional autocatalytic set/planet is sufficient. If these
emergences are a rare event (say, one in 10^3 years/planet) the second one
will never happen by virtue of radically changed environment.

> >set
> >and there was no reason to change; but there may be more to it than that.
> >Amino acids have been found in meteorites and they have the same handedness
> >as life too. Most non biological chemical processes produce an equal amount

Murchison may have had an EE of 30% of L-Ala and 50% of L-glutamic acid. This
is a single data point, and not a dramatic deviation from a racemate, if the
rate of autocatalytic set nucleation events is low.

Nature 389, 234 - 235 (1997); doi:10.1038/38392

Origins of life: A left-handed Solar System?

CHRISTOPHER F. CHYBA

Christopher F. Chyba is in the Department of Planetary Sciences, University
of Arizona, Tucson, Arizona 85721, USA.

How and why did life on Earth come to use almost exclusively laevorotatory,
or left-handed amino acids (L-enantiomers), rather than their mirror-image
dextrorotatory, or right-handed forms (D-enantiomers)? The paper by Engel and
Macko on page 265of this issue1 re-examines the Murchison meteorite, and
reinforces the surprising result that some process favouring L-enantiomers
seems to have operated before the origin of life on Earth, and probably
before the formation of the Solar System. If this is correct, our Solar
System may have formed with a built-in bias for left-handed amino acids.

An excess of L-enantiomers is an intriguing result, as it suggests that the
handedness, or chirality, of terrestrial biology might have an
extraterrestrial cause. But an L-excess indigenous to the meteorite is
difficult to prove, as it may instead be due to contamination by the
terrestrial biosphere of an originally racemic mixture. (In a racemic
mixture, L- and D-enantiomers are present in equal abundance, as in typical
laboratory chemical syntheses of amino acids.)

When amino acids were first discovered in the Murchison meteorite2, apparent
excesses of L-enantiomers ranging from 0 to 20% were found. However, the
conservative interpretation of these results was that contamination was to
blame, especially as non-protein amino acids were reported to show no
enantiomeric excess3. ('Protein' amino acids are of the type used by
terrestrial life in proteins, and are therefore the most likely to suffer
from biological contamination.) A later study4, claiming indigenous
enantiomeric excesses in Murchison of up to 70%, was criticized on similar
grounds5.

This controversy led Pillinger6 to emphasize the need for a criterion
independent of enantiomeric excess to judge the authenticity of meteoritic
amino acids. He suggested determining the isotopic composition of individual
enantiomers, because terrestrial stable isotope ratios differ from those in
meteorites.

Engel and Macko1 appear to have achieved this. These authors had previously7
used the carbon isotope ratio 13C/12C of alanine in Murchison to argue for an
excess of L-alanine over D-alanine, but reservations about these measurements
persisted8. In their present letter1, they instead examine the 15N/14N ratio
of individual Murchison amino-acid enantiomers. They find L-enantiomer
excesses in the amino acids alanine and glutamic acid . both protein amino
acids . of over 30% and 50%, respectively, with 15N/14N ratios that are too
high to include much terrestrial material. The contradiction between this and
an earlier report2 that alanine and glutamic acid have L-enantiomer excesses
of about 0 and 10%, respectively, suggests that the Murchison meteorite is
very heterogeneous.

	Figure 1 Left- and right-handed versions of the amino acid alanine.
Full legend

High resolution image and legend (43k)

Another way to establish an extraterrestrial origin is to use amino acids
that are extremely rare in the terrestrial biosphere. This was used by Zhao
and Bada9 to argue that two amino acids found above and below the
65-million-year-old Cretaceous/Tertiary boundary layer in Stevns Klint,
Denmark, were not terrestrial contaminants. The same criterion was recently
applied by Cronin and Pizzarello10 to the Murchison meteorite. These authors
discovered enantiomeric excesses of up to 9% in apparently non-biological
amino acids. So it appears that two very different approaches in two
laboratories confirm enantiomeric excesses in the Murchison meteorite.

The same isotope ratios that distinguish Murchison amino acids from their
terrestrial counterparts imply that these molecules or their chemical
precursors originated in interstellar clouds1, 8. Why should chemistry in
such a cloud have a bias towards one-handedness? One possibility10 is that an
enantiomeric preference may have been imposed on the cloud out of which our
Solar System formed by circularly polarized light, perhaps synchrotron
radiation from a neutron star11. If a substantial fraction of the organic
inventory of early Earth was then derived from comets and asteroids12, the
synchrotron-radiation hypothesis would connect terrestrial biochemistry with
the extreme physics of collapsed stars.

If the excess was indeed established by some process specific to the
molecular cloud out of which our Solar System formed, similar enantiomeric
excesses should be found in other organic-rich meteorites and in comets.
Examination of other carbonaceous chondrites could therefore test this
hypothesis. Unless there is a more widespread process operating, other solar
systems could have been formed with a preference for D-amino acids, or with
no preference at all . with possible consequences for life in those systems.
Alternatively, if the enantiomeric excess somehow arose during chemical
evolution within Murchison itself, no correlation between meteorites is
demanded.

Although these enantiomeric excesses suggest a link between extraterrestrial
organic molecules and the origin of terrestrial life, the connection is by no
means certain. A preference may have arisen during prebiotic or biotic
evolution as well. For example, consider peptide nucleic acid (PNA), a
candidate DNA precursor molecule. It has been shown that an otherwise achiral
PNA strand can have its chirality fixed by the presence of an L- or D-lysine
residue attached to its end13. One can imagine a picture in which this random
'seeding' of chirality led to a chiral preference in either prebiotic
chemistry or early life14. So even if the Solar System was born with a
preference for L-amino acids, there may have been other opportunities for
chiral choices to be made, and our understanding is far too limited to know
whether these subsequent choices might have dominated any initial
enantiomeric bias.

Finally, Lederberg15 suggested in 1965 that one criterion for detecting
extraterrestrial life in the Solar System might be to search for enantiomeric
excesses. Mounting evidence for (evidently non-biological) enantiomeric
excesses in the Murchison meteorite means that this criterion alone may be
less useful than we had hoped . whether within martian meteorites, on the
martian surface, or elsewhere.

References
1. 	Engel, M. H. & Macko, S. A. Nature 389, 265-268 (1997). | Article |
PubMed | ISI | ChemPort |
2. 	Kvenvolden, K. et al. Nature 228, 923-926 (1970). | ChemPort |
3. 	Kvenvolden, K. A., Lawless, J. G. & Ponamperuma, C. Proc. Natl Acad.
Sci. USA 68, 486-490 (1971). | ChemPort |
4. 	Engel, M. H. & Nagy, B. Nature 296, 837-840 (1982). | ISI | ChemPort
|
5. 	Bada, J. L. et al. Nature 301, 494-497 (1983). | ChemPort |
6. 	Pillinger, C. T. Nature 296, 802 (1982). | ISI |
7. 	Engel, M. H., Macko, S. A. & Silfer, J. A. Nature 348, 47-49 (1990).
| Article | PubMed | ISI | ChemPort |
8. 	Cronin, J. R. & Chang, S. in The Chemistry of Life's Origins (eds
Greenberg, J. et al.) 209-258 (Kluwer, Amsterdam, 1993).
9. 	Zhao, M. & Bada, J. L. Nature 339, 463-465 (1989). | Article | PubMed
| ISI | ChemPort |
10. 	Cronin, J. R. & Pizzarello, S. Science 275, 951-965 (1997). |
ChemPort |
11. 	Bonner, W. A. Origins of Life 21, 59-111 (1991). | ChemPort |
12. 	Chyba, C. F. & Sagan, C. in Comets and the Origin and Evolution of
Life (eds Thomas, P. J., Chyba, C. F. & McKay, C. P.) 147-173 (Springer, New
York, 1997).
13. 	Wittung, P. et al. Nature 368, 561-563 (1994). | ChemPort |
14. 	Cohen, J. Science 267, 1265-1266 (1995). | ChemPort |
15. 	Lederberg, J. Nature 207, 9-13 (1965). | PubMed | ISI | ChemPort |

> >of right and left, however if it is exposed to powerful clockwise polarized
> >microwaves it produced only right-handed sugars and left-handed 

A pulsar has two poles of opposite polarity. You will get depletion/preferred
formationi of one respective enantiomer of the racemate if irradiated with that, and equally EE
in the material accreted from that molecular cloud. But the EE is low, and
spontaneous symmetry breakings occur all the time, the smaller the scale, the
more often. 

What is interesting is chirality in the Jovian and Saturnian system (these
might be lousy with life, and life closely related to ours, though), and
beyond (where no metabolism is possible, and there should be enough primitive
material to gauge the composition of the presolar nebula).

> >amino-acids,
> >exactly what we see on Earth and in meteorites. Interestingly there is a
> >portion of the Orion Nebula that produces copious amounts of just this sort
> >of radiation; there may be a connection, if so the handedness of these
> >molecules may not be exactly universal but is the most common form in this
> >part of the galaxy.

Where should a polarised radiation source of that intensity have come from?

> Does this mean that if we encounter spacefaring aliens, we can still eat 
> them?

Sure, dry machine-phase can eat other dry machine-phase any time. For your
hypothetical case above: yes, and it will kill you. The chemistry will be
sufficiently different for it to be of no nutritional value, and diversity
will produce sufficient number of molecules acting as mutual toxins.

-- Eugen* Leitl <a href="http://leitl.org">leitl</a>
______________________________________________________________
ICBM: 48.07078, 11.61144 http://www.leitl.org
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