Daily University Science News
For a transition
to occur from the pre-biological world of 4 billion years ago
to the world we know today, amino acids--the building blocks of
proteins in all living systems--had to link into chainlike molecules.
Now Robert
Hazen and Timothy Filley of the Geophysical Laboratory of the
Carnegie Institution of Washington, and Glenn Goodfriend of George
Washington University have discovered what may be a key step in
this process -- a step that has baffled researchers for more than
a half a century.
Their work,
supported by NASA's Astrobiology Institute and the Carnegie Institution,
is reported in today's issue of the Proceedings of the National
Academy of Sciences.
The molecular
structure of all but one amino acid is an asymmetrical arrangement
grouped around carbon. This arrangement means that there are two
mirror-image forms of each amino acid; these forms are designated
left-handed (L) and right-handed (D).
All of the
chemistry of living systems is distinguished by its selective
use of these (L) and (D), or chiral, molecules. Non-biological
processes, on the other hand, do not usually distinguish between
L and D variants.
For a transition
to occur between the chemical and biological eras, some natural
process had to separate and concentrate the left- and right-handed
amino acids. This step, called chiral selection, is crucial to
forming chainlike molecules of pure L amino acids.
Hazen and
his collaborators performed a simple experiment. They immersed
a fist-sized crystal of the common mineral calcite, which forms
limestone and the hard parts of many sea animals, in a dilute
solution of the amino acid aspartic acid and found that the left-and
right-handed molecules adsorbed preferentially onto different
faces of the calcite crystal.
Most minerals
are centric, that is, their structures are not handed. However,
some minerals display pairs of crystal surfaces that have a mirror
relationship to each other. Calcite is one such mineral. It is
common today, and was prevalent during the Archaean Era some 4
billion years ago when life first emerged.
This study
suggests a plausible process by which the mixed D- and L-amino
acids in the very dilute "primordial soup" could be
both concentrated and selected on a readily-available mineral
surface.
Hazen remarks,
"Since the pioneering work of Stanley Miller in the 1950s,
prebiotic synthesis of amino acids has been shown to be relatively
easy. The real challenges now lie in selecting and concentrating
L-amino acids, and then linking those molecules into chainlike
proteins.
"Our
experiments demonstrate that crystal faces of calcite easily select
and concentrate the amino acids. Experiments now underway will
see if the calcite also promotes the formation of amino acid chains."
The Carnegie
Institution of Washington has been a pioneering force in basic
scientific research since 1902. It is a private, nonprofit organization
with five research departments in the U.S.: Terrestrial Magnetism,
Plant Biology, Observatories, Embryology, and the Geophysical
Laboratory.
Carnegie is
a member of and receives research funding for this study and other
efforts through the NASA Astrobiology Institute (NAI), a research
consortium involving academic, non-profit and NASA centers. The
NAI, whose central administrative office is located at NASA's
Ames Research Center in Mountain View, CA, is led by Dr. Baruch
Blumberg (Nobel '76). The institute also has international affiliate
and associate members. Astrobiology is the study of the origin,
evolution, distribution, and future of life in the universe.
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