Oxygen's dark side: How O2 flips personalities, linking life and energy

Oxygen does not always behave the same way. Most of the time, it’s the life-sustaining gas we breathe – stable, calm, and predictable. But under the right conditions, oxygen can flip personalities, turning into a volatile “singlet” version that chews through fats, proteins, or even battery components.

A new study has now uncovered what makes oxygen switch between its friendly and destructive sides.

By applying a simple rule from electron-transfer theory, scientists show exactly when the dangerous “singlet” form appears – and why it shows up in some systems but not others.

Singlet and triplet oxygen

Chemists talk about singlet oxygen and triplet oxygen to distinguish two spin states of the same O₂ molecule. Triplet oxygen is the calm version that fills your lungs, while singlet oxygen is reactive and short-tempered.

The precursor is often superoxide, an oxygen species with one extra electron. When two superoxides meet, a process called disproportionation can turn one into peroxide and the other into oxygen gas.

This work pinpoints how conditions decide which oxygen comes out. The team mapped when triplet oxygen dominates and when singlet oxygen takes over.

Stefan A. Freunberger and colleagues led the research at the Institute of Science and Technology Austria (ISTA). Their experiments spanned both water-based and non-aqueous systems relevant to biology and batteries.

Force behind singlet oxygen

The key knob is the reaction driving force, the energy difference that pushes electrons to move. As that driving force rises, the path to triplet oxygen first speeds up then slows, opening a window where singlet oxygen forms faster.

This counterintuitive slowdown at high push is a classic feature of Marcus theory, which predicts a normal region where reactions accelerate and an inverted region where they decelerate once the push gets too big.

Two separate kinetic curves describe the rates to triplet and singlet products, and their crossover marks the tipping point.

“While superoxide can give rise to either singlet or triplet oxygen, we still did not know what exactly causes the ‘bad’ singlet oxygen to evolve and how it can be tuned,” said Freunberger.

The upshot is practical. If you lower the driving force or raise the reorganization energy of the medium, you favor the safer triplet pathway.

pH decides oxygen’s fate

Inside cells, pH varies by compartment. Lysosomes are acidic, while mitochondria run slightly basic, according to a comprehensive review.

That matters because pH shifts the driving forces that control superoxide reactions. The new results imply that high pH in mitochondria keeps the push low, which suppresses singlet oxygen formation where energy production happens.

In contrast, acidic organelles lean the other way. The same chemistry could make singlet oxygen more likely in compartments that break down biomolecules.

This alignment between biochemistry and reaction physics helps explain why cells tune their interior conditions the way they do. Small shifts in pH can have large kinetic consequences.

When singlet oxygen attacks

Metal–oxygen batteries produce superoxide and peroxides during charge and discharge. In these devices, singlet oxygen has been identified as a major source of parasitic reactions that consume the electrolyte.

The new kinetics explain why some electrolyte and salt choices raise or lower the odds of singlet oxygen release. Conditions that push the oxygen driving force past the crossover invite trouble at the positive electrode.

Not everyone agrees about the scale of the damage in every cell design. Recent experimental work argues that singlet oxygen isn’t the dominant degradation route in some lithium–oxygen systems, highlighting the chemistry’s complexity.

These debates don’t undermine the core result here. Knowing how the spin state depends on the push gives engineers a real lever to test and adjust.

Seeing the invisible reaction

Singlet oxygen is tricky to see because it lives briefly and reacts quickly. One reliable sign is a faint near-infrared photon at 1270 nm that appears as singlet oxygen relaxes.

The team combined this optical signal with mass spectrometry and careful kinetic fits. By spanning low to very high driving forces with different mediators, they were able to separate the two rate curves that define the crossover.

That separation is key – it turns a long-running hunch into a quantitative rule. For the first time, scientists can predict when oxygen will flip into its reactive form rather than simply observing the damage afterward.

What’s coming for cells and power

For biology, the link between local pH and singlet oxygen sets up testable questions in organelles where superoxide appears. It also suggests why enzymes and buffers that tune pH can protect sensitive structures.

For energy storage, the path forward is clear. Chemists can lower the driving force of oxygen reactions, increase reorganization energy through solvent design, or avoid disproportionation routes that create superoxide under hot conditions.

The study is published in Nature.

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