Re: left-handed non-biological amino acids

SHinrichs9@aol.com
Tue, 11 Mar 1997 09:13:14 -0500 (EST)

There is a physical reason why non-racemic are not expected from
non-biological natural processes.

ďThe probability of the formation of one antipode or the other is therefore
the same. As the law of averages applies to chemical reactions the appearance
of an excess of one antipode is very improbable, (Ref. 1, pg. 59)Ē

Constant stereochemistry is an important feature that can cause polymers to
be efficient replicators. Constant stereochemistry can force certain chemical
reaction to occur on just one side of the polymer so that complimentary
pairing relationships consistently form on one side potentially resulting in
an efficient replication process. According to Eigenís paradox (Ref. 2) life
must maintain at least a certain minimum replication efficiency in order for
itís information to continue to survive through replication. The requirement
for efficiency gets greater the longer the polymer which creates a serious
challenge for abiogensis scenarios. Without constant stereochemistry it may
be essentially impossible to maintain sufficient level of efficiency in order
for chemical evolution to develop an efficient nucleic acid based cell.

Actually, I have debated the constant stereochemistry issue with Dr. Robinson
from Talk.Origins. He pointed out the fact that there are known processes
which can separate stereoisomers (crystallization of tartaric acid being the
classic example). My organic chemistry (Ref. 3) text book aside from tartaric
acid also mentioned the following naturally occurring optically active amines
(the alkaloids); brucine, strychnine, quinine and cinchonine. My textbook and
Robinson did not mention the mechanism which caused them to be formed
optically active. Also, in this discussion group it has not been mentioned
the mechanism what caused the non-racemic results from the Murchison
meteorite. It may have been caused by a mechanism that would not relate to
abiogenesis scenarios. It is important to understand the mechanism which
caused these non-racemic results in order to determine if the have any
relevance to abiogenesis on earth. Ref. 4 reports there is no evidence for
asymmetrical adsorption on clays which are the most commonly proposed
adsorbing surface for abiogenesis

A common laboratory process for separating out or resoluting constant
stereochemistry from racemic solution usually involves salt formation between
a racemic amine and an optically active acid. In the optically active
solution, one of the enantiomers have different physical properties such as
melting point so it could crystallize out of the solution; thus, be separated
out by precipitating out or filtering. A prebiotic soup which would be mostly
water; thus, would never become optically active so the development by
non-biological means of non-racemic amino acids or nucleic acids by this
process appears implausible. The enantiomers in a racemic solution have the
same physical properties which makes it apparently impossible to separate
them out under any plausible prebiotic soup.

An understanding of the process which caused these non-racemic results is
crucial in order to understand the relevance of these non-biological
non-racemic results. I do not know what the process is. It would be
appreciated if someone with knowledge in this area could report what caused
these non-biological non-racemic results so that an understanding of their
relevance to abiogenesis can be assessed.

References

1. Oparin, A. I., Life, Its Nature, Origin and Development, New York:
Academic press, 1961
2. Maynard-Smith, The Major Transitions in Evolution, Freeman, 1995
3. Cram & Cram, Organic Chemistry
4. Yockey, Information Theory and Molecular Biology, Cambridge

>One of the criticisms creationists make about the inorganic origin of life
>scenarios is that when amino acids are made in the laboratory they are 50%
>left handed and 50% right handed( a 50-50 ratio is called racemic). Since a
>mixture of left and right handed amino acids make useless proteins and
>living forms only have left handed proteins, the problem has been claimed to

>
>be fatal tothe origin of life scenario. It has been known for a while that
>non-racemic amino acids existed in the Murchison meteorite.
>
>I had posted this somewhere several months ago.
>
>***
>Unless you are willing to believe in life in outer space, there is a
>mechanism
>(unknown to us at this moment) which is quite capable of producing
>non-racemic
>amino acids. Here is data from the Murchison meteorite in which non-racemic
>amino acids were extracted.
>
>amino acid d/l ratio in Murchison meteorite
>GLU ASP PRO LEU ALA
>H2O .322 .202 .342 .166 .682
>H2O .30 .30 .30 nd .60
>HCl .176 .126 .105 .029 .307
>
>~Michael H. Engel and Bartholomew Nagy, "Distribution and
>Enantiomeric Composition of Amino Acids in the Murchison
>Meteorite", Nature , 296, April 29, 1982, p. 838.
>
>Some of these results show that something in space was able to produce
>70-90% L amino acids. This concept that Christians propagate that nature
can't
>produce anything but racemic forms is flawed. I don't know the mechanism but

>there is one. (See also Engel, Macko and Silfer, "Carbon Isotope Composition

>of Individual amino acids in the Murchison Meteorite," Nature 348, Nov. 1.
>1990, pp 47-48)****
>
>Some people still criticised these results as being due to left-handed amino

>
>acid contamination. Cronin and Pizzarello provide strong evidence that the
>chirality problem (handedness) may not be as severe as previously thought.
>Peterson in an accompanying commentary notes,
>
>"Now chemists have furnished intriguing evidence that
>certain amino acids that formed in space 4.5 billion years ago
>have a small, but significant, excess of the left-handed form.
>John R. Cronin and Sandra Pizzarello of Arizona State University
>in Tempe report their findings in the Feb. 14 SCIENCE.
>"'This is the first convincing demonstration that there may
>be some natural nonbiological process that results in a
>slight...excess of the [left-handed] amino acids,' says Jeffrey
>L. Bada of the Scripps Institution of Oceanography in La Jolla, Calif.
>"The observations suggest that extraterrestrial,
>nonbiological organic matter could have played an essential role
>in the origin of life on Earth by providing the initial asymmetry
>from which a distinctive handedness developed, Cronin and
>Pizzarello contend.
>"Using sensitive analytical techniques, the researchers
>extracted and studied in meticulous detail four amino acids found
>int he carbon-rich Murchison meteorite, a type known as a
>carbonaceous chondrite. They examined amino acids that were
>relatively common in the meteorite but were not among the 20
>amino acids found in terrestrial orangisms.
>"By focussing on these particular amino acids, Cronin and
>Pizzarello could avoid problems of contamination that would bias
>the results. In each case, the researchers found an excess of
>the left-handed form of the amino acid, ranging from 2 to 9
>percent.
>"The findings indicate that even amino acids that are never
>found in known life-forms, and so could not result from
>terrestrial biological evolution, display a lef-handed bias."~I.
>Peterson, "Left-handed Excess in Meteorite Molecules", Science
>News, Feb 22, 1997, p. 118
>
>The article itself describes their procedure
>
>"We initially targeted for study 2-amino-2,3-
>dimethylpentanoic acid (2-a-2,3dmpa), an amino acid with two
>chiral centers and, consequently, four steroisomers. This amino
>acid meets two important criteria: (i) It is present in the
>Murchison meteorite but has not been reported to occur in
>terrestrial matter, and (ii) its two chiral centers are resistant
>to epimerization because one 9C-2) lacks a hydrogen atom and the
>other (C-3) has a methine hydrogen atom of low acidity.
>Consequently, it is likely that the chiral centers retained their
>original configurationsn through the aqueous and mild thermal
>processing experienced by the meteorite parent body and that the
>original enantiomer ratios have not been compromised by
>contamination."~John R. Cronin and Sandra Pizzarello,
>"Enantiomeric Excesses in Meteoritic Amino Acids," Science
>275(February 14, 1997):951-955, p. 951
>
>and
>
>"Contamination of the 2-a-2,3-dmpa by a terrestrial source of the
>L enantiomers seems improbable. A search of Chemical Abstracts
>failed to produce a report of the natural occurrence of 2-a-2,3-
>dmpa other than that in the Murchison meteorite."~John R. Cronin
>and Sandra Pizzarello, "Enantiomeric Excesses in Meteoritic Amino
>Acids," Science 275(February 14, 1997):951-955, p. 952
>
>"The finding of enantiomeric excesses in amino acids
>indigenous to the Murchison meteorite constitutes the first
>natural evidence for the operation of an abiotic process for
>enantiomeric enrichment. The observations suggest that organic
>matter of extraterrestrial origin could have played an essential
>role in the origin of terrestrial life as a provider of the
>initial enantiomeric excesses from which homochirality
>developed."~John R. Cronin and Sandra Pizzarello, "Enantiomeric
>Excesses in Meteoritic Amino Acids," Science 275(February 14,
>1997):951-955, p. 954