Cosmic Amino Acids

Cosmic Amino Acids Also Show Left-Handed Bias, Study Says - New ... http://query.nytimes.com/gst/fullpage.html?res=9A03E2D6173FF937A...

February 14, 1997

Cosmic Amino Acids Also Show Left-Handed Bias, Study Says

By MALCOLM W. BROWNE

Scientists have long sought to explain a great mystery: the fact that nearly all living organisms on earth are made of proteins assembled exclusively from the left-handed versions of certain amino acids.

Life as it is now known could not exist without the single-handedness, or homochirality, of amino acids; proteins could not fold into living structures unless the handedness of their constituent amino acids was completely consistent.

The puzzle is that nature's choice was for left-handed amino acids rather than right-handed ones. Was it chance alone that picked the left hand, or was there a deeper reason, based either on biological evolution or on some influence pervading the depths of space where amino acids are also created?

The overwhelming preference of terrestrial life for left-handed amino acids over the right-handed kind is not likely to be explained soon. But two chemists at Arizona State University have analyzed amino acids extracted from a meteorite and found the first material evidence that some kind of cosmic influence, rather than biological evolution alone, may be responsible for the left-handed bias.

The discovery, reported today by Dr. John R. Cronin and Dr. Sandra Pizzarello in the journal Science, may shed light not only on the chemistry of terrestrial life but also on the possible basis of life elsewhere in the universe.

The main thrust of their finding is that even amino acids never found in terrestrial life forms -- and which could not have been the result of biological evolution -- show a left-handed bias. Their conjecture is that some influence outside this planet loves a southpaw.

Many organic molecules exhibit handedness, or chirality. Their paired molecular structures are analogous to pairs of gloves -- identical in the arrangement of their atoms except that they are mirror-reversed. These mirror-reversed forms often have very different chemical properties.

When amino acids are synthesized in the laboratory, roughly equal amounts of the left-handed and right-handed types are ordinarily produced, and yet most living organisms produce only the left-handed kind.

Amino acids come in many varieties, but all are carbon chains that incorporate both an amino group (a nitrogen atom and three hydrogen atoms) and a carboxyl group (a carbon atom with two oxygen atoms and one hydrogen atom). Amino acids can be linked together in endlessly complex arrays of peptides, polypeptides and proteins.

Experiments by Dr. Cronin and his colleagues over the last 20 years have focused on amino acids brought to earth in a carbon-rich rock called the Murchison Meteorite, which fell near Murchison, Australia, in 1969.

Analyses of molecules in the meteorite conducted soon after it hit the ground suggested that roughly equal amounts of right- and left-handed amino acids were present. This kind of 50-50 distribution, called a racemic mixture, is typical of mixtures created when amino acids are synthesized in the laboratory. But racemic mixtures are not characteristic of life, which depends on left-handed amino acids alone.

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Cosmic Amino Acids Also Show Left-Handed Bias, Study Says - New ... http://query.nytimes.com/gst/fullpage.html?res=9A03E2D6173FF937A...

The inference drawn from early analyses by several scientists was that the amino acids in the Murchison Meteorite were created by nonbiological processes, or at least by processes inconsistent with life dependent on homochiral amino acids.

In later experiments conducted with greater precision, however, scientists detected a slightly elevated ratio of left-handed to right-handed amino acids in the meteorite. Many biologists discounted the importance of this finding on the ground that it was probably the result of contamination by terrestrial proteins.

In their latest work, Dr. Cronin and Dr. Pizzarello sought to eliminate the problem of contamination by focusing their attention on rare amino acids never found in terrestrial organisms. Such amino acids are unlikely to have entered the meteorite as contaminants from the ground.

''Life on earth is based on 20 amino acids, but there are 50 or 60 other amino acids not associated with life on earth,'' Dr. Cronin said in an interview. ''We looked at four of these nonbiological amino acids found in the Murchison Meteorite, and they showed excesses of left-handed forms ranging from 2 percent to 9 percent.''

Dr. Cronin infers that some cosmic influence caused the imbalance between nonbiological left- and right-handed amino acids in the meteorite. Biologically important amino acids reaching earth with an enriched abundance of left-handed forms might later have undergone some process of selection that eliminated the right-handed forms.

One possible explanation of homochirality originating in space, he said, is a hypothesis proposed in 1991 by Dr. William A. Bonner, a chemist at Stanford University. Dr. Bonner suggested that light emitted from within the intense gravitational field of a neutron star would be circularly polarized, that is, the light would propagate along a kind of corkscrew path, and could influence the handedness of molecules forming along its path in the thin interstellar gas.

Dr. Jeffrey L. Bada of the Scripps Institution of Oceanography in San Diego, who also studies chemical problems related to the origin of life, said in an interview, ''This is the first convincing evidence that some influence outside the earth might have played some role in determining the handedness of biological amino acids.''

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