868 PCBs AND ASSOCIATED AROMATICS
with PCBs and related compounds. The PCB binds to a
receptor protein and is transported into the nucleus, occu-
pies a nuclear binding site and turns on gene expression
similar to the way steroid hormone-receptor complexes
turn on genes. The most toxic PCB congeners, namely iso-
mers #77, 81, 126 and 169 are dioxin-like in their coplanarconformation. The four coplanar PCBs are approximate
isostereoisomers of 2,3,7,8-tetrachloro-p-dioxin, as shown
in the following diagram. The most biologically active
compounds are substituted at the para- and at least one
meta- position of both phenyl rings and do not contain any
orthochloro substituents. These compounds, fortunately,173 2,2,3,3,4,5,6
174 2,2,3,3,4,5,6
175 2,2,3,3,4,5,6
176 2,2,3,3,4,6,6
177 2,2,3,3,4,5,6
178 2,2,3,3,5,5,6
179 2,2,3,3,5,6,6
180 2,2,3,4,4,5,5
181 2,2,3,4,4,5,6
182 2,2,3,4,4,5,6
183 2,2,3,4,4,5,6
184 2,2,3,4,4,6,6
185 2,2,3,4,5,5,6
186 2,2,3,4,5,6,6
187 2,2,3,4,5,5,6
188 2,2,3,4,5,6,6
189 2,3,3,4,4,5,5
190 2,3,3,4,4,5,6
191 2,3,3,4,4,5,6
192 2,3,3,4,5,5,6
193 2,3,3,4,5,5,6
Octachlorobiphenyls
194 2,2,3,3,4,4,5,5
195 2,2,3,3,4,4,5,6
196 2,2,3,3,4,4,5,6
197 2,2,3,3,4,4,6,6
198 2,2,3,3,4,5,5,6
199 2,2,3,3,4,5,6,6
200 2,2,3,3,4,5,6,6
201 2,2,3,3,4,5,5,6
202 2,2,3,3,5,5,6,6
203 2,2,3,4,4,5,5,6
204 2,2,3,4,4,5,6,6
205 2,3,3,4,4,5,5,6
Nonachlorobiphenyls
206 2,2,3,3,4,4,5,5,6
207 2,2,3,3,4,4,5,6,6
208 2,2,3,3,4,5,5,6,6
Decachlorobiphenyl
209 2,2,3,3,4,4,5,5,6,6134 2,2,3,3,5,6
135 2,2,3,3,5,6
136 2,2,3,3,6,6
137 2,2,3,4,4,5
138 2,2,3, 4,4,5
139 2,2,3, 4,4,6
140 2,2,3, 4,4,6
141 2,2,3,4,5,5
142 2,2,3,4,5,6
143 2,2,3,4,5,6
144 2,2,3,4,5,6
145 2,2,3,4,6,6
146 2,2,3,4,5,5
147 2,2,3,4,5,6
148 2,2,3,4,5,6
149 2,2,3,4,5,6
150 2,2,3,4,6,6
151 2,2,3,5,5,6
152 2,2,3,5,6,6
153 2,2,4,4,5,5
154 2,2,4,4,5,6
155 2,2,4,4,6,6
156 2,3,3,4,4,5
157 2,3,3,4,4,5
158 2,3,3,4,4,6
159 2,3,3,4,5,5
160 2,3,3,4,5,6
161 2,3,3,4,5,6
162 2,3,3,4,5,5
163 2,3,3,4,5,6
164 2,3,3,4,5,6
165 2,3,3,5,5,6
166 2,3,4,4,5,6
167 2,3,4,4,5,5
168 2,3,4,4,5,6
Heptachlorobiphenyls
169 3,3,4,4,5,5
170 2,2,3,3,4,4,5
171 2,2,3,3,4,4,6
172 2,2,3,3,4,5,5TABLE 11 (continued)
The International Union of Pure and Applied Chemistry numbering system for PCB congenersNo. Structure No. StructureC016_003_r03.indd 868C016_003_r03.indd 868 11/18/2005 1:12:27 PM11/18/2005 1:12:27 PM