[meteorite-list] Murchison and S & T (Part 2 of 2)

From: Bernd Pauli HD <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 10:06:56 2004
Message-ID: <3DA0069A.142D7F83_at_lehrer.uni-karlsruhe.de>

Organic Clues in Carbonaceous Meteorites
(April, 1979, Sky & Telescope, pp. 330-332)
C.R. Pellegrino and J.A. Stoff, Rockville Centre, New York

When living things finally did take over the earth, the assembly of
proteins was made possible only by the uptake of entirely right-handed
or entirely left-handed amino acids. The geometry of long-chain carbon
compounds allowed no room for random associations of both right-handed
and left-handed components in their construction. On Earth, it was the
left-handed variety that won acceptance. Hence, terrestrial proteins,
whether they be derived from trees or mosquitoes or men (except for a
special class of single-celled organisms, which utilize right-handed
amino acids in their cell walls), are composed entirely of left-handed
amino acids.
Using a beam of plane-polarized light, it is possible to determine
the right-handedness or left-handedness of a set of molecules. A
right-handed molecule will twist or rotate the plane of polarization to
the right, whereas the left-handed variety will twist it to the left.
When homogenous mixtures of amino acids from the Murray and Murchison
meteorites were examined in this manner, no such rotation was observed,
indicating that both forms were present in equal quantities. These
findings are reminiscent of ratios presumed to have existed in
Precambrian seas prior to the emergence of cellular enzyme activity, and
strongly suggest an origin held, not in the biology of cells, but in the
chemistry of atoms.
The nature of meteoritic amino acids is different from those on
your fingertips, to be sure. Equal distributions of both molecular
configurations would seem to cast serious doubts on the feasibility of
their ever having been generated by any kind of cellular activity
or by life as we know it. Nevertheless, comparison with other
carbonaceous-chondrite meteorites occasionally leads to points of
confusion. A meteorite that in 1864 fell in Orgueil, France, and a 1938
fall in lvuna, Tanganyika, contain greater traces of right-handed than
left-handed amino acids. Not only does this mixture run counter to amino
acids found in earthly proteins, but their tendency toward one polarity
points to a possible origin in cells.
We can already provide a good explanation for discrepancies of
amino-acid ratios among carbonaceous chondrites. Given irradiation
by light, heat, X-ray, or other energy sources, it is possible to
interconvert amino acids from one form to another. For example, a
solitary left-handed molecule of valine, impelled by a constant input of
energy, would eventually flip over to a right-handed configuration. An
entire vial of left-handed valine exposed to the radiations of the sun
would, given enough time, undergo total interconversion. One should not,
however, expect to recover a vial filled only with right-handed
molecules since, once produced, they are as likely to flip back to the
left-handed variety as left-handed molecules become right-handed.
The situation is analogous to laying amillion pennies heads up (to
represent left-handed molecules) in a large tray. By randomly tossing
handfuls of them into the air - our application of energy - more and
more of the pennies would land heads down until the distribution of
heads and tails was nearly equal. Probabilities being what they are,
from this point on one would always expect to find about the same number
of heads and tails no matter how many more handfuls wer tossed.
Thus, a vial of pure left-handed valine suspended in space and
irradiated (but not fried) for a million years or so would ultimately
turn up as an optically inactive mixture of left-handed and right-handed
molecules, in spite of its initially pure form.
The Murray and Murchison meteorites are among the lightest and least
densely packed of the nearly 40 carbonaceous chondrites known today. If,
during their long passage through the solar system, they were ever part
of a large asteroidal body, then surely they resided on or very near its
surface. Consequently, their contents were left naked to the raw
energies of space, and amino acids recovered from these meteorites are
presumed to have undergone many "flips of the coin" - they have become
utterly randomized. A meteorite originating in the depths of a parent
body would have received more adequate shielding against such energy.
The lvuna and Orgueil specimens reveal compression of their matrix,
suggesting the operation of mild gravitational forces exerted by
overlaying rock in their respective parent bodies. Mixtures of amino
acids extracted from these meteorites deviate sharply from the
half-and-half composition of lighter specimens (which include the Murray
and Murchison meteorites). Skewness among the denser carbonaceous
chondrites infers an initial sample consisting largely or entirely of
right-handed amino acids.
Although the origin of these substances is still a matter of
speculation, most planetary geologists and organic chemists are in
agreement that they were contained in the meteorites prior to any
contact with our atmosphere. If we assume the least glamorous
hypothesis, then some manner of preliving chemical evolution, perhaps
advancing in the direction of molecules that would one day be able to
reproduce themselves, appears to be preserved or fossilized in
meteorites.
These celestial vagrants offer the alluring possibility that the
universe is not such a lonely place in which to live. Clouds of
formaldehyde (HCHO) spread across various parts of the galaxy
seem to exemplify the trend: wherever carbon and hydrogen and
their associated counterparts lie scattered and heated at the
right temperature, it is a fair bet that they will coalesce into
compounds of higher order. That you are alive and reading these
words is evidence that such reactions can and do occur.
Received on Sun 06 Oct 2002 05:47:06 AM PDT