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.
The Carnegie Institution of Washington
[Contact: Robert Hazen, Tina McDowell]