Quantum Chemistry in the Time of Cholera

Slate magazine notes that occasional cases of cholera have been reported in the Gulf states, suggesting that the bacteria Vibrio cholerae is alive and well in that area. Recently, a group at Universtiy of Washington [O'Neal, Claire J., Jobling, Michael G., Holmes, Randall K., Hol, Wim G. J.
"Structural Basis for the Activation of Cholera Toxin by Human ARF6-GTP"
Science 2005 309: 1093-1096]has used x-ray crystallography to probe how the cholera toxin provokes such massive diarrhea when it takes up residence in the human gut. A figure from the paper shows how the overall 3-dimensional structure of the toxin changes when an activator protein binds to it. Protein chemists use the term "allosteric" to describe these mechanisms.

The picture is what interests me. I lectured on Monday about the probability density that you can compute using quantum mechanics (ψ*ψdτ). You can also measure the density experimentally using x-ray techniques, as Hol's group did for the cholera toxin. These sorts of pictures don't actually show the full density function, but a surface of constant electron density, usually around .002 e/bohr³, which seems small. A bohr is very small (0.529 x 10^-10 )meters, so this corresponds to about one mole of electrons (on the order of 10²³!) per cubic inch. This sounds like a lot, until you realize it's about the same as the number of electrons in a teaspoon of water!