Left Handed Amino Acids Discovered in 4.5x10^9 year old metorite

[Moridin]

http://www.sciencedaily.com/releases/2008/02/080228174823.htm

Pizzarello, in ASU's Department of Chemistry and Biochemistry, worked with Yongsong Huang and Marcelo Alexandre, of Brown University, in studying the organic materials of a special group of meteorites that contain among a variety of compounds, amino acids that have identical counterparts in terrestrial biomolecules. These meteorites are fragments of asteroids that are about the same age as the solar system (roughly 4.5 billion years.)

Scientists have long known that most compounds in living things exist in mirror-image forms. The two forms are like hands; one is a mirror reflection of the other. They are different, cannot be superimposed, yet identical in their parts.

This seems exciting for origin of life research.

 

[Q_Goest]

Interesting, thanks Moridin.

From the article:

When scientists synthesize these molecules in the laboratory, half of a sample turns out to be "left-handed" and the other half "right-handed." But amino acids, which are the building blocks of terrestrial proteins, are all "left-handed,"

However, in these particular meteorites, Cronin and Pizzarello -

. . . discovered a preponderance of "left-handed" amino acids over their "right-handed" form.

"The findings of Cronin and Pizzarello are probably the first demonstration that there may be natural processes in the cosmos that generate a preferred amino acid handedness,"

So when synthesizing these molecules, we get a 50/50 distribution. However, "natural processes in the cosmos" generate a preference of left handed over right handed amino acids.

Ok, any thoughts as to why? Why aren't lab synthesized molecules preferentially left handed? And what would need to be different in this process to make them mostly left handed?

 

[Moonbear]

Ok, any thoughts as to why? Why aren't lab synthesized molecules preferentially left handed? And what would need to be different in this process to make them mostly left handed?

Good question! I think this would make an excellent research topic. Perhaps it's something as simple as the naturally available precursors or catalysts for the reactions...maybe the more even distribution is simply an artifact of being able to bash things together with unnaturally high concentrations of starting compounds. But, clearly experiments would need to be done to find out. The findings of this study that show this preferential handedness occurs even outside Earth's atmosphere makes this an even more compelling question.

 

[Spirochete]

I was thinking about this question and another related one recently. It could be very profitable to discover how the universe is (apparently) able to manufacter molecules with one specific stereochemistry. Chiral synthesis is a big head ache in the pharmaceutical industry, since many drugs are only active in either D or L.

I asked a similar question in an organic chemistry forum and got some interesting responses. I was essentially asking if the universe should be completely achiral. Sadly the last person to respond posted a website which used various unanswered questions in science as a justification for creationism.

http://www.chemicalforums.com/index.php?topic=22524.0

 

[Schrodinger's Dog]

Ah yes the homochirality problem. This is something creationists like to throw about to try and disprove the origin of life from basic chemicals, unfortunately most of the people who bandy it around tend to have no real understanding of the subject.

Of course though if you have a mix of left and right chirality you have self terminating chains, so obviously areas where there were high concentrations of the left hand would produce longer chains more often and be more likely to create life. So it's obvious there must have been some sort of preference for life to happen in the first place. And of course if mostly certain handed is being used by life, then that leaves mostly the other handed unused, say temperatures then dropped to a level where formation was unlikely, but not everywhere, and an environment where the other handedness tends to occur leaked over to the new area where right handed chains had formed independently, thus a two stage process is possible, where the formation of basic parts happens to be related but did not happen at the same time.

It could be that conditions in the original soup favoured left and right equally, but the first life to develop, happened to have left amino acids and right DNA and became successful, maybe even at some point early life was almost wiped out leaving only or mostly left/right handed simple organisms, and the right/left etc, eventually became extinct. Or as said it's just the preferred and more usual combination found in the original soup, and again life tended to be expressed more often with such a chirality, again right handed chemicals in certain forms eventually disappearing, left/left, right/left, right/right essentially being less selected by some unknown mechanism. Just some of my thoughts anyway.

Another possibility is that some chemicals tend to form into homochiral crystals that could seed already more prevalent right handed forms, or exposure of such right handed forms could cause this.

There is also the fact that certain polarizations of UV light will destroy certain forms more readily than others. This is called photolysis. So if certain light conditions existed, then it could mean right handed forms were favoured and others left.

None of these are particularly convincing on their own. But who knows if they could get enough of these factors together to show why homochirality became the norm in lifeforms then they could gain a working hypothesis for the: why homochirality question.

Anyway, there is something good about reading creationist material, it makes you think about ways to get over the problems.

 

[Q_Goest]

Hi Schroedinger’s Dog,

Of course though if you have a mix of left and right chirality you have self terminating chains, so obviously areas where there were high concentrations of the left hand would produce longer chains more often and be more likely to create life. So it's obvious there must have been some sort of preference for life to happen in the first place. And of course if mostly certain handed is being used by life, then that leaves mostly the other handed unused, say temperatures then dropped to a level where formation was unlikely, but not everywhere, and an environment where the other handedness tends to occur leaked over to the new area where right handed chains had formed independently, thus a two stage process is possible, where the formation of basic parts happens to be related but did not happen at the same time.

It could be that conditions in the original soup favoured left and right equally, but the first life to develop, happened to have left amino acids and right DNA and became successful, maybe even at some point early life was almost wiped out leaving only or mostly left/right handed simple organisms, and the right/left etc, eventually became extinct. Or as said it's just the preferred and more usual combination found in the original soup, and again life tended to be expressed more often with such a chirality, again right handed chemicals in certain forms eventually disappearing, left/left, right/left, right/right essentially being less selected by some unknown mechanism. Just some of my thoughts anyway.

The fact the rocks found were 4.5 billion years old indicates these molecules were created prior to life on Earth, so life can’t be used to explain the preference unless the sample came from a location where life had existed prior to Earth. That in itself would be a fantastic discovery, but I don’t think that’s the explanation here. My very limited understanding of this is that there are processes, perhaps in space, that prefer one handedness over another.

I also wonder if life could have evolved from right-handed amino acids instead of left-handed ones. Is there any reason, in principal, that a human, animal, plant or other living organism couldn’t be constructed of molecules that were all right-handed instead of left-handed? My understanding is that there is nothing preventing this. Although a side note of interest is that a right-handed organism might not be able to digest food that was left-handed.

 

[Schrodinger's Dog]

Hi there. Yeah, I was just speculating based on the articles points about amino acids and DNA, being left and right handed respectively, does seem pretty strange. Complete and utter supposition of course.

 

[chemisttree]

We have analyzed the soluble organic composition of a carbonaceous meteorite from Antarctica that was collected and stored under controlled conditions, largely escaped terrestrial contamination and offers an exceptionally pristine sample of prebiotic material. Analyses of the meteorite diastereomeric amino acids alloisoleucine and isoleucine allowed us to show that their likely precursor molecules, the aldehydes, also carried a sizable molecular asymmetry of up to 14% in the asteroidal parent body. Aldehydes are widespread and abundant interstellar molecules; that they came to be present, survived, and evolved in the solar system carrying ee gives support to the idea that biomolecular traits such as chiral asymmetry could have been seeded in abiotic chemistry ahead of life.

http://www.pnas.org/cgi/content/abstract/0709909105v1?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=1&title=Molecular+asymmetry+in+extraterrestrial+chemistry&andorexacttitle=and&andorexacttitleabs=and&andorexactfulltext=and&searchid=1&FIRSTINDEX=0&sortspec=relevance&resourcetype=HWCIT

It seems that the authors rely the supposed ee of the 'likely' aldehyde precursor to both isoleucine and alloisoleucine (reported to be as much as 14%). I wonder how the authors get around the argument that racemization of these aldehydes occurs readily and the oxidation to carboxylic acid, effectively fixing the stereochemistry in one form or the other, should occur rapidly after whatever process caused the deviation from the racemate. One note, the chirality of amino acids is not forever once formed and the pure D or L isomers racemize over time, at least on Earth they do.

 

[Spirochete]

Of course though if you have a mix of left and right chirality you have self terminating chains, so obviously areas where there were high concentrations of the left hand would produce longer chains more often and be more likely to create life.

Could you expand on what you mean by this, maybe with a simple example? I can't quite see what would prevent a left handed amino acid from polymerizing with a right handed one.

I have a semester and a half of organic so I know the basics.

 

[Schrodinger's Dog]

Could you expand on what you mean by this, maybe with a simple example? I can't quite see what would prevent a left handed amino acid from polymerizing with a right handed one.

I have a semester and a half of organic so I know the basics.

Well its been a while since I studied this, so pardon me if I'm wrong but a left handed chain polymerising with a right handed chain, should terminate the chain, thus preventing the chain from achieving any substantial length, not sure about DNA, but I believe that's how it works with amino acids.