How did life begin? Biochemical evolution on mineral surfaces
How did life begin on Earth? University of Chicago geophysicist Joseph V. Smith, in a Proceedings of the National Academy of Sciences paper published Tuesday, March 31, provides a theory for how small organic molecules may have been able to assemble on the surfaces of minerals into self-replicating biomolecules–the essential building blocks of life.
"The problem with most theories on the origin of life is that there is too much water around for the kind of organic chemistry that needed to take place," said Smith, Louis Block Professor in the Geophysical Sciences. "Synthesis of biomolecules from organic compounds dispersed in aqueous `soups’ require a mechanism for concentrating the organic species next to each other, and biochemically significant polymers–like polypeptides and ribonucleic acids–must be protected from photochemical destruction by solar radiation."
Smith postulates that this chemistry could have been facilitated by silica-rich minerals resembling zeolites, porous crystals with channels running through them. Most zeolites are hydrophilic–water-loving–and tend to absorb water from their surroundings. But certain synthetic zeolites are organophilic, preferentially absorbing organic materials out of water.
A naturally occurring organophilic zeolite–called mutinaite–was recently discovered in Antarctica, and Smith thinks that this mineral could provide the key to the chemical evolution that led to the origin of life. It’s possible that mutinaite, which has aluminum in place of silica, loses aluminum at its surface to become silica-rich through weathering, Smith said. A small amount of remaining aluminum would provide the catalytic centers for assembling organic molecules into polymers.
"For many years, I’ve wondered if such a material could occur in nature," said Smith. If small organic molecules, like amino acids, could accumulate in the pores of a zeolite, the mineral surface could have provided the catalytic framework for assembling them into polymers and protecting them from destruction by the sun.
A famous experiment performed at the University of Chicago in 1954 by then-graduate student Stanley Miller and his advisor, the Nobel laureate chemist Harold Urey, showed that amino acids, which make up the proteins found in all living organisms, could form from chemicals in the atmosphere combined with water and lightening.
No experiment has yet demonstrated how the amino acids assembled into protein and ribonucleic-acid (RNA) chains, but Smith is planning such experiments using a synthetic, silica-rich organophilic zeolite.
Amino acids occur naturally in right-handed and left-handed forms, but only the left-handed forms are found in the proteins of living organisms. Smith said, "It’s probably an accident that only the left-handed form is used, but it may have started in a zeolite with a left-handed channel." Zeolites with one-dimensional channels could have provided the template for assembly of only one version of the amino acids into the first primitive proteins.
Smith plans a trip to Australia, where some of the oldest and least-metamorphosed rocks and minerals are found, to look for more naturally occurring organophilic zeolites like the mutinaite found in Antarctica. He’s hoping these minerals still contain evidence of primary biocatalysis. Further research will include chemical experiments to see if the zeolites actually carry out the chemistry he proposes, and the use of computer models to study the structure of the channels.
Smith’s work on zeolites was funded by Union Carbide Corporation/UOP, the National Science Foundation, the American Chemical Society, Exxon Educational Foundation, Mobil Research Foundation and Chevron Corporation.
Source: University of Chicago News Office, March 31, 1998