There are 75 possible congeners of polychlorinated dibenzo-p-dioxins, but only 7 of them have affinity for the AH receptor and are toxic via this mechanism. The crucial structures are so called lateral chlorines in positions 2,3,7, and 8. These 4 chlorines also make the congeners persistent, because they prevent microbial degradation. Additional chlorines make the<unk>  less potent, but basically the effects remain the same although at higher doses. There are 135 possible dibenzofurans, and 10 in which the lateral chlorines are<unk><unk> . Critical structures of PCDD/Fs. There are 209 PCB<unk> . Analogously to PCDDs at least two lateral chlorines in each ring in positions 3,4, and/or 5 are needed for<unk><unk>  activity. Because the AH receptor requires a planar (flat) structure, only PCB congeners that can rotate freely along the C—C axis between the rings can attach the receptor. Substituents in ortho-positions 2 and 6 prevent rotation and thus hinder the molecule from assuming a planar position. Mono-ortho congeners (one Cl in 2, 2', 6, or 6') have minimal activity. No significant<unk><unk>  activities have been noticed, if there are two or more o-chlorines. Brominated<unk>  and biphenyls have similar properties, but they have been studied much less. Structures of biphenyl and 3,3’,4,4’,5-pentachlorobiphenyl Many natural<unk>  have very high affinity to AH receptors. These include indoles, flavones, benzoflavones, imidazoles and pyridines. These<unk>  are metabolized rapidly, but continuous intake from food may cause similar receptor activation as the background levels of<unk> . They do not reach concentrations causing typical<unk><unk>  toxicity, however.
Dioxins and dioxin-like compounds
Polychlorinated naphthalene
Polychlorinated dibenzodioxins
Organobromine compound
Organochlorine compound
Polycyclic aromatic hydrocarbon
Loline alkaloid
Polychlorinated dibenzofurans