[/caption]
A wider range of asteroids were capable of creating the kind of amino acids used by life on Earth, according to new NASA research. Amino acids are used to build proteins, which are used by life to make structures like hair and nails, and to speed up or regulate chemical reactions. Amino acids come in two varieties that are mirror images of each other, like your hands. Life on Earth uses the left-handed kind exclusively. Since life based on right-handed amino acids would presumably work fine, scientists are trying to find out why Earth-based life favored left-handed amino acids.
In March, 2009, researchers at NASA’s Goddard Space Flight Center in Greenbelt, Md., reported the discovery of an excess of the left-handed form of the amino acid isovaline in samples of meteorites that came from carbon-rich asteroids. This suggests that perhaps left-handed life got its start in space, where conditions in asteroids favored the creation of left-handed amino acids. Meteorite impacts could have supplied this material, enriched in left-handed molecules, to Earth. The bias toward left-handedness would have been perpetuated as this material was incorporated into emerging life.
In the new research, the team reports finding excess left-handed isovaline (L-isovaline) in a much wider variety of carbon-rich meteorites. “This tells us our initial discovery wasn’t a fluke; that there really was something going on in the asteroids where these meteorites came from that favors the creation of left-handed amino acids,” says Dr. Daniel Glavin of NASA Goddard. Glavin is lead author of a paper about this research published online in Meteoritics and Planetary Science January 17.
“This research builds on over a decade of work on excesses of left-handed isovaline in carbon-rich meteorites,” said Dr. Jason Dworkin of NASA Goddard, a co-author on the paper.
“Initially, John Cronin and Sandra Pizzarello of Arizona State University showed a small but significant excess of L-isovaline in two CM2 meteorites. Last year we showed that L-isovaline excesses appear to track with the history of hot water on the asteroid from which the meteorites came. In this work we have studied some exceptionally rare meteorites which witnessed large amounts of water on the asteroid. We were gratified that the meteorites in this study corroborate our hypothesis,” explained Dworkin.
L-isovaline excesses in these additional water-altered type 1 meteorites (i.e. CM1 and CR1) suggest that extra left-handed amino acids in water-altered meteorites are much more common than previously thought, according to Glavin. Now the question is what process creates extra left-handed amino acids. There are several options, and it will take more research to identify the specific reaction, according to the team.
However, “liquid water seems to be the key,” notes Glavin. “We can tell how much these asteroids were altered by liquid water by analyzing the minerals their meteorites contain. The more these asteroids were altered, the greater the excess L-isovaline we found. This indicates some process involving liquid water favors the creation of left-handed amino acids.”
Another clue comes from the total amount of isovaline found in each meteorite. “In the meteorites with the largest left-handed excess, we find about 1,000 times less isovaline than in meteorites with a small or non-detectable left-handed excess. This tells us that to get the excess, you need to use up or destroy the amino acid, so the process is a double-edged sword,” says Glavin.
Whatever it may be, the water-alteration process only amplifies a small existing left-handed excess, it does not create the bias, according to Glavin. Something in the pre-solar nebula (a vast cloud of gas and dust from which our solar system, and probably many others, were born) created a small initial bias toward L-isovaline and presumably many other left-handed amino acids as well.
One possibility is radiation. Space is filled with objects like massive stars, neutron stars, and black holes, just to name a few, that produce many kinds of radiation. It’s possible that the radiation encountered by our solar system in its youth made left-handed amino acids slightly more likely to be created, or right-handed amino acids a bit more likely to be destroyed, according to Glavin.
It’s also possible that other young solar systems encountered different radiation that favored right-handed amino acids. If life emerged in one of these solar systems, perhaps the bias toward right-handed amino acids would be built in just as it may have been for left-handed amino acids here, according to Glavin.
The research was funded by the NASA Astrobiology Institute (NAI), which is administered by NASA’s Ames Research Center in Moffett Field, Calif.; the NASA Cosmochemistry program, the Goddard Center for Astrobiology, and the NASA Post Doctoral Fellowship program. The team includes Glavin, Dworkin, Dr. Michael Callahan, and Dr. Jamie Elsila of NASA Goddard.
There is some sort of symmetry breaking principle here. I am not sure about the conjectures offered here. The chirality might more likely be due to some polarization of UV light. Ammino acids can form from N_2, CH_4 etc in the presence of UV. I would conjecture there is some mechanism which has a polarization preference or some birefringence.
LC
So here we have preferential depletion depletion of the right handed enatiomer of isovaline. Valine having an isoeletric point at pH 6, and being hydrophobic.
Right… There is much more to metabolic preference than mirror symmetry, and there are a lot more symmetries where that came from.
Chemistry stereo centers are L and D by convention, so doesn’t map to mirror symmetry. Mirror symmetry may couple weakly to optical properties, so may be problematic for such selection. Finally, non-chiral molecules may have handedness, while those with several chiral centers may have none (say, one L center mirroring a D, resulting in a symmetrical molecule).
It is necessary but not certainly sufficient to break mirror symmetry in metabolism, which is pickier in picking isomers AFAIK. (Though admittedly the coevolution of protein enzymes and symmetry breaking argue that it may have been.)
Finally, life isn’t “left-handed” except maybe by weight and number of molecules, since nucleotide and most carbohydrate heteromers (polymers of different monomers, unlike plastic which is usually a homomer of one monomer) are right handed.
Which brings me to the problem with predicting a slight solar system amino acid excess (which may have been amplified by chemical selection), it doesn’t predict the way the other one or two symmetry breakings turned out AFAIU. And if those were random outcomes, why would we need to have an external excess for amino acids only?
Maybe the solar system amino acid mirror symmetry breaking set up the conditions for the other two. Maybe. As an example of the argument against, right now I’m personally leaning towards an RNA world on top of a natural metabolic like probiotic system. There is a recent IMHO excellent paper explaining selection of natural enzymes as the Earth cooled down, enabling an RNA world scenario but problematic for a “protein world” such.
In the end, correlation isn’t causation. You can’t go from observing excess to predicting an obligatory symmetry breaking.
“Chemistry stereo centers are L and D by convention, so doesn’t map to mirror symmetry. Mirror symmetry may couple weakly to optical properties, so may be problematic for such selection.”
I’m going to disagree here, it’s part of how entaiomeric properties and chirality were discovered – they react to polarized light differently. That’s where the D/L notation comes from – it reflects the direction that the enatiomer rotates plane polarized light in (as opposed to R/S notation which has a different basis).
“Finally, non-chiral molecules may have handedness, while those with several chiral centers may have none (say, one L center mirroring a D, resulting in a symmetrical molecule).”
Naw, not really, because the chiral centers aren’t neccessarily going to rotate the plane polarized light by the same amount – you just end up with DD, DL, LD, or LL enantiomers.
“Which brings me to the problem with predicting a slight solar system amino acid excess (which may have been amplified by chemical selection), it doesn’t predict the way the other one or two symmetry breakings turned out AFAIU. And if those were random outcomes, why would we need to have an external excess for amino acids only?”
Different enatiomers have slightly different properties – one method of seperating enatiomers is by fractional crystalization, which reading between the lines may be all we are dealing with here.
Valine is hydrophobic.
If D-Valine is more or less soluble in water (for a given temperature) than L-Valine, then that alone may prove sufficient to explain all of the observations.
Um, eh, right, thanks! I confused the notations obviously.
Yes, that could happen. But I was trying to point out that there are mirror symmetric centers that gives a perfectly symmetric molecule.
I think we are talking cross purposes here. Just after the quote you made I said: “Maybe the solar system amino acid mirror symmetry breaking set up the conditions for the other two.” So I agree that there is a breaking, and your suggestion is one way it may affect solutions.
But I was trying to discuss that such a breaking (AFAIK) wouldn’t decide the remaining observed ones (for nucleobases and carbohydrates).
“I think we are talking cross purposes here. Just after the quote you made I said: “Maybe the solar system amino acid mirror symmetry breaking set up the conditions for the other two.” So I agree that there is a breaking, and your suggestion is one way it may affect solutions.”
My point was though, you seem to be assuming that the symetry breaking – if you want to keep calling it that, is as a result of the steps involved in the synthesis on the Isovaline in the first place – that there’s some process at work that favours the formation of L-Isovaline over D-isovaline.
My point was that this doesn’t have to be the case, because the evidence as outlined in the UT article suggests that the enrichment occurs as a result of some refractory process operating after the synthesis of the initialy racemic mixture. I quote from the article:
“The more these asteroids were altered, the greater the excess L-isovaline we found…”
“In the meteorites with the largest left-handed excess, we find about 1,000 times less isovaline than in meteorites with a small or non-detectable left-handed excess…”
Admittedly, they do talk about non-raceamic mixtures, and I do recall reading something somewhere about how if the early solar system encountered a source of polarized radiation that destroyed one enatiomer over another preferentially, but this would still count as a refractory process.
Essentially, what we have is this:
Synthesis
Enrichment by supernovae.
Further enrichment by hydrothermal fluids.
As far as the Supernova thing goes:
V.A. Tsarev, 2008, published in Kratkie Soobshcheniya po Fizike, 2008, Vol. 35, No. 8, pp. 35–40.
Supernovae, neutron stars and biomolecular chirality; William A. Bonnera and Edward Rubenstein
Supernovae and the Chirality of the Amino Acids R.N. Boyd, T. Kajino, and T. Onaka.
Last time I checked, a recent supernovae during the nascent phase of the solar system also happened to be the best way of explaining several isotopic anomalies, was it not?
Trivia: The solubility of D-Valine in water at 20°C is 56 g/l, the solubility of L-valine in water at 20°C is 85 g/l
I should have looked a little harder – one of those, I think it was Boyd, deals with the weak interation between neutrinos from core collapse supernovae and N-14 as a refractory process for producing an excess of L-Amino acids.
A bit off topic perhaps, but what is the story behind the cube with the W in the picture?
*It’s pretty much just to show for scale. It’s probably of a standard size across (an inch or centimeter or what have you), though it should probably be more specific. Or, it may have been at the source of the image, but that wasn’t duplicated in UT’s caption.
Yeah, these are typically 1 cm across and show Top, Bottom, North South East West (TBNSEW) on their sides so there is a reference for what ‘side’ the picture is taken from. You`ll usually (you should) see these in any photos of a rock sample, meteorite, etc.
Neat! Most of the times you see reference scale thingies from biological findings, and in the field that is usually the photographer’s pen…
“… proteins, which are used by life to make structures like hair and nails, and to speed up or regulate chemical reactions.”
Something like: metal is used in the making of cars, in parts such as wheel rims and electrical wiring.
Also, this might help understand the discussion:
http://en.wikipedia.org/wiki/Chirality_%28chemistry%29#Naming_conventions
Further reading: Roger Zelazny’s Doorways in the Sand. He made chirality a reality.
The chirality or parity symmetry is more general than just mirror symmetry. For mirror symmetry you have an azimuthal coordinate change z –> -z, whereas with partiy you change a vector components (x, y, z) –> (-x, -y, -z). Mirror symmetry is a subroup action.
LC
In Chemistry, Chirality is defined by mirror symmetry.
In Chemistry, for a molecule to become chiral requires the presence of at least one carbon center with 4 different centers around it. In amino acids, with the exception of Glycine, this is provided by the ?-carbon. Glycine is the exception because the ?-carbon has to hydrogens bound to it, and so is non chiral.
If any other form of symmetry exists at this center, it’s not chiral.
Build yourself four tetrahedra.
For the first pair,
Take the first tetrahedon, paint one side red, one side green, one side blue, and the fourth yellow. Take the second tetrahedron and pain it in the mirrored colour scheme.
To a chemist, these tetrahedra are chiral, and are enantiomers. There is no (other) way of mapping the first tetrahedron onto the second tetrahedron without pulling one of them apart and reassembling it. So it is with Enantiomers – there is no way to map the d-enantiomer onto the l-enantiomer without breaking and remaking chemical bonds (IE carrying out a chemical reaction).
Take the second pair of tetrahedra, repeat as above, except paint two faces the same colour. These two tetrahedra can be mapped onto each other by rotation (and translation). These tetrahedra are not chiral, it’s just the same tetrahedron pointing in two different directions.
Enatiomerism, in brief is simply a special kind of isomerism. Isomerism being where you have the same molecule with the same atoms and the same groups, just rearranged differently. It’s why we have n-Butane and Isobutane. the N just means that it’s a straight chain CH3CH2CH2CH3 but the Iso (or i-Butane – even though technically it’s methylpropane) is arranged as a branched isomer HC(CH3)3. Both have exactly the same emperical formula, exactly the same numbers of atoms, however they’re arranged in different ways, and have differing properties.
To some up – mirror symmetry is special to chemists because interchanging it requires chemical reactions (with the exception of Amines – but the antiomers of chiral Amines require cold or to be in the form of quaternary anions to isolate), and anything else is just the same molecule pointing in a different direction.
I was thinking a bit more generally. Thanks for the explanation.
LC