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Author Topic:   Excellent paper-peptide self assembly
singularity
Inactive Member


Message 1 of 50 (50932)
08-18-2003 9:40 PM


Hey everyone
Just thought you would like to be alerted to a juicy paper in Science (A Possible Primordial Peptide Cycle
Claudia Huber,1 Wolfgang Eisenreich,1 Stefan Hecht,1 Gnter Wchtershuser2* ). Url for the summary is:Just a moment...
Basically the researchers have found that an environment of warm water and CO in the presence of amino acids and colloidal iron and nickel sulfides catalyzes the polymerisation of amino acids.
Any comments?

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 Message 2 by sos, posted 08-23-2003 12:29 AM singularity has not replied

  
sos
Inactive Member


Message 2 of 50 (51950)
08-23-2003 12:29 AM
Reply to: Message 1 by singularity
08-18-2003 9:40 PM


Peptide self-assembly is one thing...
...producing a chirally pure polypetide length of significance is another.
Since the link provided only gets me to the login page of the magazine, I do not know what it said. Could you either copy a short summery from the article or summerize it yourself?
Some thoughts before even finding out what the article said:
Chirality is still, to my knowledge, a BIG problem for abiogenesis as all modern living cells use exclusively L-form amino acids. Since one of the shortest protein chains known, insulin, is about 150 peptides in length, it would seem reasonable to look for a naturalistic process that can produce polypeptide chains of at least 150 all L-form amino acids... I don't think they have found it yet but there isn't much sense in discusssing the next level of problems unless a significant answer for naturally producing pure chirality is found first.
Does this article address this problem?
Is the faith of naturalists any better than the faith of creationists?

This message is a reply to:
 Message 1 by singularity, posted 08-18-2003 9:40 PM singularity has not replied

Replies to this message:
 Message 3 by Percy, posted 08-23-2003 11:36 AM sos has not replied

  
Percy
Member
Posts: 22388
From: New Hampshire
Joined: 12-23-2000
Member Rating: 5.2


Message 3 of 50 (51967)
08-23-2003 11:36 AM
Reply to: Message 2 by sos
08-23-2003 12:29 AM


Re: Peptide self-assembly is one thing...
sos writes:
Chirality is still, to my knowledge, a BIG problem for abiogenesis as all modern living cells use exclusively L-form amino acids. Since one of the shortest protein chains known, insulin, is about 150 peptides in length, it would seem reasonable to look for a naturalistic process that can produce polypeptide chains of at least 150 all L-form amino acids... I don't think they have found it yet but there isn't much sense in discusssing the next level of problems unless a significant answer for naturally producing pure chirality is found first.
You describe chirality as a problem for abiogenesis, but your example has to do with protein size. I must be missing your point, because I don't see what one has to do with the other in relation to abiogenesis.
Is the faith of naturalists any better than the faith of creationists?
This question is better addressed in either the Is It Science? or Faith and Belief forum, but the short answer is that the positions of science are supported by evidence. When this isn't the case then it isn't science. Science believes that the physical laws in place today and for which we have much evidence were the same ones operating when life first formed. Creationism believes that a being for which there is no evidence using mechanisms for which there is no evidence created the first life.
You can get free access to Science Online, but it doesn't provide access to most content, including the mentioned article.
--Percy

This message is a reply to:
 Message 2 by sos, posted 08-23-2003 12:29 AM sos has not replied

Replies to this message:
 Message 4 by Rei, posted 09-18-2003 8:32 PM Percy has not replied
 Message 6 by DNAunion, posted 11-01-2003 12:11 AM Percy has replied

  
Rei
Member (Idle past 7012 days)
Posts: 1546
From: Iowa City, IA
Joined: 09-03-2003


Message 4 of 50 (56381)
09-18-2003 8:32 PM
Reply to: Message 3 by Percy
08-23-2003 11:36 AM


Re: Peptide self-assembly is one thing...
Isn't the left-handed molecule simply named as the left-handed molecule because it is the first one discovered, and then the inherent mirror to that molecule around a given point would be the right-handed molecule? Otherwise, how would one decide, given a previously never-witnessed molecule, that it was left or right handed?
------------------
"Illuminant light,
illuminate me."

This message is a reply to:
 Message 3 by Percy, posted 08-23-2003 11:36 AM Percy has not replied

Replies to this message:
 Message 5 by DNAunion, posted 11-01-2003 12:04 AM Rei has not replied

  
DNAunion
Inactive Member


Message 5 of 50 (63762)
11-01-2003 12:04 AM
Reply to: Message 4 by Rei
09-18-2003 8:32 PM


Re: Peptide self-assembly is one thing...
quote:
Rei: Isn't the left-handed molecule simply named as the left-handed molecule because it is the first one discovered, and then the inherent mirror to that molecule around a given point would be the right-handed molecule?
/*DNAunion*/ No.
quote:
Rei: Otherwise, how would one decide, given a previously never-witnessed molecule, that it was left or right handed?
/*DNAunion*/ Two methods (simplified here). One deals with rotation of light - either to the left or to the right - when passed through a sample of molecules. The other is based on a standardized method of orienting a molecule in three-dimensional space and then noting which side a certain molecular group is found on: the left or the right.

This message is a reply to:
 Message 4 by Rei, posted 09-18-2003 8:32 PM Rei has not replied

  
DNAunion
Inactive Member


Message 6 of 50 (63763)
11-01-2003 12:11 AM
Reply to: Message 3 by Percy
08-23-2003 11:36 AM


Re: Peptide self-assembly is one thing...
quote:
Persipient: You [note, not me] describe chirality as a problem for abiogenesis, but your example has to do with protein size. I must be missing your point, because I don't see what one has to do with the other in relation to abiogenesis.
/*DNAunion*/ Probability. If there is some mechanism directing the selection of amino acid enantiomers - ensuring that the L form is always picked - then length is irrelevant. But when the process occurs by undirected, non-biological processes, an easily made assumption is that both chiral forms - being present in equal quantities and being chemically equivalent - would be incorporated at random in a growing chain. Thus, the longer the chain, the less likely it would be to have all left-handed amino acids. It's like flipping a fair coin: it's not that hard to flip 4 consecutive heads, but try flipping 40 heads in a row.

This message is a reply to:
 Message 3 by Percy, posted 08-23-2003 11:36 AM Percy has replied

Replies to this message:
 Message 7 by Rei, posted 11-01-2003 12:57 AM DNAunion has replied
 Message 12 by Percy, posted 11-02-2003 9:01 AM DNAunion has not replied

  
Rei
Member (Idle past 7012 days)
Posts: 1546
From: Iowa City, IA
Joined: 09-03-2003


Message 7 of 50 (63766)
11-01-2003 12:57 AM
Reply to: Message 6 by DNAunion
11-01-2003 12:11 AM


Re: Peptide self-assembly is one thing...
And what evidence do you have that amino acids of different chiralities join readily?
------------------
"Illuminant light,
illuminate me."

This message is a reply to:
 Message 6 by DNAunion, posted 11-01-2003 12:11 AM DNAunion has replied

Replies to this message:
 Message 8 by DNAunion, posted 11-01-2003 2:21 AM Rei has replied

  
DNAunion
Inactive Member


Message 8 of 50 (63777)
11-01-2003 2:21 AM
Reply to: Message 7 by Rei
11-01-2003 12:57 AM


Re: Peptide self-assembly is one thing...
quote:
Rei: And what evidence do you have that amino acids of different chiralities join readily?
/*DNAunion*/ And what evidence do you have that they don't?
*******************************
The following will suffice (at least for those who aren't arguing just to argue).
quote:
Reactions can proceed enantio-selectively if chiral reactants or catalysts are involved, or if some external chiral influence is present. But because chiral reactants and catalysts themselves require an enantioselective production process, efforts to understand the homochirality of life have focussed on external chiral influences. (G. L. J. A. Rikken & E. Raupach, Enatioselective Magnetochiral Photochemistry, Nature, vol 405, June 22 2000, p932-935)
For those that the above quote does not convince, then just use simple logic. If both enantiomers of amino acids would not be incorporated into polypeptides produced by undirected, non-biological processes alone, then why are OOL researchers searching for a solution for homochirality?

This message is a reply to:
 Message 7 by Rei, posted 11-01-2003 12:57 AM Rei has replied

Replies to this message:
 Message 9 by Rei, posted 11-01-2003 1:04 PM DNAunion has replied

  
Rei
Member (Idle past 7012 days)
Posts: 1546
From: Iowa City, IA
Joined: 09-03-2003


Message 9 of 50 (63809)
11-01-2003 1:04 PM
Reply to: Message 8 by DNAunion
11-01-2003 2:21 AM


Re: Peptide self-assembly is one thing...
1) This doesn't cover how readily opposite chiralities bond - it just briefly mentions that it's possible. Of course it's possible - the issue here is how readily they will. You can even cause xenon to bond in the right conditions Did you not read my post? I stated "readily".
2) The body of the summary undercuts your position - it states that for opposite chiralities to bond, there would need to be a production process which creates a mix of chiral forms, and as a consequence, researchers are only concerned with the production process (i.e., it doesn't seem realistic that there would be a production process that would create such a mix)
------------------
"Illuminant light,
illuminate me."

This message is a reply to:
 Message 8 by DNAunion, posted 11-01-2003 2:21 AM DNAunion has replied

Replies to this message:
 Message 10 by DNAunion, posted 11-01-2003 1:21 PM Rei has replied

  
DNAunion
Inactive Member


Message 10 of 50 (63812)
11-01-2003 1:21 PM
Reply to: Message 9 by Rei
11-01-2003 1:04 PM


Re: Peptide self-assembly is one thing...
quote:
Rei: 2) The body of the summary undercuts your position
/*DNAunion*/ Nope, it supports my position.
Your further statements, however, expose your lack of knowledge of the topic.
quote:
Rei: ... it states that for opposite chiralities to bond, there would need to be a production process which creates a mix of chiral forms, and as a consequence, researchers are only concerned with the production process (i.e., it doesn't seem realistic that there would be a production process that would create such a mix)
/*DNAunion*/ Wrong. The rule is that undirected, non-biological processes produce racemic mixtures of the amino acid enantiomers (i.e., mixtures in which both enantiomers are present in equal number). The problem facing OOL researchers is trying to get a chirally pure solution by undirected, non-biological processes.
quote:
"Prebiotic syntheses of amino acids either on Earth or elsewhere would be expected to produce racemic mixtures (equal amounts of the L and D enantiomers, or D/L = 1.0). (Jeffrey L. Bada, Enhanced: Extraterrestrial Handedness?, Science, Volume 275, Number 5302 Issue of 14 Feb 1997, pp. 943)
And the one you didn't understand the first time...
quote:
Reactions can proceed enantio-selectively if chiral reactants or catalysts are involved, or if some external chiral influence is present. But because chiral reactants and catalysts themselves require an enantioselective production process, efforts to understand the homochirality of life have focussed on external chiral influences. (G. L. J. A. Rikken & E. Raupach, Enatioselective Magnetochiral Photochemistry, Nature, vol 405, June 22 2000, p932-935)
/*DNAunion*/ Some processes have been found that result in slight enantiomeric excesses (for example, CPL , circularly polarized light, can preferentially destroy one enantiomer over the other), but they are far short of producing a enantiomerically pure product under prebiotically plausible conditions.
PS: If anyone has any recent findings on homochirality I'd be interested. I stopped keeping up with the topic a couple of years ago.
[This message has been edited by DNAunion, 11-17-2003]

This message is a reply to:
 Message 9 by Rei, posted 11-01-2003 1:04 PM Rei has replied

Replies to this message:
 Message 11 by Rei, posted 11-01-2003 1:56 PM DNAunion has replied

  
Rei
Member (Idle past 7012 days)
Posts: 1546
From: Iowa City, IA
Joined: 09-03-2003


Message 11 of 50 (63816)
11-01-2003 1:56 PM
Reply to: Message 10 by DNAunion
11-01-2003 1:21 PM


Re: Peptide self-assembly is one thing...
quote:
*DNAunion*/ Wrong. The rule is that undirected, non-biological processes produce racemic mixtures of the amino acid enantiomers (i.e., mixtures in which both enantiomers are present in equal number).
My apologies, I misread your article summary as saying something to the opposite. However, you still have not answered part 1 - i.e., my initial question. Note that it is a question, not a statement, but I have yet to see a valid response to it. One would think that there is a difference in bonding affinities between different chiralities, because a common methods of separating racemic mixtures include seeding with a crystal of one enantiomer and chemical reaction with a chiral reactant. It would seem that once a hypercycle began with one chirality, that chirality would be locked in.
quote:
The problem facing OOL researchers is trying to get a chirally pure solution by undirected, non-biological processes.
Are you unfamiliar with the Murray and Murchison meteorites? Of course, that is just one possibility. I hope you're not playing a game of "God of the Gaps" here.
quote:
/*DNAunion*/ Some processes have been found that result in slight enantiomeric excesses (for example, CPL , circularly polarized light, can preferentially destroy one enantiomer over the other), but they are far short of producing a chrirally pure product.
Would you, perchance, happen to have the ratios of chiralities in the aforementioned meteorites on hand? I'm having trouble tracking it down.
This topic always reminds me of a quote by Albert Einstein, when asked about why there are more electrons than positrons in the universe:
"The electron got there first."
------------------
"Illuminant light,
illuminate me."

This message is a reply to:
 Message 10 by DNAunion, posted 11-01-2003 1:21 PM DNAunion has replied

Replies to this message:
 Message 13 by DNAunion, posted 11-02-2003 3:30 PM Rei has replied

  
Percy
Member
Posts: 22388
From: New Hampshire
Joined: 12-23-2000
Member Rating: 5.2


Message 12 of 50 (63916)
11-02-2003 9:01 AM
Reply to: Message 6 by DNAunion
11-01-2003 12:11 AM


Re: Peptide self-assembly is one thing...
DNAunion writes:
Probability. If there is some mechanism directing the selection of amino acid enantiomers - ensuring that the L form is always picked - then length is irrelevant. But when the process occurs by undirected, non-biological processes, an easily made assumption is that both chiral forms - being present in equal quantities and being chemically equivalent - would be incorporated at random in a growing chain.
And is the assumption that left and right handed amino-acids bond readily to one another valid?
Whether they do or not, I still can't see how this is a "BIG problem for abiogenesis", which is how Sos described it. My own perspective is that the actual "BIG problem for abiogenesis" is that so little is known, dwarfing problems like chirality by comparison.
--Percy

This message is a reply to:
 Message 6 by DNAunion, posted 11-01-2003 12:11 AM DNAunion has not replied

  
DNAunion
Inactive Member


Message 13 of 50 (63980)
11-02-2003 3:30 PM
Reply to: Message 11 by Rei
11-01-2003 1:56 PM


Re: Peptide self-assembly is one thing...
quote:
/*DNAunion*/ The problem facing OOL researchers is trying to get a chirally pure solution by undirected, non-biological processes. Some processes have been found that result in slight enantiomeric excesses (for example, CPL , circularly polarized light, can preferentially destroy one enantiomer over the other), but they are far short of producing a chrirally pure product.
quote:
Rei: Are you unfamiliar with the Murray and Murchison meteorites? Of course, that is just one possibility. Would you, perchance, happen to have the ratios of chiralities in the aforementioned meteorites on hand? I'm having trouble tracking it down.
/*DNAunion*/ I am familiar with the Murchison one, which involved CPL (a mechanism I mentioned originally). The enantiomeric excesses were small. Here are some personal notes I still have on CPL, which reference the Murchison meteorite.
CPL (Circularly Polarized Light)
It has been suggested that CPL (circularly polarized light) may have caused a ratio in chirality different than 50/50 for an amino acid contained in a meteorite that was studied. However, there are a few issues that cast some degree of doubt upon this explanation, at least if it is intended to explain biological homochirality.
(1) The amino acid that was examined is not one of the 20 biological amino acids
quote:
The reported L amino acid excesses are very small and would need to be amplified by some process in order to generate homochirality. Even if this did take place, the L amino acid homochirality would be associated with alpha-dialkyl amino acids, which are not major players in modern protein biochemistry. (Jeffrey L. Bada, Enhanced: Extraterrestrial Handedness?, Science, Volume 275, Number 5302 Issue of 14 Feb 1997, pp. 942 — 943)
(2) There was only a 2% to 9% difference between the two optical isomers
quote:
as reported by Cronin and Pizzarello on page 951 of this issue, some unusual amino acids present in the Murchison meteorite apparently do have small excesses of the L enantiomers (that is, D/L < 1.0). These measurements have minimized the contamination problem by focusing on amino acids that either are extremely rare or have never been reported in terrestrial organisms or the geosphere.
Four alpha-dialkyl amino acids--alpha-methylisoleucine and alpha-methylalloisoleucine (2-amino-2,3-dimethylpentanoic acid), alpha-methylnorvaline (2-amino-2-methylpentanoic acid), and isovaline (2-amino-2-methylbutanoic acid)--are reported to have an L enantiomeric excess of 2 to 9%. Only isovaline has been reported in some microbial peptides, where it is present as either the L or D enantiomer. The alpha-hydrogen analogs of two of these alpha-dialkyl amino acids, norvaline and alpha-amino-n-butamoic acid, are racemic, suggesting that enantiomeric excess is only preserved in amino acids that are resistant to racemization (Jeffrey L. Bada, Enhanced: Extraterrestrial Handedness?, Science, Volume 275, Number 5302 Issue of 14 Feb 1997, pp. 942—943)
quote:
Then, in 1996, John Cronin and Sandra Pizzarello at Arizona State University found that some unusual nonprotein amino acids in the Murchison meteorite show a small excess of the L-isomer (on the order of a few percent). (Christopher Wills and Jeffrey Bada, The Spark of Life: Darwin and the Primeval Soup, Perseus Publishing, 2000, p120-121)
quote:
Recently, a new approach to this problem has been taken that is not subject to some of the criticisms of the earlier work (Cronin and Pizzarello). In these studies, the targets for analysis were Murhcison amino acids that (1) either have no known terrestrial source or are very restricted in occurrence in the biosphere, and (2) have chiral centers that are resistant to racemization (epimerization). Consequently, it is likely that the chiral centers of these amino acids retained their original configuration through the aqueous and mild thermal processing experienced in the meteorite parent body and that the original enantiomer ratios were not compromised by terrestrial contamination. An L-enantiomeric excess was observed in each case: [alpha]-methylisoleucine (7.0%), [alpha]-methylalloisoleucine (9.1%), isovaline (8.4%), and [alpha]-methylnorvaline (2.8%). Interestingly, the [alpha]-H analogues of these last two amino acids, that is, [alpha]-amino-n-butyric acid and [alpha]-aminopentanoic acid, were found to be racemates. (John R. Cronin, Clues from the Origin of the Solar System: Meteorites, chapter 6 of The Molecular Origins of Life: Assembling Pieces of the Puzzle, 1998, Cambridge University Press, p134)
(3) CPL produces the slight deviation from equality by preferentially breaking down one form — that is, CPL-produced variance from a 50/50 ratio is a destructive process.
quote:
Although very small enantiomeric excesses (~2%) have been generated from racemic leucine irradiated with circularly polarized ultraviolet light, there was extensive decomposition (59 to 75%) during the experiment. This implies that amino acid survival during this type of enantiomeric enrichment process would be extremely poor. (Jeffrey L. Bada, Enhanced:Extraterrestrial Handedness?, Science, Volume 275, Number 5302 Issue of 14 Feb 1997, pp. 942 — 943)
quote:
A circularly polarized light wave has an electric field that rotates either clockwise or counterclockwise about the wave’s direction of motion. Ultraviolet light with these types of polarization are absorbed unequally by right-handed and left-handed molecules and can break down or destroy one form more easily than the other, creating an asymmetry in the relative populations of the two forms. (Ron Cowen, Starlight Shows Life the Right Path, Science News, August 1, 1998 v154 n5 p68(1))
With these objections in mind, it is not without reason to state that this kind of light does not yet hold the complete answer.
One final note. The enantiomeric excesses of the chiral biological amino acids in the Murchison meteorite have changed over the years due to terrestrial contamination. Consequently, one needs to be wary not to consider numbers for such amino acids when looking for non-biological causes of enantiomeric excesses.
quote:
If the amino acids detected in Nakhla had been formed by abiotic processes on Mars, they would originally have been racemic (D/L ~1). Contamination of the meteorite by terrestrial L-amino acids after it fell to Earth could have significantly lowered the initial D/L ratios of any endogenous amino acids, perhaps to values close to those we measured. We have observed a similar lowering of D/L ratios in an interior sample of Murchison during its residence on Earth. In measurements carried out in 1970, soon after Murchison fell, the D/L ratios for alanine, aspartic acid, and glutamic acid were close to 1.0. Analyses of the same Murchison sample today yielded D/L ratios in the range of 0.3 to 0.5 for aspartic acid, 0.7 to 0.9 for glutamic acid, and 0.7 to 1.0 for alanine. Concentrations of the abiotic nonprotein amino acids in Murchison, such as AIB and racemic isovaline, were found to be similar in both the 1970 and 1998 analyses, indicating that these amino acids have not been affected by terrestrial contamination. (Daniel P. Glavin, Jeffrey L. Bada, Karen L. F. Brinton, & Gene D. McDonald, Amino Acids in the Martian Meteorite Nakhla, Proceedings of the National Academy of Sciences, Vol. 96, Issue 16, August 3, 1999, p8835-8838)

This message is a reply to:
 Message 11 by Rei, posted 11-01-2003 1:56 PM Rei has replied

Replies to this message:
 Message 14 by Rei, posted 11-02-2003 4:41 PM DNAunion has replied

  
Rei
Member (Idle past 7012 days)
Posts: 1546
From: Iowa City, IA
Joined: 09-03-2003


Message 14 of 50 (63999)
11-02-2003 4:41 PM
Reply to: Message 13 by DNAunion
11-02-2003 3:30 PM


Re: Peptide self-assembly is one thing...
Thanks for digging up the percentage on the Murchison meteorite. The Murray meteorite also had a non-racemic mixture, although I would imagine that the percentages were also not *too* high of a L/R ratio.
quote:
With these objections in mind, it is not without reason to state that this kind of light does not yet hold the complete answer.
As you'll notice from my post on this subject, however:
quote:
Of course, that is just one possibility.
You didn't address the key points of my posts (and I would be interested in your response).
1) The same point that I've brought up from the beginning that you haven't addressed: How readily enantioisomers react; I presented evidence that suggests that they don't react as readily.
2) How once a hypercycle starts with a certain set of amino acids, it is effectively "locked in".
------------------
"Illuminant light,
illuminate me."
[This message has been edited by Rei, 11-02-2003]

This message is a reply to:
 Message 13 by DNAunion, posted 11-02-2003 3:30 PM DNAunion has replied

Replies to this message:
 Message 15 by DNAunion, posted 11-02-2003 8:45 PM Rei has replied

  
DNAunion
Inactive Member


Message 15 of 50 (64038)
11-02-2003 8:45 PM
Reply to: Message 14 by Rei
11-02-2003 4:41 PM


Re: Peptide self-assembly is one thing...
quote:
Rei: You didn't address the key points of my posts (and I would be interested in your response).
/*DNAunion*/ No, I have addressed your key points. I showed at least one to be wrong and have addressed the others, just not to your satisfaction.
quote:
Rei: 1) The same point that I've brought up from the beginning that you haven't addressed: How readily enantioisomers react; I presented evidence that suggests that they don't react as readily.
/*DNAunion*/ You keep missing the point, or slightly altering it. The point is that long polymers are required for complex life functions, such as self-replication, and that undirected, non-biological processes are not likely to form them, one reason being that homochirality is needed.
Since you now reference enentiomers in general, that is what I will respond to.
Here is a reference to the first paper on the problem of enantiomeric cross inhibition: note what happens when both enantiomers are present and undirected, non-biological processes are used to try to form polymers.
quote:
The aforementioned studies of the selectivities and limitations of template-directed oigomerization of oligonucleotides have been conducted under carefully controlled conditions using templates, as well as monomers, whose structural and chiral purity were rigorously controlled. In an experiment designed to test the requirement for chiral purity, it was demonstrated that incorporation of even a single mononucleotide of opposite chirality into the end of a growing chain in template-directed oligomerization is sufficient to terminate the reaction (Joyce et al., 1984). This observation (the phenomenon is referred to as enantiomeric cross-inhibition) has had serious consequences for theories of the origin of life (Alan W. Schwartz, Origins of the RNA World, chapter 11 of The Molecular Origins of Life: Assembling Pieces of the Puzzle, Cambridge University Press, 1998, p247)
/*DNAunion*/ Here’s some others on the need for homochirality.
quote:
The chirality problem of the template-directed reaction was circumvented (and postponed) by the use of homochiral nucleotides. Based on laboratory experiments, it has been established that nucleotides synthesized by prebiotic reactions are always racemic — that is, they contain equal concentrations of D and
L enantiomers. Thus a better simulated prebiotic experiment should use D and L ribose rather than D-ribose alone. When the same kind of experiment was carried out with the racemic mixture of activated
ribonucleotides, the template-directed reaction did not proceed, because of enantiomeric cross-inhibition
(Joyce et al, 1984). (Biogenesis: Theories of Life’s Origin, Noam Lahav, Oxford University Press, 1999, p207)
quote:
Considerable effort has been directed toward finding an amplification mechanism by which a
homochiral polynucleotide template, however produced originally, might selectively catalyze the
oligomerization of like-handed monomers. This hope has been frustrated by the phenomenon of
‘enantiomeric cross-inhibition’ — the inhibition of template-directed oligomerization of activated
mononucleotides when both D- and L- enantiomers are present in the reaction mixture. (Alan W. Schwartz, Selecting for Homochirality before RNA, Current Biology, Vol. 7 No. 8,
August 1 1997, r477-r479)
quote:
Laboratory experiments with nonenzymatic replication of RNA strands (see Joyce 1987, 1989, for a review) pointed out to the problem of enantiomeric cross-inhibition, the poisoning of chain growth on a template following the addition of building blocks with enantiomers of ribose. The chiral purity of the building blocks of RNA, the sugar moiety, is crucial for their participation in the self-replication reactions and hence in the establishment of the RNA World. (Biogenesis: Theories of Life’s Origin, Noam Lahav, Oxford University Press, 1999, p201—202)
quote:
Why are chiral molecules in a state of absolute chiral purity so essential to the existence of life? The synthesis of specific proteins within living cells, mediated by DNA and RNA, as well as the replication of DNAs to pass on genetic information, all depend on an extremely precise ‘fitting together’ (complementarity) of the polymeric molecular chains involved. It was suggested early on that replicating double-stranded nucleic acid polymers would be impossible with a mixture of D- and L-monomer subunits. This has since been confirmed experimentally in studies which showed inhibition of nucleotide
polymerization with chirally impure monomers. Similarly, molecular models have shown that the presence of ‘unnatural’ L-sugars in nucleotide polymers prevents them from adopting the complementary double-stranded helical structures necessary for self-replication. Thus, it is now generally accepted that the absolute homochirality for enantiomeric purity of biopolymer subunits are essential for self-replication and, by implication, for the origin of life. (William A. Bonner, Chirality Cosmochemistry and Life, Chemistry and Industry, n17, September 7 1992, p640-645)
quote:
The origin of homochiral structures between the chemical and biological evolution on the Early
Earth is a missing link in the process of the origin of life. Therefore the 'first asymmetric synthesis'
continues to receive considerable scientific attention in biology, chemistry, as well as in physics. Recent experiments gave evidence for the feasibility of prebiotic polymerization reactions: The growth of chain length of special chiral molecules could be simulated successfully in the laboratory to reach the size of biopolymers. But for these "artificial" polymerization reactions monomers of the same handedness were required. The insertion of a "wrong" stereochemical configuration of only one enantiomer inhibits the whole polymerization process immediately. This has been well known state of the art and is defined as 'enantiomeric cross inhibition'.
Because of the phenomenon of the 'enantiomeric cross inhibition' we assume, that life could only arise in an environment, in which a certain enantiomeric excess already came to existence. The origin of this enantiomeric excess is still unknown. (Uwe J. Mejerhenrich and Wolfram H. P. Thiemann [both from the University of Bremen], Enantiomer Separations Planned in Cometary Matter in Situ, Page not found - Astrobiology PS: Old link, may not still be present)
quote:
Finally, there are no efficient prebiotic syntheses that produce purine ribosides or pyrimidine
nucleosides. Added to this are the problems with the synthesis of ribose or nucleosides in that any prebiotic
synthesis would generate racemic mixtures. Any template polymerization with such mixtures would show
enantiomeric cross inhibition. Enantiomeric inhibition occurs if an activated L-nucleoside is polymerized
into a template polymerization of activated D-nucleosides. Experimental evidence has clearly demonstrated
that such conditions cause complete chain termination during polymerization. (http://campus.arbor.edu:8880/~michaelb/orglif.htm PS: Old link, may not still be present)
quote:
Nucleosides are assembled from the ribose and bases, nucleotides are formed from nucleosides and phosphates, and oligonucleotides are chains of nucleotides. The hypothetical formation of the oligonucleotide components was described above; however, the locations for formation of these components are separate, so the components must be transported to a common location. Morever, for ribose and the bases, they must be separated from complex witches’ brews of sugar enantiomers and similar compounds, many of which would have hindered nucleotide and oligonucleotide formation (Joyce, 1989). In particular, Joyce et al. (1994) found that template-directed oligomerization reactions are inhibited in racemic mixtures by molecules of opposite-handedness. (italic emphasis in original, John Washington, The Possible Role of Volcanic Aquifers in Prebiologic Genesis of Organic Compounds and RNA, Origins of Life and Evolution of the Biosphere, Vol. 30 No. 1, February 2000, p72)
quote:
Since the template directed polymerization of one enantiomer is likely to be inhibited strongly by the presence of the other (enantiomeric cross-inhibition), it is hard to see how replication could get started, even from a racemic solution of pure beta-DL-nucleosides. (Byron Wingerd, RNA as a Prebiotic
Precursor, 4/27/1997, http://www.msu.edu/user/wingerdb/rna.htm)

This message is a reply to:
 Message 14 by Rei, posted 11-02-2003 4:41 PM Rei has replied

Replies to this message:
 Message 17 by Rei, posted 11-03-2003 2:03 PM DNAunion has replied

  
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