Mirror image molecules in Orion

Pick of the Pics and Updating Magic Universe

Mirror-image molecules sorted in the Orion Nebula

But Pasteur’s hope for a cosmic driver comes true only locally

A predominance of either left-handed or right-handed versions of molecules is likely within huge dust clouds imaged by Japanese astronomers. The electric field of light rays coming from the clouds corkscrews to the left or corkscrews to the right, with “circular polarization”. The different kinds of clouds are clearly distinguishable in a massive star-forming region within the Orion Nebula, called BN/KL. Yellow denotes left-handed light, and red, right-handed. The largest yellow and red features are about 100 times wider than the Solar System, and the astronomers suggest that the polarized light will favour the formation of left-handed or right-handed molecules. The conspicuous dots left of centre near the bottom are bright young stars of the Trapezium group — strong winds  from which have helped the astronomers by blowing away dust that otherwise would obscure the BN/KL region of interest. Credit: Near-infrared (2.14 μm) image with the SIRPOL polarization instrument, NAOJ.

On seeing this report by Tsubasa Fukue and Motohide Tamura of the National Astronomical Observatory of Japan (with colleagues in Japan, UK, Australia and USA) my mind went straight back to Louis Pasteur.

Alanine, an amino acid, has mirror-image forms. L (laevo) rotates the electric field of light to the left and and D (dextro) to the right. Image NAOJ.

Although immortalized for the germ theory of disease, Pasteur’s initial claim to fame came from a discovery he made as a young student – namely that molecules from living sources have effects on the polarization of light, but the same molecules made synthetically do not. This is the phenomenon of chirality, or handedness. Chemists had to learn to think three-dimensionally about versions of molecules that are mirror images of each other. In the example shown here, every amino acid molecule in living things on Earth is of the left-handed (L) kind.

Molecular handedness is a fundamental feature of life and Pasteur suspected that some fundamental feature of the Universe was responsible for it. The phenomenon has been both a puzzle and a spur for investigators of the origin of life. The fact that carbon compounds in meteorites show the same bias in handedness as that seen on Earth suggests that some physical process was at work throughout the Solar System, at least.

The astronomers now offer an answer. Circularly polarized light pervading the dust cloud in which the Sun and its planets were born would have prompted our molecular bias. The scenario is made convincing by the sheer size of the clouds in Orion possessing one polarity or the other. But it ‘s not the Universe-wide mechanism that Pasteur expected. It seems that if the Solar System had originated in a cloud with the opposite kind of circularly polarized light, all our amino acids would be dextro.

Here’s a relevant extract from the story in Magic Universe, called “Handedness: mysteries of left versus right that won’t go away” — with an update appended.

The mystery of life’s biases

Pasteur’s discovery of molecular handedness still resounds through chemistry, biochemistry and astrochemistry, and in speculations about life’s origin, where handedness greatly complicates the chemical problem. The shapes of crystals and the effects on light both trace back to the arrangements of atoms in molecules. And as scientists revealed the chemistry of life during the 19th and 20th Centuries, they reconfirmed again and again that organisms are extremely choosy.

The spiral of deoxyribonucleic acid, the DNA helix that builds the genes, nearly always twists in the same direction as an ordinary corkscrew. Oppositely twisted DNA, called Z-DNA, sometimes shows up, and may have special genetic functions. But it is not a mirror image of the normal kind of DNA. It cannot be, because it has to fit together chemical sub-units that all have twists of their own.

Similarly, the twenty kinds of amino acids used to build proteins are all left-handed molecules. When proteins act as enzymes in the manufacture of other molecules, they communicate their chiral biases. That is why natural sugar, made in plants with the help of enzymes, is one-handed with a predictable effect on polarized light.

The nutritional value of food, the efficacy of medicinal drugs and the potency of scents all depend on the handedness of the molecules. One form of the molecule limonene smells of the lemons, and its mirror image, of oranges. More commonly, the wrong versions of biologically active molecules are useless to living things, and sometimes poisonous.

Basic questions about chemical handedness nevertheless remain unanswered. The fact that all living organisms share the same patterns of molecular handedness is maybe unsurprising, given that they seem to be descended from common ancestors of long ago. But how the very first ultra-primitive cells contrived to assemble and live on selected versions of molecules, and avoid the mirror images, remains a puzzle.

It has even provided a recurring argument, not easily refuted, for those wishing to insist on divine intervention in the primordial biochemistry. Pasteur himself was worried enough to seek a physical cause of the handedness in the living world. He was disappointed when he experimented with powerful magnets and detected no effect on molecular handedness, yet he was convinced that some cosmic explanation would be found. Pasteur insisted: ‘L’univers est dissymétrique.’

The physicists’ worlds and anti-worlds

His attempt to pasteurize the cosmos with magnets was remembered in 1957, when the Universe was found to be asymmetrical in a different respect. Physicists discovered a bias in ordinary matter, such that all of the ghostly particles called neutrinos spin to the left around their direction of travel. This violated a principle called parity, requiring that particles should be equally capable of spinning in either direction. To find a neutrino spinning to the right, you have go into the looking-glass world of antimatter, and look for anti-neutrinos.

The fall of parity provided, for the very first time, an objective way of telling left from right. Human beings have to learn the distinction subjectively, as every drill sergeant knows. In the bowdlerized version: ‘Do me the kindness, young gentleman, of showing me in which hand you usually hold a knife. Thank you. So next time, when I invite you to turn right … .’

If you were to contemplate a rendezvous with an alien astronaut, in an imaginary universe where matter and antimatter are equally common, how would you avoid an accident? Matter and antimatter annihilate each other. So you might be well advised to send a message explaining our custom of shaking hands, and pointing out that we use the hand away from which neutrinos always rotate. If the alien comes from an antiworld, physicists back at his base will be most familiar with antineutrinos. Richard Feynman of Caltech, who first recommended this precaution using parity violation, concluded: ‘If he puts out his left hand, watch out!’

Some chemists claimed that parity violation was Pasteur’s wished-for asymmetry in the Universe, which solved the problem of handedness of molecules at the dawn of life. Physical-chemical effects, they said, made one form of a mirror molecule slightly more stable than the other. The left-handed spin of the neutrino favoured the left-handed bias in amino acids.

This sounded impressive until you looked at the numbers. The difference in stability between left-handed and right-handed molecules due to parity violation is only a million-millionth of a per cent. If that were meaningful, why have chemists not been able to exploit it in laboratory experiments? Those should be far more propitious for differences to show up, than in the random chemistry on the young Earth.

A long-standing hope was that light might have a more powerful effect. If handedness in molecules affected polarized light, why should polarized light not reciprocate in the course of chemical reactions, and so influence the shaping of molecules? News of success on those lines came in 1997 from Yoshihisa Inoue of Osaka.

He and his colleagues used circularly polarized ultraviolet light, in which the orientation of the wave spontaneously rotates, either clockwise or anticlockwise. Shining it on a material called 4-cyclooctene, with a twisted ring of eight carbon atoms, they produced a majority of molecules with a direction of twist that was sympathetic to the direction of rotation of the polarized light. The team soon obtained similar results with amino acids, key ingredients of life.

This research has great basic theoretical significance,’ Inoue said, ‘which is related to the origin of chirality in the biosphere.’ There had been a suggestion that polarized light from intensely magnetic neutron stars could affect the handedness of molecules forming in interstellar space. That in turn might conceivably be an origin for materials with a certain bias turning up on the young Earth, before the origin of life. William Bonner and Edward Rubenstein of Stanford had suggested this scenario, and Inoue thought his experiments confirmed its feasibility.

Other scientists considered it more likely that the source of biased molecules needed for life was on the Earth itself. The fact that Pasteur’s racemic acid divides neatly into tartrate and anti-tartrate crystals shows that simple molecules can recognise one another’s handedness and settle down, like with like. In other settings, the pre-existence of molecules of a certain handedness may, by their very presence, induce newly forming molecules to follow suit. When the molecules are densely packed, such induction is far more likely than in a dilute solution, a gas, or interstellar space.

On the primordial Earth, it seems plausible that the selection of handedness in the molecules of life happened in the solid state or on the surface of a solid,’ said Reiko Kuroda of Tokyo. She began the 21st Century leading a team effort on what she called chiromorphology — meaning the chiral shape, which is expressed at all levels in nature, whether microscopic or macroscopic, and whether animate or inanimate. Her starting point was the chemistry of solids.

Here the aim is to analyse how molecules gather in a crystal, and to see exactly how their handedness affects their interactions at close quarters. The hope is for the invention of new kinds of solid-state chemical reactions that favour a chosen handedness, as the organometallic compounds do. Kuroda found such examples and also designed and made a novel instrument for the study of chirality in the solid state. Besides their possible usefulness for the chemical and pharmaceutical industries, the reactions may make events on the early Earth more comprehensible.

The update for Magic Universe To add after the passage quoted.

Tsubasa Fukue and Motohide Tamura, at the National Astronomical Observatory of Japan, took Pasteur’s quest for the origin of molecular handedness out into cosmic space. In a star-forming region called BN/KL, within the famous Orion Nebula, they identified huge gas clouds, a hundred times wider than the Solar System, each pervaded by polarized light in one sense or the other. Such light would favour the formation of molecules with a bias in their handedness.

Reporting the discovery in 2010, the Japanese astronomers rejected the idea that special neutron stars were responsible for the polarizations. Instead they considered them to be a natural by-product of the formation of massive stars, which are present in the BM/KL region.  For this to be relevant to the Solar system, the Sun had to be born alongside massive stars. Citing evidence for nearby a supernova explosion almost coincidental with with the origin of the Sun,  Fukue and Tamura said that this criterion was satisfied.

This wasn’t the fundamental cosmic asymmetry that Pasteur looked for with his magnet, or that others wanted to link to particle physics. It was a regional effect. If the Sun had been born in a different dust cloud our biochemistry could have been the other way around. Either way, the presence of a majority of molecules with distinctive handedness, by courtesy of astrophysics, would have made it that much easier for primitive life-like assemblies of molecules to take their pick.

References

T. . Fukue et al., “Extended High Circular Polarization in the Orion Massive Star-Forming Region: Implications for the Origin of Homochirality in the Solar System”, Origins of Life and Evolution of Biospheres, Vol. 40, p. 335, 2010. pdf available at http://www.springerlink.com/content/q0k1k74u76451557/fulltext.pdf

N. Calder, Magic Universe, pp. 362-4, Oxford UP 2003

If you find circular polarization hard to imagine, Wikipedia has an entry with animations at http://en.wikipedia.org/wiki/Circular_polarization

Other discussions of the origin of life on this blog are to be found in the category Comets.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: