THE PHOSPHORUS OF LIFE, BORN AMONG THE STARS

Phosphorus is scarce in the cosmos. In the Sun’s atmosphere there are about 3 phosphorus atoms for every 10 million hydrogen atoms — roughly a hundred times fewer than sulfur atoms. Yet life cannot do without it: phosphates are essential to DNA and RNA, the carriers of genetic information, to ATP and to phospholipids.

How phosphorus became available to the first chemistry of life is still an open question. One clue cited by the authors: grains unusually rich in phosphorus, unlike those of other meteorites, were found in the samples brought back from the asteroid Ryugu — a hint that space rocks may have delivered phosphorus to the young Earth.

A short list of molecules

Radio astronomers have identified more than 40 molecules containing sulfur in interstellar space. For molecules that bond phosphorus to carbon, the list stops at three: CP, CCP and HCP, all found in the envelopes of old, carbon-rich stars, plus one tentative detection. Chemists have proposed many more complex candidates, but how they form and are destroyed is poorly known.

In diffuse clouds of gas, phosphorus exists mainly as a positive ion, P⁺. One reaction looked especially promising: P⁺ meeting acetylene (C₂H₂), a common carbon molecule. Its only measurement dates from 1989–1990, at room temperature, and astrochemistry databases simply assume the same rate at every temperature. A 2012 calculation, meanwhile, predicted a rate ten times lower. Something did not add up.

Ions meet acetylene at SOLEIL

Matteo Michielan, Daniela Ascenzi (University of Trento, Italy), Alexandre Zanchet (CSIC, Madrid) and colleagues in France, Spain and Czechia went back to the lab. At the SOLEIL synchrotron near Paris, they produced phosphorus ions by breaking phosphorus trichloride molecules with ultraviolet light, choosing the energy carefully so that only ions in their lowest-energy state were made — a choice they checked with a separate test reaction.

They then guided the ions through a cell of acetylene at very low pressure, so that each ion collided at most once, and counted the products with mass spectrometers, at collision energies from 0.1 to 10 electronvolts. Quantum chemistry calculations and an improved model of how the two partners attract each other completed the picture.

One product dominates

At low energy:

• 99% of reactions produce the ion HCCP⁺ plus a hydrogen atom;
• 1% produce CCP⁺ plus a hydrogen molecule — ten times more than the 2012 calculation predicted.

The CCP⁺ channel should not even be open for ions in their usual magnetic “spin” state. Its presence shows that the colliding pair can flip its spin during the encounter, a process called intersystem crossing. The authors explain it by weakly bound complexes that linger long enough for the flip to happen — all the more at low energy.

A third product reported in 1989–1990, a combined molecule, never appeared. The team attributes it to the higher pressure of the older technique, and does not expect it in space.

Faster in the cold

Converting their measurements into reaction rates from 10 to 5,000 kelvins, they find 1.1 × 10⁻⁹ cm³ per second at room temperature — close to the old value — but a rate that rises as temperature falls, reaching 1.3 × 10⁻⁹ at 10 kelvins, instead of the constant value used until now. The reason: acetylene’s elongated electric charge distribution pulls the ion in more strongly than the standard model assumes. They provide the formulas ready to plug into astrochemistry databases.

Around an old carbon star

The team added the reaction to a chemical model of IRC +10216, an evolved, carbon-rich star, surrounded by an envelope where several phosphorus molecules have been detected. Phosphorus ions live mostly in the outer layers, acetylene in the inner ones, but they overlap at about 3 × 10¹⁶ centimetres from the star. There, the reaction becomes the main doorway to phosphorus-bearing ions and the main source of HCCP⁺. In interstellar clouds, HCCP⁺ capturing an electron could then yield CP and CCP — two of the three molecules already seen.

The model predicts HCCP⁺ and CCP⁺ in comparable amounts. Checking it will require telescopes to find them — and, first, laboratory measurements of their radio signatures, which, as far as the authors know, have never been made.