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In a 2017 study, researchers reported that highly charged ions of the proteins cytochrome c and myoglobin were the most acidic organic species they had isolated and detected. That result applies to proton transfer in the gas phase inside a mass spectrometer—not to acids dissolved in water or a bottleable acid. [Zenaidee et al., 2017]
What does “strongest organic acid” mean here?
Acid strength depends on the environment and the measurement. Zenaidee and colleagues measured how readily highly protonated protein ions transferred protons to other species in gas-phase ion–molecule reactions. Their paper called the ions “The Strongest Organic Acids to Date,” and its discussion described highly charged cytochrome c and myoglobin ions as “the most acidic organic species to be isolated and detected.” That wording is bounded to the species isolated and detected in the study; it is not an unqualified record for every organic acid under every condition. [Primary study]
This is not an aqueous pKa ranking. A pKa describes acid dissociation in solution, while the study compared proton-donating behavior in the gas phase. The protein ions were not ordinary acids that could be handled as liquids.
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They were highly protonated, gas-phase ions of cytochrome c and myoglobin, two proteins. Researchers generated the ions by electrospray ionization and studied them in a modified ion trap. In the cytochrome c preparation described in the paper, the solution contained acetic acid and 1,2-butylene carbonate as a supercharging agent. These experimental ions are distinct from the proteins dissolved in that starting solution. [Primary study]
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How did the experiment show acidity?
The researchers combined tandem mass spectrometry, ion–molecule reactions and theoretical calculations, varying the ions’ charge states. They exposed highly protonated protein ions to gases and non-polar molecules and observed proton-transfer reactions. Reported reaction partners included argon, oxygen and nitrogen. In the described cytochrome c experiments, highly charged ions lost protons in argon; a helium control showed essentially no reaction under those conditions. [Primary study]
The result is notable because the ions could donate protons even in reactions with non-polar species, including inert gases. The claim concerns what happened in the instrument under the study’s gas-phase conditions; it does not mean argon, nitrogen or oxygen behave as bases in ordinary aqueous chemistry.
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What did the researchers measure and calculate?
| Finding | What it means | Evidence type |
|---|---|---|
| More than 130 kJ mol⁻¹ less basic | The most highly charged protein ions observed were reported to be over 130 kJ mol⁻¹ less basic than the least basic known neutral organic molecules, including tetrafluoromethane and methane. In this gas-phase comparison, lower basicity corresponds to greater proton-donating tendency. | Reported comparison from Zenaidee et al. (2017). [Primary study] |
| About one proton per four amino-acid residues | The highly charged protein ions formed in the study averaged this charge density. | Reported experimental average from Zenaidee et al. (2017). [Primary study] |
| One proton per three residues | At this charge density, the authors’ calculations predicted that protein ions should spontaneously lose a proton to vacuum. | Theoretical prediction, not the experimentally achieved average. [Primary study] |
Does this settle how electrospray creates highly charged protein ions?
No. The authors interpreted their results as more consistent with the chain-ejection model than the charge-residue model for forming highly charged protein ions under the conditions they studied. That is their interpretation of those experiments, not a universal resolution of how electrospray ionization works in every system. [Primary study]
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The primary paper by Muhammad A. Zenaidee, Michael G. Leeming, Fangtong Zhang, Toby T. Funston and William A. Donald appeared in Angewandte Chemie International Edition, volume 56, issue 29, pages 8522–8526. PubMed records electronic publication on May 4, 2017, and an issue date of July 10, 2017. The study supports the authors’ claim for highly charged protein ions isolated and detected in their gas-phase work. It does not, on its own, establish that no later study has surpassed that record. [Primary study]
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