To illustrate in class the interpretation of various types of models of molecules, we presented Aflatoxin B2, one of the less toxic members of this extremely toxic family of compounds.

Some of the more toxic relatives of B2 have an additional double bond in the righthandmost five-membered ring, and an OH at the junction of those two rings. Since we have examined the phenomenon of chirality, we might note that B2 has two stereogenic centers, and hence is capable of existing in four stereoisomeric forms. The one shown is the naturally occurring one. (Can you find the stereogenic centers?)

The aflatoxins are fungal metabolites produced by several species of Aspergillus fungus, which grow on peanuts, cottonseeds, corn, and other crops. Their toxicity and carcinogenicity are believed to be related to inhibition of nucleic acid synthesis.

If you have eaten 40 tablespoons of peanut butter, you have experienced a one in a million risk of dying from aflatoxin poisoning or an induced cancer.

Here is a "ball and stick" picture of B2, oriented just about the same as the line structure above:

The color code used is fairly standard: oxygen is red, carbon is gray (sometimes black) and hydrogen is white (sometimes light gray), although since none of the three elements absorbs visible light, the choice is arbitrary.

The balls represent the positions of the nuclei, and give some indication of the relative size the isolated atom would have. The spherical shape of the balls really is meaningless, as are the connecting "sticks", which serve the same purpose as the bond lines in our line structure at the top of the page.

What we can see in images from atomic force microscopes suggests that the model closest to what molecules actually look like is this:

This model is called a space-filling or van der Waals model. The relative sizes of the atoms are derived from internuclear distances found in simple compounds of those elements. Placing the atoms in contact as in this picture also eliminates the misleading idea of two-electron bonds. After all, electrons in molecules reside in molecular orbitals delocalized over multiple atoms.

For those using a Chime-enabled browser, here follows a Chime window containing Aflatoxin B2.