Fig 1.
Chemical structures of the bisphenol compounds examined for estrogen receptor ERα and ERβ.
3,3’-Dimethyl bisphenol A (DM-BPA) is often and confusingly labeled “bisphenol C (BPC)”, and was not tested in this study. 17β-Estradiol (E2) was tested as a standard natural agonist compound.
Fig 2.
Competitive radio-ligand receptor-binding assays.
Dose-response receptor binding curves are shown for the assays for (A) ERα and (B) ERβ. 17β-estradiol (E2) was used as an internal standard compound in the assays for both ERα and ERβ, in which tritiated [3H]E2 was used as a radiolabeled receptor tracer. The Y axis is expressed by the normalized binding data from 100% (no competitor chemical) to 0% (nonspecific binding at maximal concentrations of competitor). Log[chemicals (M)] is the logarithm of the concentration of competitor chemicals plotted on the X-axis.
Table 1.
Receptor-binding affinities and selectivities of the bisphenol compounds.
Fig 3.
Luciferase reporter gene transcription activation assays.
Dose-response curves are shown for the assays for (A) ERα and (B) ERβ. E2 was used as an internal standard compound in the assays for ERα and ERβ. Log[chemicals (M)] is the logarithm of the concentration of test chemicals plotted on the X-axis.
Table 2.
Transcriptional activity of bisphenols for estrogen receptors ERα and ERβ.
Fig 4.
Luciferase reporter gene transcription inhibition assays for ERβ.
The dose-response curves are shown for (A) the residual activity of 10 nM E2 for ERβ in the presence of serial concentrations of BPC, and (B) E2 from the assays for ERβ in the presence of serial concentrations of BPC.
Table 3.
Specific activities of halogen-containing bisphenols for ERα and ERβ.
Fig 5.
The bar chart diagrams of specific activities among halogen-containing bisphenols.
The activities of the bisphenols in the receptor-binding assays and luciferase reporter gene assays for ERα and ERβ were estimated by referring to the activity of BPE-Br as the standard (= 100). BPE-Cl is exactly the same compound as HPTE (see Fig 1).
Fig 6.
Dehydrochlorination (–HCl) and hydrochlorination (+ HCl) between BPC and BPE-Cl (HPTE).
This chemical reaction formula is shown only for form’s sake. Synthesis of BPC by the dehydrochlorination of BPE-Cl (HPTE) was described by Cleveland et al. [39]. Ball and stick models are shown. The 3D-conformers of BPE-Cl [43] and BPC for this reaction formula were obtained from the chemical information resource PubChem [35].
Fig 7.
Schematic diagram of halogen bonding.
Halogen bonds exist between the halogen atom X (F, Cl, Br, or I) and the electron-rich δ– and/or electron-poor δ+ sites in the receptor protein. The electrons of atom X in the ligand molecule are maldistributed to the electron-rich δ– and electron-poor δ+ portions.
Fig 8.
Interactive chemical structure models of BPC.
Chemical constitution formulas are shown together with the electronic formula of chlorine and oxygen atoms (A), while the canonical forms or resonance structures are expressed for the vinylidene chloride moiety of BPC (B).
Fig 9.
Resonance structures feasible for BPC.
BPC is in a trans no-πbenzene ring-πC = C-nCl electron conjugation system. Resonance A shows the structures starting, at the top-left corner, from the n→π* transition of the chlorine atom 3p nonbonding orbital, whereas resonance B shows the structures starting from the n→π* transition of the oxygen atom 2p nonbonding orbital. R is another phenol group that is also involved in a similar trans no-πbenzene ring-πC = C-nCl electron conjugation. The resonance would take place also intersectingly in the cis arrangement.