NO(subscript x)ious PAH-lution: precarious prediction.

[NO.sub.x.ious] PAH-lution: Precarious prediction

Chemists keep looking for generic rules tying a compound's features to its toxicity. Rules that apply to all known members of a chemical class offer a good chance of accurately predicting the toxicity of new members as they are discovered. Tocicologists thought they had one such rule for the "nitro-PAHs" found in urban air, says William A. Pryor, an environmental chemist at Louisiana State University in Baton Rouge. These air pollutants form whtn nitrogen oxides [(NO.sub.X.)] marry into another family of ubiquitous air pollutants, the polycyclic aromatic hydrocarbons (PAHs). But in a chance focus on one particular offspring of such a union, Pryor's team discovered the fallibility of that rule. They also found what they say appears to be a new route for nitro-PAH formulation: free-radical reactions.

Several [NO.sub.x] species, such as nitrogen dioxide, carry an odd number of electrons. Because of their unpaired, or odd, electron, such highly reactive species are called free radicals. Nitro-PAHs formed when nitrogen dioxide merges with PAHs through free-radical reactions differ freatly from those created in more conventional ways, the researchers found, and include some of the most mutagenic pollutatns in urban soot. "So we're on the track of something that is environmentally important," Pryor says.

The work also provides new clues to the toxicological significance of shape. Pryor's group studied nitro-PAHs based on three linked phenyl rings from which a nitrogen dioxide group dangles. Others had found that the mutagenicity of a nitro-PAH appeared linked to its reduction potential, or the ease with which it accepts an electron, Pryor explains. So the Baton Rouge researchers compared twin nitro-PAHs featuring identical chemical compositions and reduction potentials: 3-nitrofluoranthene (3NF) and 1-phenyl-4-nitronaphthalene (1P4NN). The only difference between the planar 3NF (left) and 1P4NN is a severing of one bond anchoring 1P4NN's top and bottom rings (right). Thus freed, the top ring twists out of the plane.

Though the molecules' low reduction potential suggested both should be strongly mutagenic in bacteria (a gauge of potential carcinogenicity), only 3NF was. In fact, the chemists report in the May/June CHEMICAL RESEARCH IN TOXICOLOGY, 3NF proved the most mutagenic compound they studied -- and its twisted twin the most innocuous.