A explosão de estrelas estilhaçou o espelho da vida?

domingo, maio 30, 2010

Did exploding stars shatter life's mirror?

19 May 2010 by Marcus Chown

Magazine issue 2760.

Explosive twist (Image: Robert Mallozz/Marshall Space Flight Center)

MR SPOCK is dying. Fortunately for the crew of the USS Enterprise, the Spock in question is not the real one, but an evil mirror-image version created in a freak transporter malfunction. This Spock's back-to-front body can digest only right-handed amino acids; meanwhile, like all organic matter, the food around him is made of left-handed amino acids. He is starving in the midst of plenty.

This plot line from the 1970 novel Spock Must Die! - the first literary spin-off from the Star Trek TV series - highlights one of life's fundamental mysteries. Why does biology use only one of two mirror-image forms in which most complex molecules can occur? The latest pop at an answer weaves astrophysics, particle physics and biochemistry into a startling proposal: that the stellar explosions known as supernovae are to blame.

"It is an intriguing idea," says Daniel Glavin, an astrobiologist at the NASA Goddard Flight Center in Greenbelt, Maryland. It is certainly a novel turn in this twistiest of tales: the story of how life came to be left-handed.

The property of handedness, known to chemists as chirality, is a feature of many molecules whose arrangement of atoms is not completely symmetrical. A chiral molecule comes in two forms that are rather like a pair of gloves. Right and left-handed gloves are essentially identical, with the same basic components, four fingers and a thumb, and the same function of keeping our hands snug and protected. They are not exactly the same, however: you cannot rotate or flip a glove of one type so that it will superimpose perfectly on the other. But look in a mirror, and a left-handed glove becomes right-handed.

Similar molecular mirror-image forms are called enantiomers. They are made from the same atoms and have the same chemical and physical properties. Most chemical reactions produce equal quantities of both.

That makes nature's predilection for one form - its "homochirality" - all the more strange. Only left-handed or "l" amino acids make up the proteins that provide our cells with structure and regulate their functions, and only right-handed or "d" sugars play an active part in biochemistry. It is like keeping a drawer full of only one sort of glove, while stubbornly refusing to wear the other.

Star turn

Perhaps homochirality is the result of a chance asymmetry in life's early history on Earth, amplified by time and evolution. In that case, you might expect it to be non-existent or even reversed elsewhere. Yet the builder's rubble left over from the construction of the solar system tells a different story. "For every type of amino acid found in meteorites there is an excess of the left-handed form over the right-handed of between 2 and 18 per cent," says Uwe Meierhenrich of the Nice Sophia Antipolis University in France. "An excess of the right-handed form has never been found."

That alone does not prove anything: meteorites might have become contaminated when they came into contact with the Earth's surface and before they were picked up. But the strong implication is that the left-handed bias pre-dates the existence of life, our planet and indeed our solar system, even if life on Earth amplified it to an extreme.

So is the asymmetry simply a question of basic physics? That is certainly a possibility (see "Disturbance in the force"), but there are other attractive suggestions too. One was identified in 1998, when a team led by Jeremy Bailey of the University of New South Wales in Sydney, Australia, discovered regions in the Orion nebula, a star-forming zone 1300 light years away from Earth, that are suffused with circularly polarised infrared light (Science, vol 281, p 672).

Light becomes circularly polarised when its associated electric field vibrates in a plane that rotates clockwise or anticlockwise about its direction of travel. In a nebula, such polarisation could happen when light is scattered off the atoms and molecules, including amino acids, floating around in the gas clouds.

Circularly polarised light interferes with the arrangement of electrons that bind atoms together in such a way that it can selectively break up molecules of one or other chiral form, depending on which way it is rotating. The regions of the Orion nebula identified by Bailey and his colleagues could therefore have an excess of one form of amino acid. A similar situation in the cloud from which our solar system formed could have been the chiral seed from which asymmetric life on Earth grew.

It is a seductive possibility, but it has its problems. The selective destruction of amino acids only kicks in if the light has enough energy to trigger the necessary chemical reactions - in practice requiring the presence of ultraviolet light, rather than the less energetic infrared light seen in the Orion nebula. "No one has detected any of this light yet," says Meierhenrich - although this might be because the gas clouds scatter ultraviolet light so effectively that little of it makes it to our telescopes.

The new scenario sketched by Richard Boyd of the National Ignition Facility in Livermore, California, along with Toshitaka Kajino and Takashi Onaka of the University of Tokyo, Japan, sidesteps this problem. It too starts with a cloud in which molecules, including amino acids, have already formed. But light is not the catalyst for change; instead, it is the combined effect of the immense magnetic fields and the vast fluxes of high-energy particles that are produced in a supernova explosion.

A core-collapse, or type II, supernova occurs when a massive star, its fuel spent, collapses within seconds under its own weight to form a superdense neutron star just tens of kilometres across. This remnant generates an incredibly intense magnetic field, with field lines emerging from its north pole and returning to its south pole, as is the case with Earth's magnetic field.

Atomic nuclei have a quantum-mechanical property known as spin which, all things being equal, aligns itself with a magnetic field. The crux of Boyd's idea is the effect such magnetic fields have on nitrogen-14 nuclei in an amino acid, where a nitrogen atom attaches the defining amine (NH2) group to a carboxyl group. Within a molecule, nitrogen spins do not have the latitude of movement they would if they were free, and calculations performed by the chemist A. D. Buckingham of the University of Cambridge in 2004 show how switching on a magnetic field in fact produces a rotational effect in different directions for molecules of opposite chiralities (Chemical Physics Letters, vol 398, p 1).

As a result, Boyd suggests, when the magnetic field of a supernova remnant starts up, amino acids of one chirality end up with their nitrogen spins pointing along the magnetic field lines, away from the star at the north pole and towards it at the south, while those of the opposite chirality will be forced to align with their nitrogen spins in the opposing direction.

This sets the stage for fireworks as the dying star collapses in on itself, sending an intense blast of neutrinos and antineutrinos spewing out radially in all directions, including along the magnetic field lines. Antineutrinos in particular react readily with nitrogen-14 nuclei, producing a carbon-14 nucleus and a positron. In a similar, energetically less-favoured reaction, neutrinos turn nitrogen-14 into oxygen-14 and an electron. In both cases, once the nitrogen nucleus in an amino acid is hit, the amine group is blown apart and the amino acid disintegrates.

Read more here/Leia mais aqui: New Scientist