Ortho-PCBs
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ortho-PCBs, congeners of PCBs that have one or more chlorines in ortho-position (positions 2 or 6; see chemical structures). Position of a group in aromatic ring as related to some other group can be ortho-, meta- (positions 3 or 5), or para- (position 4). In PCB congeners (see congener) this is counted from the carbon-carbon bridge between the two phenyl rings. ortho-Congeners mean congeners that have one (mono-ortho) or several (di-ortho, tri-ortho, tetra-ortho) chlorines in ortho positions. ortho-Positions affect the conformation of the molecule; non-ortho PCBs can assume a completely flat (planar) conformation, which is close to that of dioxins. Space-requiring ortho chlorines are a steric hindrance for the flat conformation, and therefore only non-ortho or to some extent mono-ortho PCBs can mimic PCDDs to have similar toxic effects based on binding to AH receptor (see this). Chlorines in para-positions seem to be preferably biodegraded. This means easier degradation of non-ortho congeners, and ortho-congeners are present in the environment at higher concentrations than non-ortho congeners.
