Life's first handshake: Chiral molecule detected in interstellar space

June 14, 2016
Scientists applaud the first detection of a "handed" molecule, (propylene oxide) in interstellar space. It was detected, primarily with the NSF's Green Bank Telescope, near the center of our Galaxy in Sagittarius (Sgr) B2, a massive star-forming region. Propylene oxide is one of a class of so-called "chiral" molecules -- molecules that have an identical chemical composition, but right- and left-handed versions. Chiral molecules are essential for life and their discovery in deep space may help scientists understand why life on Earth relies on a certain handedness to perform key biological functions. Sgr A* in this image indicates the supermassive black hole at the center of our Galaxy. The white features in the composite image are the bright radio sources in the center of our Galaxy as seen with the VLA. The background image is from the Sloan Digital Sky Survey. The two "handed" versions of propylene oxide are illustrated. The "R" and "S" designations are for the Latin terms rectus (right) and sinister (left). Credit: B. Saxton, NRAO/AUI/NSF from data provided by N.E. Kassim, Naval Research Laboratory, Sloan Digital Sky Survey

Like a pair of human hands, certain organic molecules have mirror-image versions of themselves, a chemical property known as chirality. These so-called "handed" molecules are essential for biology and have intriguingly been found in meteorites on Earth and comets in our Solar System. None, however, has been detected in the vast reaches of interstellar space, until now.

A team of scientists using highly sensitive radio telescopes has discovered the first complex organic chiral molecule in . The molecule, propylene oxide (CH3CHOCH2), was found near the center of our Galaxy in an enormous star-forming cloud of dust and gas known as Sagittarius B2 (Sgr B2).

The research was undertaken primarily with the National Science Foundation's Green Bank Telescope (GBT) in West Virginia as part of the Prebiotic Interstellar Molecular Survey. Additional supporting observations were taken with the Parkes radio telescope in Australia.

"This is the first molecule detected in interstellar space that has the property of chirality, making it a pioneering leap forward in our understanding of how prebiotic are made in the Universe and the effects they may have on the origins of life," said Brett McGuire, a chemist and Jansky Postdoctoral Fellow with the National Radio Astronomy Observatory (NRAO) in Charlottesville, Virginia.

"Propylene oxide is among the most complex and structurally intricate molecules detected so far in space," said Brandon Carroll, a chemistry graduate student at the California Institute of Technology in Pasadena. "Detecting this molecule opens the door for further experiments determining how and where molecular handedness emerges and why one form may be slightly more abundant than the other."

McGuire and Carroll share first authorship on a paper published today in the journal Science. They also are presenting their results at the American Astronomical Society meeting in San Diego, California.

Forming and Detecting Molecules in Space

Complex organic molecules form in interstellar clouds like Sgr B2 in several ways. The most basic pathway is through gas-phase chemistry, in which particles collide and merge to produce ever more complex molecules. Once organic compounds as large as methanol (CH3OH) are produced, however, this process becomes much less efficient.

To form more complex molecules, like propylene oxide, astronomers believe thin mantles of ice on dust grains help link into longer and larger structures. These molecules can then evaporate from the surface of the grains and further react in the gas of the surrounding cloud.

To date, more than 180 molecules have been detected in space. Each molecule, as it naturally tumbles and vibrates in the near vacuum of the interstellar medium, gives off a distinctive signature, a series of telltale spikes that appear in the radio spectrum. Larger and more have a correspondingly more complex signature, making them harder to detect.

The S (Latin for sinister, left) and R (Latin for rectus, right) versions of the chiral molecule propylene oxide, which was discovered in a massive star-forming region near the center of our Galaxy. This is the first detection of a chiral molecule in interstellar space. Credit: B. Saxton (NRAO/AUI/NSF)

To claim a definitive detection, scientists must observe multiple spectral lines associated with the same molecule. In the case of propylene oxide, the research team detected two such lines with the GBT. The third was at a frequency difficult to observe from the Northern Hemisphere due to satellite radio interference. Carroll, McGuire, and their colleagues used the Parkes telescope to tease out the final spectral line needed to verify their results.

The current data, however, do not distinguish between the left- and right-handed versions of the molecule. In additional to the same chemical composition, chiral molecules have the same melting, boiling, and freezing points, and the same spectra. "These spectra are like your hands' shadows," said Carroll. "It's impossible to tell if a right hand or a left hand is casting the shadow." This presents a challenge for researchers trying to determine if one version of propylene oxide is more abundant than the other.

Chirality in Space, a Helping Hand to Biology on Earth

Every living thing on Earth uses one, and only one handedness of many types of chiral molecules. This trait, called homochirality, is critical for life and has important implications for many biological structures, including DNA's double helix. Scientists do not yet understand how biology came to rely on one handedness and not the other. The answer, the researchers speculate, may be found in the way these molecules naturally form in space before being incorporated into asteroids and comets and later deposited on young planets.

"Meteorites in our Solar System contain chiral molecules that predate the Earth itself, and have recently been discovered in comets," noted Carroll. "Such small bodies may be what pushed life to the handedness we see today."

"By discovering a in space, we finally have a way to study where and how these molecules form before they find their way into meteorites and comets, and to understand the role they play in the origins of homochirality and life," McGuire said.

The researchers believe it may eventually be possible to determine if there is an excess of one handedness of over the other by examining how polarized light interacts with the molecules in space.

"The Prebiotic Interstellar Molecular Survey is the culmination of a nearly decade-long research campaign with the GBT," said Anthony Remijan, an astrochemist with the NRAO and head of the research team. "It is an invaluable resource and helps us understand the cosmic origins of this and other similarly elusive molecules."

The 100-meter Green Bank Telescope is the world's largest fully steerable radio telescope.

Explore further: Mining for Molecules in the Milky Way

More information: "Discovery of the interstellar chiral molecule propyleneoxide (CH3CHCH2O)," B. McGuire & P.B. Carroll et al., Science, June 2016. DOI: 10.1126/science.aae0328

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torbjorn_b_g_larsson
4.7 / 5 (13) Jun 14, 2016
"Scientists do not yet understand how biology came to rely on one handedness and not the other."

Actually it seems to be understood, but the biology isn't well known and consensus agreed on. The ribosome needs chiral nucleotides and amino acids both to evolve a genetic triplet code for (chiral selected) proteins read off from (chiral selected) messenger RNA strings.

Observably the ribosome has a frozen in two-tier chirality selection mechanism, which it doesn't need today since its products control a chiral selecting metabolism. Likely then the early cells were non-chiral in their chemistry, and chirality selection evolved with the code for functional, genetically produced proteins. That could not only work, we now know since last year that early non-chiral RNA ribozymes works better, are shorter, as cross-chiral RNA polymerases.

But the radio astronomer astrobiologists are not cell biologists...
ogg_ogg
1.9 / 5 (7) Jun 14, 2016
I strongly disagree with larsson's post.There is no clear reason that the overwhelming majority of chiral alpha-amino acids found in protein are left-handed. Don't take his claim seriously unless he provides a reference to back it up. The actual report has no clear connection to why life prefers left handed AAs. More click bait from Phys.org -from Science site:"it isn't directly involved in biochemical reactions". And oh, by the way, there IS consensus that some of the methane existing in space has chirality, you'll note the careful insertion of the word "complex" in the claim. (CH4 will have one or more H atoms replaced by a D atom (deuterium, an isotope of H) proportional to its abundance and stability (CD4 is more stable than CH4, in most circumstances).
Whydening Gyre
4.9 / 5 (8) Jun 14, 2016
Just to be clear on this chirality thing -
Does LH mean clockwise rotation and RH mean CCW?
Or vise versa?
Da Schneib
4.7 / 5 (12) Jun 14, 2016
Errr, methane cannot be chiral with 1, 2, or 3 deuterium atoms as you will understand if you realize it is a tetrahedron of hydrogen atoms surrounding a carbon atom. Replace one H with a D and you can always turn it so it is not a mirror image; replace two, and you can always turn it so the pair do not form a mirror image.

The two-tier chirality mechanism in ribosomes is well-documented. The chirality of both nucleic and amino acids is well-known.

@torbjorn speculates and clearly identifies it as such ("Likely,...") and I see little reason to doubt his statements about recent research since they are so easily supported; @torbjorn, this would be a good time to step up with a link to that research.

Chill, @ogg.
Da Schneib
5 / 5 (9) Jun 14, 2016
Just to be clear on this chirality thing -
Does LH mean clockwise rotation and RH mean CCW?
Or vise versa?
It's defined by the twist imparted in polarized light traveling from the sample to the analyzer; therefore "RH" or "R" molecules twist the plane to the right, implying clockwise rotation of the polarized beam with its axis through the sample and analyzer, with the beam traveling from the sample to the analyzer.

Not all chiral molecules are optically active; chiral molecules are stereoisomers; optically active stereoisomers are called enantiomers. All enantiomers are stereoisomers, but not all stereoisomers are enantiomers (i.e. optically active stereoisomers). Since non-enantiomer stereoisomers are not optically active their handedness cannot be defined.
Manfred Particleboard
2.5 / 5 (8) Jun 15, 2016
Umm did they find a racemic mixture of propylene oxide or any particular preference for one stereoisomer? The article labors the point about the significance of chirality as being mysteriously Levo in nature, yet nothing about the distribution of the two forms as found in space....
And methane cannot be chiral with just the isotopes of hydrogen, the rules for chirality takes the lightest substituent on the carbon and then consider the remaining three. If they go from heaviest to lightest in a clockwise manner, it is an R form of the molecule.(i've summarised somewhat) That said, you can take a Hydrogen and be left with a H, D and T form of hydrogen. Taking one H you can have a clockwise form of T, D and H, or taking the other H you can have an anticlockwise form of T, D and H. So.. No! There is no such thing as a chiral methane.
BigAhk
1.3 / 5 (6) Jun 15, 2016
I have taken two science courses in the past 28 years. It is just so odd to me that we are basing the existence of other life in the universe, on our biological map of how the lives we know exist.
Whydening Gyre
5 / 5 (7) Jun 15, 2016
Just to be clear on this chirality thing -
Does LH mean clockwise rotation and RH mean CCW?
Or vise versa?
It's defined by the twist imparted in polarized light traveling from the sample to the analyzer; therefore "RH" or "R" molecules twist the plane to the right, implying clockwise rotation of the polarized beam with its axis through the sample and analyzer, with the beam traveling from the sample to the analyzer.

So.... the opposite of what I envisioned in my left hand world....:-)
Does that make DNA helix RH?
And thanks, Da...
Da Schneib
5 / 5 (7) Jun 15, 2016
Umm did they find a racemic mixture of propylene oxide or any particular preference for one stereoisomer?
Radiotelescopes aren't enough to tell alone whether one enantiomer dominates or whether the mixture is racemic. They'll need polarized light.

...you can take a Hydrogen and be left with a H, D and T form of hydrogen. Taking one H you can have a clockwise form of T, D and H, or taking the other H you can have an anticlockwise form of T, D and H. So.. No! There is no such thing as a chiral methane.
Yes, you can get a stereoisomer from three species plus the carbon, but that's not what was being discussed, as you say.
Da Schneib
5 / 5 (8) Jun 15, 2016
Just to be clear on this chirality thing -
Does LH mean clockwise rotation and RH mean CCW?
Or vise versa?
It's defined by the twist imparted in polarized light traveling from the sample to the analyzer; therefore "RH" or "R" molecules twist the plane to the right, implying clockwise rotation of the polarized beam with its axis through the sample and analyzer, with the beam traveling from the sample to the analyzer.

So.... the opposite of what I envisioned in my left hand world....:-)
Does that make DNA helix RH?
And thanks, Da...
Most DNA is right handed; there is a form that's left handed. Watson and Crick's sample was right handed because that's the most common form. Handedness is defined looking down the strand in the direction it's read by the cellular machinery; right handed is clockwise.
Whydening Gyre
5 / 5 (9) Jun 15, 2016
I have taken two science courses in the past 28 years. It is just so odd to me that we are basing the existence of other life in the universe, on our biological map of how the lives we know exist.

It's because that type of life would be most recognizable and thusly, relatable, to us...
"Life as we know it..."
torbjorn_b_g_larsson
5 / 5 (8) Jun 18, 2016
Sorry for a late response.

@ogg: "I strongly disagree with larsson's post.There is no clear reason that the overwhelming majority of chiral alpha-amino acids found in protein are left-handed.

I provided a clear reason.

"Don't take his claim seriously unless he provides a reference to back it up. "

Correct. The comment boxes are small and sometimes I am rushed. I provided a longish comment on reddit with references: https://www.reddi.../d49gvsv .

The central reference is the frozen in two-tier selection in the genetic machinery, i.e.http://www.ncbi.n...2926754/ .

[tbctd]
torbjorn_b_g_larsson
5 / 5 (8) Jun 18, 2016
[ctd]

That it is viable to have a racemic (non-chiral) RNA World is central, of course. Citing myself: "That the RNA world started out with molecules of all kinds including "mirror molecules" seems to have been a benefit, not a problem, as it made for shorter polymerase strands in a replicating pool of them. ["A cross-chiral RNA polymerase ribozyme", Jonathan T. Sczepansk]i & Gerald F. Joyce, Nature, 2014.]"

@BA: It is a time tested procedure of empirical based science, start with what you know and go from there. Moreover it is like evolution in that sense, and 4 billion years of empirical testing is quite enough proof of concept, if you ask me. =D

In this case, the simplest form of genetic machinery (string based, if you ask computer science) involving template readout - from RNA to protein - needs a chiral choice, go left or right, which enforces a chemical chiral choice of the nucleotide and protein linkages (their "backbones").
torbjorn_b_g_larsson
5 / 5 (8) Jun 18, 2016
I meant polynucleotide and poly-amino acid linkages, to compare like with like.
john berry_hobbes
1.5 / 5 (4) Jun 19, 2016
S and R? I guess I'm old. Thought it was L and D.
BongThePuffin
Jun 19, 2016
This comment has been removed by a moderator.
torbjorn_b_g_larsson
5 / 5 (1) Jun 19, 2016
@john: DS described the chemistry, but didn't give the chemical notation, which is a mess.

The Wikipedia articles on this are currently a mess too. Here is a better article, with history:

"An early procedure assigned a D prefix to enantiomers [chemically related to a right-handed reference compound and a L prefix to a similarly related left-handed group of enantiomers. Although this notation is still applied to carbohydrates and amino acids, it required chemical transformations to establish group relationships, and proved to be ambiguous in its general application. A final solution to the vexing problem of configuration assignment was devised by three European chemists: R. S. Cahn, C. K. Ingold and V. Prelog. The resulting nomenclature system is sometimes called the CIP system or the R-S system.

[tbctd]
torbjorn_b_g_larsson
5 / 5 (1) Jun 19, 2016
[ctd]

In the CIP system of nomenclature, each chiral center in a molecule is assigned a prefix (R or S), according to whether its configuration is right- or left-handed. No chemical reactions or interrelationship are required for this assignment. The symbol R comes from the Latin rectus for right, and S from the Latin sinister for left. The assignment of these prefixes depends on the application of two rules: The Sequence Rule and The Viewing Rule."'

" It is important to remember that there is no simple or obvious relationship between the R or S designation of a molecular configuration and the experimentally measured specific rotation of the compound it represents. In order to determine the true or "absolute" configuration of an enantiomer, as in the cases of lactic acid and carvone reported here, it is necessary either to relate the compound to a known reference structure, or to conduct a rather complex X-ray analysis on a single crystal of the sample."

[tbctd]
torbjorn_b_g_larsson
5 / 5 (1) Jun 19, 2016
[ctd]

[ https://www2.chem...ism3.htm ]

Here is my understanding: Adding up the S & R centers may or may not give you a molecule with rotation. Then there is the added complexity that a crystal with rotation may or may not be sufficiently optically active to make a difference.

[Since I am not working with this, it feels a bit overwhelmingly handwavy. I can usually work my way out of a chemical description if I must, but that is all. Maybe DS can fill in the gaps/correct my errors.]
Da Schneib
5 / 5 (1) Jun 19, 2016
It's very unclear (at least in chemistry and particularly in chemical terminology) how D vs. L compounds' internal symmetries link up with R vs. S centers.

R and S are basically arbitrarily assigned as far as I can see; they have more to do with regularizing chemical nomenclature than with the actual characteristics, either in spatial terms of the organization of the radicals, or in optical activity terms, of the molecules they describe.

This has little or nothing to do with D and L, which denote Dextro- (right) and Levo- (left) rotary isomers, which can be measured for their optical activity in solution, D giving a clockwise or right rotation to polarized light, and L giving a counterclockwise or left rotation.

How the D and L isomers behave optically also is not necessarily related to the actual spatial organization of the radicals.
[contd]
Da Schneib
5 / 5 (1) Jun 19, 2016
[contd]
So basically we have two arbitrary nomenclature systems neither of which actually has anything obvious to do with the real spatial arrangement of radicals or atoms in the molecules, one based on relatively mysterious and arbitrary rules (IUPAC directs querents of the R/S system to the original papers proposing the system, which is not very rigorous) and the other based on a criterion that seems to have little to do with the actual arrangement but is justified by the optical activity of the molecules measured in a solution.

The second (D/L) has some sort of physical justification since it is measurable using other physical quantities, but this justification requires extensive computer modeling to determine for a postulated but undetected compound. One cannot simply look at a stereoisomer pair and say "this one is D and that one is L."

C'est la chimie. C'est la vie. ;)
Da Schneib
5 / 5 (1) Jun 19, 2016
In case anyone wondered I was deliberately vague on this aspect of the subject; now you see why. There is no simple explanation for any of it.
torbjorn_b_g_larsson
5 / 5 (1) Jun 20, 2016
DS, thanks! Now I know why the rare examples I have seen described seems so constructed, the conventions are a mess _and_ the chemistry is messy.
Manfred Particleboard
not rated yet Jun 22, 2016
Do you realize how many people do not read beyond that?
Yeah- about as many as the number of petty, slack jawed trolls with hair trigger emotional problems who hang around this site.

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