Preparing the Primordial Soup
The notion that the raw materials for life—perhaps even life itself—simply fell from the sky has been batted about for decades. In this view, organic matter—known to exist in interstellar clouds of gas and dust—was brought to Earth by asteroids, comets, stardust, or other cosmic bodies crashing into our young planet. The extraterrestrial influx of ready-made molecules could have seeded the barren world with the organic ingredients needed to get biology rolling.
Questionable? Perhaps. But one experiment answers a thousand questions, and now, Los Alamos chemists James Boncella and Jonathan Cape have shown that aqueous mixtures of organic molecules detected in outer space can arrange themselves into simple cell-like structures able to capture and store energy from the Sun—important steps in the prebiotic (before life) chemical pathway that led to the more complicated chemical systems associated with life on Earth.
A Marvelous Event
Extraterrestrial seeding gained prominence in 1969 with the fall of a meteorite (witnesses reported an exploding fireball) near the Australian village of Murchison. More than 100 kilograms of charcoal-colored fragments were collected, and early analyses revealed the presence of hydrocarbons and a number of common amino acids. Scientists have since concluded that the interiors of well-preserved fragments were unaltered by terrestrial contaminants, and many believe that the Murchison meteorite is a pristine relic from the early solar system.
Murchison (and other carbon-containing meteorites) harbored a cornucopia of complex organic molecules, including amino acids, an abundance of different fatty acids, and aromatic hydrocarbons (hydrocarbons containing at least one six-carbon-atom ring). Just last year, a team of European researchers used ultra-high-resolution mass spectrometry to uncover more than 14,000 unique molecular compositions in Murchison's organic extracts.
The wealth of the interstellar organics encouraged Boncella and Cape to explore the prebiotic possibilities. They joined with former Laboratory colleague Pierre-Alain Monnard, now at the University of Southern Denmark, to create and evaluate prebiotic structures made from the same organic molecules found within Murchison.
They began by fashioning primitive cell-like structures from mixtures of short-chain fatty acids, the most plentiful water-soluble organic compounds in the meteorite. The structures will form spontaneously in aqueous solutions because of the push-me-pull-you nature of the fatty acid—the "head" of the lollipop-shaped molecule mixes happily with water, the hydrocarbon tail doesn't. To shield their tails from water, a group of fatty acids will arrange themselves into double-walled, hollow vesicles, the heads forming the inner and outer wall surfaces, with the tails sandwiched between the two surfaces, sheltered from water.
The chemists found that, of the fatty acids found within Murchison, decanoic acid, with a ten carbon-atom tail, was the best vesicle former—its longer tail providing a greater hydrophobic driving force. They also found that vesicles formed more readily from messy mixtures of short-chain fatty acids than from single components, an intriguing discovery given the presumed complexity of the primordial soup.
Interestingly, the researchers showed that the tiny mixed-component vesicles—only about a hundred nanometers in diameter—are able to encapsulate large negatively charged compounds, such as the electron-grabbing ferricyanide, a common laboratory oxidant. Held in solution inside the interior vesicle volume, the compounds are effectively segregated from the external environment. This act of containment, or compartmentalization, is a key characteristic of living cells and is thought to have arisen before life itself.
Los Alamos chemists have shown that organic molecules found in carbon-bearing meteorites can form primitive cell-like structures capable of harvesting energy from the Sun: Short-chain fatty acids (brown and orange) self-assemble into double-walled vesicles. Light-sensitive PAH molecules (blue) become excited by ultraviolet light to a higher energy state, and one of its electrons hops to a dissolved ferricyanide anion encapsulated within, thus storing the energy as chemical potential. The PAH molecule then accepts an electron from an external reducing agent, completing the charge-transfer cycle.
Boncella and Cape were also able to create fatty-acid vesicles exhibiting a rudimentary form of metabolism—the ability to capture and store energy for chemical transformations. The key insight was to include light-sensitive polycyclic aromatic hydrocarbons (PAHs) in the model system. Shaped like chicken-wire cutouts, PAHs are flat molecules made of fused carbon rings saturated with hydrogen atoms. PAHs are abundant in the Murchison meteorite and throughout the interstellar medium. They are also completely hydrophobic, so when added to premade vesicle solutions, PAHs migrate to the interior of vesicle walls, mingling with their fellow hydrophobes, the fatty-acid tails.
The membrane-bound PAHs serve in essence, as solar spark plugs, initiating the conversion of the Sun's energy into stored chemical potential. The process begins with ultraviolet light exciting an electron in a PAH to a higher energy state—just the boost the electron needs to move across the inner vesicle wall and transfer to an encapsulated ferricyanide ion. The PAH becomes a positively charged radical, but returns to its original state after it accepts an electron from a reducing agent in solution outside the vesicle. The process is therefore repeatable. So energy (from the Sun) was transferred (by an electron) across the vesicle membrane and used to reduce an ion (ferricyanide)—a very simple metabolic-like sequence of events.
While their prebiotic chemical model shares fundamental attributes with living things, Boncella and Cape point out that they cannot know for sure whether they are on the right track. No physical evidence of prebiotic structures exists, so scientists may never know for certain what they looked like or how they functioned. But the lack of a fossil record also elevates the importance of laboratory experiments. "We can only speculate," says Boncella, "and then head into the lab to see what is plausible."
—Craig Carmer and Jay Schecker