Phosphorus is a key component of DNA, energy-carrying adenosine triphosphate, and other important molecules of life. Now chemists suggest ice floating in space might have brought phosphorus to Earth in a form compatible with terrestrial biochemistry (Nature Communications 2018, DOI: 10.1038/s41467-018-06415-7).
Scientists still aren’t sure how early life incorporated phosphorus into its biomolecules. Many phosphorus-containing molecules thought to have existed on Earth when life first popped up aren’t soluble in water or require a hefty energetic push to take part in biochemical reactions. Some scientists have proposed meteorites as a source of more biologically accessible phosphorus. Ralf Kaiser and colleagues now show how reactions in ices similar to those found on comets could produce oxygen-containing phosphorus acids that are credible precursors to biological molecules.
Kaiser’s group started with phosphine, which has been observed around stars, in the atmospheres of Jupiter and Saturn, and on the comet 67P/Churyumov-Gerasimenko. While phosphine is poisonous to many organisms and insoluble in water, the group’s results suggest it may have played a fundamental role in the beginnings of life on Earth.
To simulate interstellar ices, the scientists mixed phosphine with carbon dioxide, water, and oxygen at 5 K and under a high vacuum. Next they bombarded these ices with 5 keV electrons. Kaiser explains that high-energy electrons are thought to drive most chemistry in outer space. His group has done similar experiments to simulate other molecules and reactions thought to happen in space.
Using photoionization time-of-flight mass spectrometry, gas chromatography-mass spectrometry, and electronic structure calculations, the researchers identified phosphinic acid (H3PO2), phosphonic acid (H3PO3), phosphoric acid (H3PO4), and pyrophosphoric acid (H4P2O7) as products of the reactions. The group was particularly excited to find phosphoric acid because it contains phosphorus(V), the same oxidation state of phosphorus found in DNA and RNA.
Matthew A. Pasek, a geochemist at the University of South Florida who has explained how a meteorite mineral called Schreibersite could be a source of biological phosphorus, says Kaiser’s group did a good job of figuring out the reaction stoichiometry needed to produce these acids. This work could allow them to accurately estimate how much of the acids could form depending on how much oxygen, water, or carbon dioxide are available. Such an understanding of the conditions and ingredients needed to make these phosphorus compounds could help astronomers narrow down their likely sources in space. But Pasek cautions that all work simulating interstellar ices is limited by scientists’ estimates of their composition.
Kaiser says he hopes next to investigate whether phosphorus acids can react with organic molecules, like alcohols or glycerol, under these conditions. Reactions with glycerol could produce phospholipids, the major component of cell walls.