02/10/93
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STANFORD -- A turbulent theory involving supernovae, cosmic nebulae and synchrotron radiation suggests the extraterrestrial origin of chemical handedness, a crucial prerequisite for the evolution of life on Earth.
Stanford chemistry Professor Emeritus William A. Bonner concludes that the source of biomolecular handedness must have been extraterrestrial because the experimental and theoretical attempts to account for its origin on Earth aren't viable.
Molecular handedness, or chirality, is a stereochemical feature of most carbon-based molecules that form bioorganic matter. Chiral molecules exist as mirror-image pairs, with one partner called left-handed, the other one right-handed, depending on the way specific atoms are arranged around a central, asymmetric carbon atom.
Bonner suggests a sequence of astrophysical events for the creation of chirality in space and its constant flow to the prebiotic earth. He will present his theory Monday, Feb. 15, in the session "Handedness in the Scientific Domain" at the annual meeting of the American Association for the Advancement of Science in Boston.
His scenario is the product of scientific imagination grounded on experimental findings in different fields. Bonner developed it in collaboration with Edward Rubenstein, a professor at Stanford Medical School and an expert on synchrotron radiation.
The crucial biomolecules of life - such as amino acids, RNA and DNA - are chiral. In order for these polymeric molecules to replicate themselves, their individual components have to be of one kind, either right- or left-handed.
"It is generally agreed that you need homochirality - either all left-handed or all right-handed - for life to get off the ground," Bonner said. "Therefore, a preponderance of one handedness must have evolved in prebiotic times."
The scientists, however, cannot explain how this happened because they have never succeeded in creating chiral molecules of only one kind in laboratory experiments that simulated prebiotic conditions.
Since chiral molecules are necessary to breed new chiral molecules, how did the first ones come about?
Bonner and Rubenstein's extraterrestrial scenario begins with the gravitational collapse of certain types of massive stars, an event that is thought to occur in our galaxy as frequently as every 50 years, according to Bonner. The shock wave rebounding from that implosion causes an incredibly violent explosion known as a supernova. It blows away the star's exterior, compresses the surrounding interstellar medium to form dust grains and new astral bodies, and it leaves behind a small, spinning neutron star.
Colossal electric fields around this star accelerate electrons in the spatial environment to almost relativistic velocities. These electrons, however, can't race straight out into space because a strong magnetic field forces them to circle around the neutron star as in a giant synchrotron accelerator, and these speeding electrons give off synchrotron radiation.
Part of that electromagnetic radiation is circularly polarized; it spirals along its direction of travel to either the right or the left and thus represents chiral radiation - just like right- or left-handed staircases represent chiral physical objects.
Experiments of numerous scientists demonstrated that subjecting equal mixtures of right- and left-handed molecules to circularly polarized light creates an excess of one kind of handedness in that mixture. The radiation simply induces molecules of one handedness to decompose faster than the other, leaving behind a larger proportion of the other-handed version.
That mechanism of producing biased handedness is experimentally the most viable explanation so far. (It applies universally to every light-absorbing molecule because the circularly polarized light emitted from neutron stars spans the entire wavelength spectrum from radio waves to gamma rays.)
The interstellar dust grains produced by the explosion swirl around in molecular clouds of primitive gases such as water vapor, ammonia and methane.
In the cold interstellar temperatures of near absolute zero, these gases condense to "ices" covering the dust grains. Under the influence of ultraviolet radiation, the ices convert to more complex, solid molecules. Initially, they are mixtures of right- and left-handed pairs, but as they are exposed to intense circularly polarized radiation, an excess of one single-handed kind forms and is instantly frozen in action by the cold temperature.
Part of this scenario was simulated earlier by the chemical experiments of J. Mayo Greenberg at the University of Leiden.
How did these grains reach Earth? According to Bonner, Earth might have passed through the clouds and picked up the grains as it traveled with the solar system around the center of the galaxy.
In another possible, more violent explanation, much of the chiral material might have crashed to Earth as comet-type bodies or asteroids that were compressed from dust grains after the supernova.
"A difficulty with that idea is that the comet or asteroid impact on Earth is a dramatic event, possibly so devastating that chiral molecules just might have been incinerated. There needs to be some means for soft landings to occur," Bonner said.
Bonner speculates that since Earth's oceans are thought to have arisen from such comet collisions, primordial oceans from which life arose could have been well stocked with chiral molecules imported from space.
Once arrived on Earth, however, chirality isn't yet ready to nurture evolution of life. The relative excess of one handedness over the other must first be amplified until all precursor molecules for life are of one handedness, a precondition for the efficient self-replication of biomolecules.
Bonner also is trying to interest astronomers to look for circularly polarized radiation near recent supernovae. For example, the crab nebula (the remains of a supernova that occurred about 900 years ago) is known to emit linearly polarized synchrotron radiation.
He said that researchers might not have detected circularly polarized radiation there because it is confined to angles above and below the plane of the electrons orbiting around the neutron star.
From another point of view, they might see the light.
-jns-
This story was written by Gabrielle Strobel, a science writing intern at the Stanford News Service.
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