In last week’s column I described how Bill Irvine uses radio astronomy techniques to detect and identify organic compounds in interstellar space. Why is it so important for the origin of life on Earth that organic compounds are scattered throughout our galaxy?
The reason is that they represent a possible source for the organic compounds required for life to begin, delivered by a rain of comets and dust particles after the moon-forming event. The alternative source is synthesis by chemical reactions on the Earth’s surface. We don’t yet know which source was primary, but we do know one thing with certainty: ALL of the carbon atoms present in living organisms were delivered to the early Earth during accretion, which is a fancy name for the process by which gravity makes planets out of interstellar dust and planetesimals, including comets and asteroid-sized objects roaming around in the early solar system. Because accretion and the moon-forming event were such violent, high energy procesess, at first any organic carbon compounds were broken down into gases such as carbon dioxide.
But after our planet reached its final mass with oceans and atmosphere present, we are also certain that intact organic carbon compounds continued to be delivered. We know this because it is still happening today, although much less frequently than four billion years ago.
Infrared astronomy is also used to learn about the chemistry of interstellar space, and this is Lou Allamandola’s research specialty. Organic compounds diffusing as gases in the vacuum of space absorb light in the infrared region. We can’t see infrared energy, but sensitive instruments can measure the spectrum, which gives us information about the source. Allamandola and his colleagues at NASA Ames observed the spectrum of light from stars that passed through the edges of molecular clouds and discovered a strange infrared absorbance peak with a wavelength of 3.4 micrometers. In the laboratory, this peak could be produced by light passing through a gaseous mixture of polycyclic aromatic hydrocarbons (PAH), so a reasonable conclusion is that the peak is due to PAH molecules. We really do live in an organic universe.
Simulating interstellar cosmochemistry
How could all this stuff be synthesized in the cold vacuum of interstellar space, and how did it get into meteorites? That question is the focus of studies carried out at NASA Ames Research Center in Mountain View California. Louis (Lou) Allamandola was inspired by the pioneering work of Mayo Greenberg in the 1960s, who became convinced that interstellar dust was more than just an annoying fog that frustrated astronomers as they attempted to visualize faraway stars. Dust, in fact, could provide a place for water, ammonia, carbon dioxide, methanol and other gaseous compounds to come together in one place, rather than roaming endlessly as lonely molecules in interstellar space.
Furthermore, after the gases formed a thin film by condensing on the dust particles, the concentrated chemicals could be acted upon by photons of high energy ultraviolet light, and the activated molecules then had the potential to react and form more complex molecules.
Finally, when the dust gathered into a solar nebula during the early stages of solar system formation, they carried the carbon compounds with them, which would explain how complex organics become components of meteorites and comets.
But how to test this hypothesis? That’s where a simulation became essential. Lou Allamandola worked with Scott Sandford, Max Bernstein and other talented young scientists at NASA Ames to build a simulation of dust grains being acted upon by ultraviolet light. They could not actually simulate dust particles, which are microscopic in size, so Lou decided to use a smooth metal surface cooled to the temperature of interstellar space by liquid helium. The metal is in a vacuum chamber, and small amounts of water vapor, ammonia, and methanol are injected into the chamber where they condense on the ultracold metallic surface to form a thin film of mixed ice. A high intensity UV laser light was directed into the center of the metal surface with its frozen film. Then they waited for days and even weeks, condensing several million years of time in a molecular cloud into a fraction of a human lifetime. Finally the light is turned off, the vacuum is released and the metal surface slowly returns from the deep chill of outer space to that of Mountain View, California, increasing over 300 degrees C in just an hour or so.
At the start of the experiment there was nothing in the chamber except four simple gases. If no reactions occurred, the gases would evaporate along with the film of ice, and the metal surface would be just as clean as when it started. But something did happen. Instead of a shiny surface, the metal was coated with a thin yellowish film, just a few micrograms. In his earlier experiments Mayo Greenberg called the product “yellow stuff,” perhaps revealing the fact that he was an astronomer, not an organic chemist. Lou, Max and Scott analyzed their yellow stuff and found all sorts of interesting complex molecules, including the amino acid glycine in one of their more recent experiments.
In 1990, Lou showed me some of the product on the metal surface. I could see droplets that reminded me of the droplets of “yellow stuff” I extracted from the Murchison meteorite. I asked Lou to do a run, but to use an organic solvent, chloroform, instead of water to rinse the surface. A few days later, I had a small sample of material to work with.
Lou and his co-workers had already analyzed compounds in the film and established that quite a few water-soluble compounds had been synthesized. As a membrane biophysicist, my first question was whether there were any components that were NOT soluble in water, which is why I suggested using chloroform as a solvent. The easiest way to check this was to dry some of the chloroform extract on a glass slide and add a dilute solution of lightly alkaline buffer, just as I had with the Murchison extracts. When I looked at the material under the microscope the results were astonishing. Not only were there water-insoluble products, but they self-assembled into obvious vesicular compartments, just like the Murchison extracts! This meant that not only had the UV photochemistry produced complex products, but some of them had the properties of hydrocarbon derivatives that allowed them to form cell-like boundary membranes.
I showed the micrographs to Lou, but at the time there was no context that allowed us to attach any significance to the observations, so it was just a curiosity. But then, a few years later, astrobiology appeared on the scene as a program supported by NASA, and the link became obvious according to the following logic: Life requires membranes; meteorites contain compounds that can form membranes and deliver them to the Earth; the parent bodies of meteorites in the asteroid belt are composed of dust with organic components produced by UV photochemistry in the molecular cloud. Suddenly we had a narrative that made sense.
In 1998, Jason Dworkin completed his PhD at UC San Diego under the guidance of Stanley Miller and decided to come to NASA Ames to carry out his post-doctoral research. What a rare opportunity! We had Lou, Scott, and Max, all astronomers with an interest in interstellar organics, we had Jason, fully versed in prebiotic chemistry, and I could add my own background in membrane biophysics to the mix. Something good had to happen. Jason was the lead member of the team, producing and analyzing tiny amounts of organic compounds in the simulator that had been assembled by Lou and Scott, and I was able to observe the self-assembly process by microscopy. The result was a paper, Self-assembling amphiphilic molecules: Synthesis in simulated interstellar/precometary ices, in the Proceedings of the National Academy of Sciences with Jason as first author. The take home message is that carbon anywhere, even in interstellar space, can form compounds that are relevant to life on the Earth, some of them even capable of self-assembly into cellular compartments.
Several readers had very thoughtful comments related to last week’s column. Patrick asked whether “bootstrap” molecules like uracil and organic phosphate compounds can be synthesized in volcanic regions, and Gerhard proposed that icy conditions might promote interesting prebiotic reactions. Gerhard also noted that life uses only a miniscule fraction of all possible proteins, then suggested that some sort of evolutionary selection of successful molecules could explain why this is. Next time around I will address these comments.