Relationship between the biological potency of polychlorinated dibenzo-p-dioxins and their electronic states

The molecular orbital (MO) method, as one approach explaining the structure-activity of polychlorinated dibenzo-p-dioxins (PCDDs), was used to understand the relationship between the biological activities and electronic state or highest occupied molecular orbital (homo) phase symmetry of PCDDs. The structure-activity relationship of PCDDs could be explained by the difference between the homo and lowest unoccupied molecular orbital (lumo) energy levels (delta epsilon: difference of epsilon homo and epsilon lumo) which indicates orbital mixing with other molecules. The magnitude of biological activity was consistent with the following order: 2,3,7,8-tetraCDD greater than 1,2,3,7,8-pentaCDD greater than 1,2,3,7,8,9-hexaCDD greater than 2,3,7-triCDD greater than 2,8-diCDD, with the order of decreasing delta epsilon by Hückel MO calculation. Our method can also estimate a magnitude of biological activity for PCDDs. The structure requirement for high toxicity suggested that the electronic states of PCDDs showed a small value difference (delta epsilon) and S1S2 phase symmetry on the homo for PCDDs. These results indicate that the electron states of PCDDs, which are based on the binding interaction of PCDDs to a cytosolic Ah receptor, play an important role in the appearance of their activity.