Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on Screening for PBT Chemicals among the “Existing” and “New” Chemicals of the EU n his comment on our paper on “Screening for PBT Chemicals among the “Existing” and “New” Chemicals of the EU”, Rayne1 raises two points: (1) several structures shown in our SI show incorrect aromatic rings; (2) we list several anhydrides, isocyanates and acyl fluorides in our list of potential PBT substances although anhydrides, isocyanates and acyl fluorides in many cases may undergo rapid hydrolysis. Regarding the first point, Rayne’s statement “some of the compounds predicted to be potential PBT chemicals in the Supporting Information of ref 1 are incorrectly drawn and include aromatic rings where none exist” is not correct. In organic chemistry, it is well-established knowledge that all of these rings are aromatic. Examples are cyanuric acid (both the enol and keto form),2 cytosine,3 coumarin (both rings),4 and pyrone and pyridone5 (see Figure 1). In all these cases, the N
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mentioned by Rayne1 are classified as potentially persistent in our screening exercise. A case in point is tetrabromophthalic anhydride. A review of tetrabromophthalic anhydride prepared for the U.S. National Institute of Health8 says: “Tetrabromophthalic anhydride was found to be rapidly hydrolyzed to tetrabromophthalic acid but not further degraded in moist soil (1% was identified in 0 and 28 day samples). When tested at 1 and 10 μg/g, little volatilization occurred and a large proportion (24% and 32%, respectively) became soilbound after 28 days. Tetrabromophthalic anhydride is, therefore, expected to be persistent in soils.″ In conclusion, Rayne’s comments incorrectly maintained that there were problems with specific chemicals in our data set. More generally, we would like to emphasize that, as our title suggests, our paper presents a methodology for and results of screening for potential PBT chemicals. Screening should always be used as a way to identify chemicals that may possess a certain set of predefined properties (here P, B, and T characteristics). The chemicals thus identified should then be subject to rigorous evaluation. We would caution readers against using the results of screening studies as a definitive statement about the properties of a specific chemical.
Martin Scheringer* Sebastian Strempel Carla A. Ng Konrad Hungerbühler Figure 1. Structures of cyanuric acid (keto and enol form), cytosine, coumarin, 4-pyridone and 4-pyrone. All rings of these structures are aromatic.
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AUTHOR INFORMATION
Corresponding Author
and O atoms are sp2 hybridized and possess lone electron pairs in p orbitals that are available for delocalization. The exocyclic double bonds do not break aromaticity, as is pointed out in the description of the SMILES algorithm provided by Daylight, the developers of the SMILES code.6 The extent of the delocalization of the π electrons varies among heterocyclic aromatic compounds, but the SMILES code is not intended to reflect this point or any other aspects of the electron density distribution, chemical reactivity, or spectral properties of the substances. In the context of aromatic rings, the only purpose of the SMILES code is to identify atoms that form a planar ring with the possibility of π electrons to be delocalized. We would like to point out that the SMILES codes listed in the SI of our article are correct. Regarding the second point, our intention was to base our score for the P dimension on information about the ultimate biodegradation of chemicals, as stated on p. 5681 of our article.7 Therefore, rapid hydrolysis of anhydrides, isocyanates, or acyl fluorides does not imply that a chemical receives a low P score. As long as there are any recalcitrant moieties in the molecule, ultimate biodegradation is slow and this is why the substances © 2013 American Chemical Society
Institute for Chemical and Bioengineering, ETH Zurich, 8093 Zurich, Switzerland
*Phone: +41-44-632 30 62; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Rayne, S. Comment on Screening for PBT chemicals among the existing and new chemicals of the EU. 2013, DOI: 10.1021/ es401204q. (2) Pérez-Manriquez, L.; Cabrera, A.; Sansores, L. E.; Salcedo, R. Aromaticity in cyanuric acid. J. Mol. Model. 2011, 17, 1311−1315. (3) Cysewski, P. An ab-initio study on nucleic bases aromaticity. J. Mol. Struct.: THEOCHEM 2005, 714, 29−34. (4) Ilic, P.; Mohar, B.; Knop, J. V.; Juric, A.; Trinajstic, N. The topology and the aromaticity of coumarins. J. Heterocyclic Chem. 1982, 19, 625−631. (5) Gupta, R. R.; Kumar, M.; Gupta, V. Aromatic heterocycles. In Heterocyclic Chemistry; Gupta, R. R., Kumar, M., Gupta, V., Eds.; Chapter 3; Springer-Verlag: Berlin Heidelberg, Germany, 1998.
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(6) Daylight Chemical Information Systems, Inc., 120 Vantis, Suite 550, Aliso Viejo, CA, USA, 2008. http://www.daylight.com/dayhtml/ doc/theory/theory.smiles.html#RTFToC28 (accessed 22 April, 2013) (7) Strempel, S.; Scheringer, M.; Ng, C. A.; Hungerbühler, K. Screening for PBT chemicals among the “existing” and “new” chemicals of the EU. Environ. Sci. Technol. 2012, 46, 5680−5687. (8) Tice, R. Tetrabromophthalic Anhydride [CASRN 632−79−1], Review of Toxicological Literature. Integrated Laboratory Systems: Research Triangle Park, NC, USA, 1999. http://ntp.niehs.nih.gov/ ntp/htdocs/Chem_Background/ExSumPdf/Tetrabromophthalic.pdf (accessed April 22, 2013).
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