Chirality (was Re: [extropy-chat] HISTORY: Solved & Unsolved Riddles)

[Amara Graps]

From: Eugen Leitl <eugen at leitl.org>, Fri, 14 Nov 2003

>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

Yes

>>  >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?

Yes, If one relies on neutron stars for chirality, then we need a
larger number than what the universe supplies. Neutron stars are
not common.

Moreover, Pascale Ehrenfreund writes:

"However, neither theory
takes into account that amino acids are very fragile compounds,
which are easily destroyed by particle radiation and even by low
energy UV photons (Ehrenfreund et al 2001b). An extraterrestrial
origin of chirality is strongly debated."

and

"The fragility of amino acids (left unprotected) in
interstellar environments and the lack of a plausible robust
mechanism (neutron stars are rare) for producing enantiomeric
excesses in amino acids, casts doubt on their interstellar origin."


See the full chirality excerpt from here:

INSTITUTE OF PHYSICS PUBLISHING REPORTS ON PROGRESS IN PHYSICS
Rep. Prog. Phys. 65 (2002) 1427-1487 PII: S0034-4885(02)04039-3

Astrophysical and astrochemical insights into the origin of life

P Ehrenfreund 1 2 , W Irvine 3 , L Becker 4 , J Blank 5 , J R
Brucato 6 , L Colangeli 6 , S Derenne 7 , D Despois 8 , A Dutrey 9 ,
H Fraaije 2 , A Lazcano 10 , T Owen 11 , F Robert 12 , an
International Space Science Institute ISSI-Team 13


10.6. Chirality and the origin of extraterrestrial organics

A fundamental characteristic of life is the homochirality of most of
its building blocks. Various theories have been proposed to explain
its origin; most of them require a chemical amplification scheme,
but they differ in the origin of the initial excess to be amplified:
random fluctuation, electroweak interaction effect or
extraterrestrial input. Detailed reviews can be found in Bonner
(1991, 1996).

Enantiomeric excesses have up to now been found only in meteoritic
matter (e.g. Engel and Macko (1997), Cronin and Pizzarello (1997),
Pizzarello and Cronin (1998, 2000)) but are searched for in
micrometeorites (e.g. Vandenabeele-Trambouze et al (2001)) and
comets (COSAC experiments on-board ESA mission ROSETTA, Thiemann and
Meierhenrich (2001)).

Early experimental approaches on the syntheses of organic compounds
do not produce chiral products (Miller and Urey 1959). Recent
evaluation of abiotic mechanisms proposed for the origin of chiral
molecules on the primitive Earth concluded that such approaches are
not likely to occur in nature (Bonner et al 1999). The absence of
chirality in products of prebiotic evolution experiments
strengthened the presumption that natural abiotic synthesis
invariably produces racemic compounds. Indeed, early analysis of
Murchison amino acids declared them racemic (e.g. Kvenvolden et al
(1970)).

In 1997, Cronin and Pizzarello reported modest L-enantiomeric
excesses of 29% in some amino acids in the Murchison meteorite.
They avoided the problems of contamination by making measurements on
2-amino-2,3-dimethylpentanoic acid,  -methyl norvaline and
isovaline. All three of these compounds are  -methyl substituted;
the first two have no known biological counterparts and the third
has a restricted distribution in fungal antibiotics. Bailey et al
(1998) have suggested that the observed enantiomeric excesses could
have been induced by circularly polarized light arising from dust
scattering in regions of high-mass star formation. These sources
occur more widely than do the supernova remnants or pulsars that
were first proposed by Rubenstein et al (1983) as sources of
circularly polarized synchrotron radiation. However, neither theory
takes into account that amino acids are very fragile compounds,
which are easily destroyed by particle radiation and even by low
energy UV photons (Ehrenfreund et al 2001b). An extraterrestrial
origin of chirality is strongly debated. Racemic mixtures of amino
acids may be turned by an amplification mechanism into (L- or
D-dominant) non-racemic mixtures under conditions that exist on
planets or through catalytical processes on the surface of minerals
(Hazen 2001). The fragility of amino acids (left unprotected) in
interstellar environments and the lack of a plausible robust
mechanism (neutron stars are rare) for producing enantiomeric
excesses in amino acids, casts doubt on their interstellar origin.
On the other hand, the production of amino acids on the parent body
via the Strecker-cyanohydrin synthesis is a robust reaction that
results in essentially racemic mixtures (Kvenvolden et al 1970).
There are, no doubt, many questions on this topic that are still
unanswered and discussions are far from resolved. However, the
possibility of coupling the delivery of extraterrestrial organic
compounds to planets with the appropriate conditions like the early
Earth, sheds new light on the importance of homochirality and the
role of exogenous delivery of organic compounds to the origin of
life.
-- 

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Amara Graps, PhD          email: amara at amara.com
Computational Physics     vita:  ftp://ftp.amara.com/pub/resume.txt
Multiplex Answers         URL:   http://www.amara.com/
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"Oh you damned observers, you always find extra things."
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