This will help me get a better handle on this, to look at these again. The full texts of many journals going back to the '60s and '70s became available online through University-based access, and this makes things more convenient. I have photocopies of these somewhere, but I actually just got these online. Here's the reference for the first article:
http://pubs.acs.org/doi/abs/10.1021/ic00177a016
That's a great article, and the authors say that the complexes of the methylmercury ion (CH3-Hg+) with different sulfur- or selenium-containing ligands (a ligand is just something that binds to the CH3-Hg+ ion) last about 0.01 seconds. That's the kinetic side, the kinetic lability. But each complex (an example of a complex is CH3-Hg-SCH2CH2CH(+NH3)CO2-, which is "methylmercury cysteinate") that lasts about 0.01 seconds is thermodynamically stable, meaning that the binding to sulfur or selenium (sulfur and selenium have very similar chemical properties) is strongly "favored." When someone talks about "stability," this usually refers, in the absence of a more specific statement, to the thermodynamic side of chemistry and is only talking about equilibrium conditions (F.A. Cotton and G. Wilkinson. Advanced Inorganic Chemistry, 5th Ed.). (I'm not saying that book is something to buy, but I have an old copy. It's a great book, and that's where I got these references on methylmercury.) A quantitative measurement of thermodynamic stability is the formation constant for an organometallic complex like methylmercury cysteinate (cysteine is an amino acid that's in proteins and in glutathione):
CH3-Hg+ (CH3-Hg+) + -SCH2CH2CH(+NH3)CO2- (cysteinate) <---> CH3-Hg-SCH2CH2CH(+NH3)CO2- (CH3-Hg-cysteinate)
K1 = the formation constant = [CH3-Hg-cysteinate]/([CH3-Hg+][cysteinate]) = 10(14)-10(18) (10(14) = 10 to the 14th power--I'm combining data from the three sources, here)
Those really large numbers mean that, at equilibrium, almost all of the methylmercury, in a water solution (like a cup of water) at any one time would be the methylmercury cysteinate complex (starting with only cysteinate and methylmercury). The problem is that it never really gets to equilibrium in the body, and that's the reason the kinetic side dominates the chemistry in the body (the reason the complexes last 0.01 seconds and allow a methylmercury ion to migrate around the brain, etc., for a long time). This is partly because, in the body, there are many more sulfur atoms (and a few selenium atoms) than mercury atoms (as part of methylmercury).
Chemical kinetics is still hard for me to get my head around, but here's one example of kinetic stability in the face of thermodynamic instability (the opposite of the situation with methylmercury, even though other types of complexes, not involving methylmercury, can be both thermodynamically and kinetically stable or labile, etc.). This helps me understand it. If you pour oil into a glass of water, there will be many small droplets of oil that will coalesce into a large droplet and then into an "oil phase" (a layer) on top of the water (the equilibrium state). But when a food manufacturer makes an oil-in-water emulsion, the droplets of oil can be microscopic and suspended in the water. An emulsion is thermodynamically unstable, meaning that the oil and water phases are separating but are doing so very slowly and are not at equilibrium. In some emulsions, the separation of the oil and water might take a year. But the emulsion is kinetically stable because it might take that year for a lot of the droplets to "bind," or coalesce, with one another. It's the "opposite" with methylmercury. In any case, this is probably not of interest to most people, but this helps me. Here's the other article:
http://pubs.acs.org/doi/abs/10.1021/om00124a008
Here are some articles on the incident with the poisoning and death, from dimethylmercury, of a Dartmouth chemistry professor. This is a really disturbing story [dimethylmercury is much more toxic than methylmercury, but I think many of the mechanisms of toxicity are similar or the same (meaning that the difference is a matter of degree of toxicity, on a per-unit-mass basis)], but I think it's important to be aware of how toxic organic mercury compounds are. They're more toxic, on a per-unit-mass basis, than lead and just behave in an extremely insidious way. I think there's a tendency to associate poisoning with acute illness or vomiting or other overt symptoms, but that's not the way it works with organic mercury toxicity:
http://collaborations.denison.edu/naosmm/topics/dartmouth.html
http://content.nejm.org/cgi/content/full/338/23/1672
(pubmed: http://www.ncbi.nlm.nih.gov/pubmed/9614258?dopt=Abstract)
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