ASU Research E-Magazine: Life's Left-handed Molecular Mystery

An honors class on chemical evolution and the origin of life led to research that might help to better explain how a molecular quirk developed in all life on Earth.

The research suggests that the light emitted by the remnants of an exploded star more than 4.5 billion years ago could have produced the quirk during organic chemical evolution that took place before the origin of life. ASU chemists John R. Cronin and Sandra Pizzarello published their findings in the Feb. 14, 1997 issue of the journal Science.

Scientists have known for a long time that many compounds in living things exist in mirror-image forms. “The best way to explain that is to say that the two forms are like your two hands. One is a mirror reflection of the other,” Cronin says. “They are different. They don’t coincide, and yet they are similar.”

One-handed compounds are needed because without them, large biological molecules, like proteins, would be unable to form reproducible structures. When scientists make the same compounds in the laboratory, half the molecules come up left-handed, half right-handed. But in Earth organisms, amino acids, which are the building blocks of life, are all left-handed.

“Life is extremely fussy about which one of these it uses,” Cronin explains. “So the question is, how did this ever come about that life was able to make this selection of one-handed forms over the other-handed forms, when they are equally probable?”

Two major theories propose answers. According to one theory, there was some sort of influence acting on chemical evolution before the origin of life that either promoted the development of left-handed amino acids, or the breakdown of right-handed amino acids.

The other theory says that molecular handedness developed as a matter of biological evolution, after the origin of life. Fortunately, meteorites rich in organic compounds provide a way to test the first theory. Life probably arose on Earth about 3.8 billion years ago. The organic compounds found in some meteorites go back 4.5 billion years.

Researchers have tried to conduct those tests for decades, but there’s always been a problem. It is all too easy to contaminate the meteorite with amino acids from Earth, creating a bias in the results.

Things changed during the spring 1995 semester, when Cronin taught a course in the ASU Honors College. He learned of an intriguing suggestion by William Bonner, an organic chemist at Stanford University, regarding neutron stars. Neutron stars are all that is left after a star has exploded in a supernova. Neutron stars emit circular polarized light. This means that the stars’ light waves describe a narrow spiral as they travel. The light waves can spiral to the left or to the right.

Left-handed organic compounds tend to absorb light waves spiraling in one direction. Right-handed compounds tend to absorb those spiraling in the other direction. Bonner and others had therefore proposed that circular polarized light emitted by a neutron star could have influenced molecular handedness in the great cloud of interstellar gas and dust from which the sun and its planets formed more than 4.5 billion years ago.

If there is any evidence of this process having taken place, Cronin figured, “we really ought to look at the meteorites and do it in a way that hasn’t been done before.” To get around the contamination problem that had plagued researchers in the past, he and Pizzarello analyzed amino acids in the Murchison meteorite that are unknown on Earth.

“There are many of those in the meteorite,” Cronin says. “We know of 40 or 50 different amino acids that have no terrestrial occurrence.” The ASU researchers have studied the organic chemistry of the Murchison meteorite, which fell in Australia in 1969, for more than two decades.

“Sure enough, when we did this, we found excesses of the left-handed form that range from 2 percent to about 9 percent over the right-handed form,” Cronin says.

Jeffrey L. Bada of the Scripps Institution of Oceanography in La Jolla, Calif., says that there has been little convincing experimental evidence to previously support either of the competing theories explaining the one-handedness of organic compounds. Adds Bada, “The findings of Cronin and Pizzarello are probably the first demonstration that there may be natural processes in the cosmos that generate a preferred amino-acid handedness.” —Steve Koppes