Miller–Urey experiment revisited

Originally posted by FabianFnas
What did the Miller–Urey experiment show?

That aminoacids can be produced spontaneously in an well defined environment?
or
That life began in an environment like the one in the experiment?

What have we learned from the experiment?

Well it is most likely not the second option (that life began in an environment like the one in the vial) as we now know that the environment of the early Earth was not highly reducing as the Miller-Urey experiment recapitulated. Although the significance of the Miller-Urey data for the origin of life may now be in question, it began the new scientific discipline of prebiotic chemistry, and has been enormously influential in the development of ideas about the origin of life. In essence, the Miller-Urey experiment established that a primitive atmosphere was capable of producing the building blocks of life from chemicials.

Originally posted by Diodorus Siculus
Yes, I agree it was probably an accident that was subsequently perpetuated in all progeny, however, D-amino acids do however exist as tetrapeptides in the NAG-NAM cell wall of bacteria:
http://en.wikipedia.org/wiki/Peptidoglycan
This may be of interest to you too Mr. Hamilton:
http://sandwalk.blogspot.com/2008/05/amino-acids-and-racemization-problem.html

http://sandwalk.blogspot.com/2008/05/amino-acids-and-racemization-problem.html

That is brilliant! Thanks for that 🙂

So, if I have read this correctly, it basically says that, before life even began, there could have been a kind of spontaneous formation of an amino acid synthesising process that by chance favoured left-handed amino acids over right-handed ones and, one this process formed, it tended to block the equivalent process that could have resulted in mainly right-handed amino acid synthesis -thus there was a bios already in place for life to use left-handed amino acids BEFORE that life began.

Originally posted by Andrew Hamilton
[b] http://sandwalk.blogspot.com/2008/05/amino-acids-and-racemization-problem.html

That is brilliant! Thanks for that 🙂

So, if I have read this correctly, it basically says that, before life even began, there could have been a kind of spontaneous formation of an amino acid synthesising process that by chance favoured left-handed amino aci ...[text shortened]... here was a bios already in place for life to use left-handed amino acids BEFORE that life began.[/b]

Yes, that is my understanding of it too and centres on the strong stereospecificity of the enzymes involved in metabolic pathways has lead to the almost exclusive use of L-forms of amino acids in the Earth's biosphere. However, D-amino acids are not unknown in certain organisms. Pencillin actually inhibits the transpeptidase that incorporates the tetrapeptide of D-amino acids in the NAG-NAM polymer of bacterial cell walls and as humans don't need these enzymes as we do use exclusively the L- forms this explains why the drug is toxic to certain microorganisms but not human cells which lack cell walls.

However, the famous Aussie Murchison meteorite (and also the Murray meteorite which I didn't know about) has a surprisingly, slight bias of the L- over D- form (left over right enantiomer) of some non-terrestial amino acids. These meterorites could of course have been contaminated by amino acids from earth, but obviously not for non-natural amino acids found in the meteorites, so the work has focused on these to unambiguously see if another factor(s) might influence non-racemic amino acid abundances:
http://www.daviddarling.info/encyclopedia/M/Murchison.html

This argues for what Cronin and Pizzarello who did the re-analysis: "...because carbonaceous chondrites formed 4.5 billion years ago, the results are indicative of an asymmetric influence on organic chemical evolution before the origin of life".
http://www.ncbi.nlm.nih.gov/pubmed/9020072?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_DiscoveryPanel.Pubmed_Discovery_RA&linkpos=1&log$=relatedarticles&logdbfrom=pubmed

This non-racemic abundance of amino acids clearly points to another influence:
http://www.ncbi.nlm.nih.gov/pubmed/9305838?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_DiscoveryPanel.Pubmed_Discovery_RA&linkpos=2&log$=relatedarticles&logdbfrom=pubmed

This is obviously a hot topic and.. "One explanation for this observation has been the asymmetric photolysis of racemic extraterrestrial amino acids by circularly polarized light (CPL) in the synchrotron radiation from orbiting electrons around the pulsar remnants of supernovae" though these papers are cross-disciplinary written by chemists, geologists and biologists suggesting the evolution of a new field of science. So while there is argument about the mechanism there appears now to a consensus that there is independent supporting evidence for the observation of the enantiometeric excess seen in overtly non-terrean amino acids.

Please see this too and related links to the left:
http://www.ncbi.nlm.nih.gov/pubmed/10465720?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_DiscoveryPanel.Pubmed_Discovery_RA&linkpos=4&log$=relatedarticles&logdbfrom=pubmed