Suck on one compound found in diesel exhaust, and you may get cancer. Suck on another, and nothing will happen. That may not seem surprising until you consider that the compounds are nearly identical. Both are made of four six-carbon rings, but they differ in the position of the nitrogen compound on the top ring: The harmless compound, 2-NBA, wears the nitrogen on the second carbon in the ring, while the toxic compound, 3-NBA, attaches it to the third. That slight shift makes all the difference when these sooty molecules bind to human enzymes, researchers report this month in Chemical Research in Toxicology. When 3-NBA attaches to an enzyme, its nitrogen (the fuzzy blue area in the right-hand image) is 3.5 angstroms—or about 3-and-a-half atom lengths—away from the closest hydrogen; that's close enough for the hydrogen to swing to 3-NBA, activating it to bind with DNA and start the cell on the slide to cancer. But in 2-NBA (left), the nitrogen is 4.2 angstroms away from the closest hydrogen--far enough to keep our enzymes safe.