Why So Many Left-Handed Compounds?
Posted February 1, 1998
Many molecules, including amino acids, have isomers that are left-handed (levorotatory) or right-handed (dextrorotatory). When amino acids are formed in the laboratory,
about half are right-handed and half are left-handed. But in living things, this is not the case. Almost all amino acids used by living things are left-handed. Perhaps there is some
as yet unidentified process in living things that favors the formation of these molecules over their right-handed counterparts. Or perhaps the very formation of living things was fostered
by left-handed molecules that formed when our solar system formed.
Many of the organic compounds on Earth are believed to have derived from meteorite impacts. Some meteorites are rich in organic compounds. The Murchison meteorite
is the most recently observed and studied of the organic meteorites known to have hit Earth. A study of the organic compounds in this meteorite has produced some surprising results.
More of the amino acids in the Murchison meteorite are left-handed than right-handed. Some of the amino acids in the meteorite show a preponderance of left-handedness, whereas others
show just a slight increase in left-handedness.
What does this mean? Some scientists hypothesize that life could not have originated on Earth without an abundance of left-handed amino acids. Because there are more
left-handed than right-handed compounds in living things as well as in organically rich meteorites, some scientists suggest that life on Earth may have derived from the compounds that
were formed in space. It is possible that the molecular cloud from which the solar system condensed produced a preference for left-handed compounds. How this occurred is still unknown.
Some scientists hypothesize that a preference for left-handed compounds was imposed on the solar system by a nearby neutron star that caused light to be polarized in a circular pattern
(causing compounds to form with distinct handedness). If this happened, then scientists looking for life elsewhere in the solar system may examine rocks from other planets to determine
if they, too, have greater numbers of left-handed chemical compounds.
References
Engel, M.H. and S. A. Macko. "Isotopic Evidence for Extraterrestrial Non-Racemic Amino Acids in the Murchison Meteorite." Nature, Vol. 389, No. 6648, pp.
265-268.
Chyba, Christopher F. "A Left-Handed Solar System?" Nature, Vol. 389, No. 6648, pp. 234-235.
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