Tuesday, November 12, 2013

Counterion Conundrum

(For Popkin's great Science News writeup, click here)
(Update - Also, Chemistry World!)

Update (11/13/13) - Blog buddy Lila Guterman of Science News has the inside scoop:
"An answer! Authors via : cerium(III)chloride heptahydrate (99.9% pure) from Sigma (cat. nr. 228931-25G)"


Fascinating news for the inorganic biochemistry fans out there: Scientists have ID'd a bacterium (Methylacidiphilum fumariolicum) living in highly acidic volcanic pools that seems to use rare earth metals in one of its enzymes. A multinational team modeled the enzyme with a variety of rare earth cores, and the bacterium appeared to selectively take them up in cell culture. Cool!

Just one small problem: What's the counterion?!?

My crack online reporting team has scoured the manuscript, finding only mentions of a mysterious Ce(III), along with triply-oxidized La* and Pr. Nowhere in the Supporting Information do they mention reagents used, and the reporter has confirmed that this subject wasn't broached.

Given the other salts the researchers added to the media, it's likely that they used either cerium (III) sulfate or cerium (III) chloride. However, Sigma-Aldrich sells no less than 10 different cerium (III) salts (Strem has over 20!), and I'm willing to bet they have markedly different bioavailability, oxidation potential, aqueous solubility...the works.

Readers, does anyone know what the cerium source is in this paper?** I certainly don't wish to draw unwarranted conclusions, but we're all still touchy over another recent dust-up having to do with a miraculous trace element.

Please let me know in the comments.

*IUPAC police: Throughout the paper, the authors refer to "Ln ions." Do you suppose they mean La (lanthanum)? Are elements abbreviated differently in other places?

**Interestingly, the authors note that their acidic growth media leached trace rare earths out of the glassware. I've never looked at that as a reaction contaminant, but I guess I'll have to start!


  1. Ln ions might mean lanthanides. I've seen that abbreviation as a catchall for the entire top row of the f-block, much like X is a generic halogen.

    ~Science Isn't Scary Steve

    1. Sure. But, they actually say "lanthanum, Ln" in the abstract. : (

    2. Oh. Maybe microbiologists happened?

      ~SIS Steve

  2. Seems they use "Ln" where they should say "La" twice on page 1 (including abstract), and twice on page 4, second column. Elsewhere they use "Ln" as I would understand it, as a shorthand for any lanthanide (or for the IUPAC police, lanthanoid). I can't see any mention of the counterion of the Ln salts in the paper.

  3. As my name suggests, I'm an inorganic chemist by trade. The issue that you point out is fairly commonplace in the bioinorganic literature. Some researchers assume that one metal salt is as good as another, even though this couldn't be farther from the truth. Zinc pyrithione, for instance, can traverse cellular membranes; zinc chloride, on the other hand, can't readily enter cells. The counterion also impacts solubility, as you point out, and measured binding affinities (since the ligands will compete for metal coordination sites). I've reviewed quite a few papers where authors neglect to identify the salts that they used. I always ding them for it, but unfortunately, the above case shows that not everyone realizes the importance of this particular detail.

    One small nit to pick: in your post you worry about the effects of the ligand on the cerium's redox properties. I would expect the cerium to be effectively redox-inactive no matter what ligands were coordinated. The Ce(IV/III) reduction potential is pretty high and positive.