LEFT & RIGHT
Geochemical origins of life's molecular handedness
Carnegie Institution of Washington & George Mason University
About the Lecture
Life arose on Earth as a geochemical process from the interaction of rocks, water, and gases. Prior to the origin of life, the necessary organic molecules had formed abundantly, but indiscriminately, both in space and on Earth. A major mystery of life's origin is how an idiosyncratic subset of those diverse molecules was selected and concentrated from the prebiotic soup to form more complex structures leading to the development of life. Rocks and minerals are likely to have played several critical roles in this selection, especially as templates for the adsorption and organization of these molecules. The author's recent experimental and theoretical studies on interactions between crystals and organic molecules reveal that crystals with chiral surface structures may have facilitated the separation of left- and right-handed biomolecules - the possible origin of life's distinctive homochirality.
About the Speaker
ROBERT M. HAZEN, research scientist at the Carnegie Institution of Washington's Geophysical Laboratory and Clarence Robinson Professor of Earth Science at George Mason University, received the B.S. and S.M. in geology at the Massachusetts Institute of Technology (1971), and the Ph.D. at Harvard University in earth science (1975). Hazen is author of more than 250 articles and 18 books on science, history, and music. He has received a number of awards for writing and research, including the American Chemical Society's Ipatieff Prize and the Mineralogical Society of America Award. He is currently President of the Mineralogical Society of America. Hazen's recent research focuses on the role of minerals in the origin of life. Robert Hazen is also a professional trumpeter who has performed with numerous ensembles including the Metropolitan Opera, the Royal Ballet, the Boston Symphony, and the National Symphony Orchestra.
President Robert Hershey called the 2,188th meeting to order at 8:15 pm February 25, 2005. The minutes of the 2,187th meeting were read and approved.
Mr. Hershey introduced the main speaker of the evening, Mr. Robert Hazen of the Carnegie Institution of Washington and George Mason University. Mr. Hazen spoke about “Left and Right: Geochemical Origins of Life's Molecular Handedness.”
One of the great mysteries, Mr. Hazen said, is how life became handed, how the molecules in our bodies and other life forms, selected from the vast supply of right-handed and left handed molecules, became almost exclusively left-handed.
This presumably began with the combination of mineral crystals and organic molecules. These combinations are critical to the formation of teeth and bones, biomineralization and biofilms, fossilization, weathering and soil formation, and the hardening of paints, glues, and dyes. They appear to have considerable potential for environmental monitoring and clean-up, nanotechnology, and drug synthesis and purification. The process also appears to be a key to discovering the origin of life, the subject of Mr. Hazen's study.
A particularly interesting example of this interaction is bone, which is a composite material. Calcium, phosphate and all sorts of protein such as collagen and osteocalcin bind together to give bone its strength and resilience. The combination with the minerals in a crystal structure preserves the protein for centuries, making possible the study of ancient human remains.
It is an idiosyncrasy of life that it uses, of all the millions of molecules available, only a few hundred. How does that selection occur? Life has very unusual synthetic patterns. Industrial processes of building carbon chains are simple processes of adding carbon molecules. Life does not do that. When life makes a molecule, it is by a complex process whereby three carbon atoms are added and one taken away. Because of that, life molecules almost always have an even number of carbon atoms.
Left-handed refers to the relative positions of the atoms in a molecule. Like a bowling ball that has holes that fit a right or a left hand but not both, molecules have a certain orientation of atoms to each other. If a structure is chiral, once it is begun with one molecule, other molecules fit into the structure only if they have the right “handedness.”
Handedness sometimes has interesting and important biological effects. R-limonene smells like lemons, L-limonene like oranges. In thalidomide, R-enantiomer is an analgesic; L-enantiomer produces major birth defects.
Mr. Hagen demonstrated that chiral faces are common in nature and that different crystal forms display very different surface structures. His research made extensive use of calcite crystals, or Iceland spar, the same material that led to the discovery of polarization of light, as Mr. Orozco told us two weeks earlier. It is readily available in large crystals, it adsorbs amino acids, and it has the most chiral surface of any crystal known.
To do his studies, he used a machine called a ChipWriter, which can work on up to 126 minerals and put up to 49,152 spots, or pits, on each. These are 100-micron spots, which are apparently large enough for the relevant observations and small enough to allow a great number of observations relative to surface area. The next step involves his Chip Reader, which simultaneously analyzes 100,000 spots.
His work indicated that calcite crystal surfaces effectively select D-aspartic and L-aspartic acid. Experiments on alanine, by contrast, show no selective adsorption. He speculated that the alignment of selected amino acids on calcite may lead to homochiral polymerization – an essential step in producing biologically functional peptides.
To learn how the molecules actually bond to a surface, he used a computer model that constructed progressive pictures and presented them like a movie. He called this a time-of-flight simulator (ToF-SIM). This gave a very clear visual impression of the process of a molecule approaching and docking on a surface.
From his studies, he concluded that:
Many mineral surfaces have the potential for chiral selection of plausible prebiotic molecules,
Microarray technology coupled with ToF-SIM provides a powerful experimental means for combinatoric studies of mineral-molecule interactions, and
Density functional theory combined with modeling of molecules and geometric matching of molecule-crystal interactions provides a general theoretical approach.
Finally, he expressed his appreciation to the NASA Astrobiology Institute, the National Science Foundation, and the Carnegie Institution of Washington for supporting his research.
Mr. Hagen agreed to answer questions and there were many. The discussion brought out the following points:
There's nothing essential about the terms right-handed and left-handed as he used them. They are arbitrary ways of conveniently distinguishing the two types of molecules.
There are some, though very few, right-handed molecules involved in life. There are no rules without exceptions.
He imagines the origin of life going from the simple to the complex, with simple molecules forming, then metabolism getting going on a surface, then layers of complexity building on that. His main point is that minerals have the capacity to selectively pick molecules out of the ocean.
He was intrigued with the question of whether the hemisphere, southern or northern, might have influenced the left-handedness of life, but he doubts it. He thinks the field was too weak to affect it.
He guesses there once were both kinds of life and the left hand side won out, like VHS over Beta. There may have been some life forms that used combinations of the two.
He speculated that some early forms of life may have been more heat-tolerant, perhaps up to 100°C.
He expressed his appreciation of Tommy Gold and his ideas about a whole different regime of life at the dark, little-known depths of earth only a few hundred feet down. It's made us think, he said.
About a healing effect of crystals, he did not want to discourage believers, but he, personally, thinks there is probably nothing physical to it. There well may be some effect of the beliefs of the users on their well-being.
Mr. Hershey announced the next meeting and made the parking announcement. He adjourned the 2,188th meeting at 9:35 to the social hour.
Ronald O. Hietala,