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Response to Comment on “PCB Association with Model Phospholipid Bilayers” In their comment, Jonker and van der Heijden (1) agree with us that how different PCBs associate with phospholipid bilayers must be reasoned from the available experimental evidence since direct structural determination does not yet exist. Jonker and van der Heijden do not disagree with our experiments but claim to find inconsistency in the discussion and tentative model that we had proposed. Unfortunately, the objections of Jonker and van der Heijden (1) result mainly from factual inaccuracies. First, Jonker and van der Heijden disagree with our premise that the dipole moment of PCB-52 is close to zero and present their alternative calculation of 16 D. But their calculation of 16 D is inconsistent with the literature value of 0.03 D calculated using the Gasteiger-Hu ¨ ckel method (2); it is obvious that dipole moments calculated from different methods vary. Furthermore, Jonker and van der Heijden claim in supporting materials that PCB-77 can have zero dipole moment when it is in “trans” configuration with torsional angle of 0, but they omit to show that the probability of PCB-77 to have this configuration within the phospholipid bilayer is large enough to matter. A full calculation must include the full statistical mechanics of the relative probabilities of all the torsional isomers. Jonker and van der Heijden also err in claiming that our measurements imply an unrealistically high PCB surface coverage, 80-160% (1). Using the same calculation and assumptions as Jonker and van der Heijden, from our statement in the main text that the PCB concentration was 7.2 µM we deduce a surface coverage of 1-2%. We apologize for a mistake in one figure caption in which the incorrect number of 20% was mentioned. Third, Jonker and van der Heijden are mistaken in asserting that PCB-77 is “a factor of about three more hydrophobic” than PCB-52, as indicated by octanol-water partition coefficients (kow) (3). Their reasoning becomes unclear when, upon reading the original publication (3), one notices that only the partition coefficient for PCB-77 (log (kow) ) 6.21) was from experiment, while that for PCB-52
10.1021/es9011959 CCC: $40.75
Published on Web 05/18/2009
2009 American Chemical Society
(log (kow) ) 5.84) was calculated based on a formula assuming a planar structure for all PCBs. But Jonker and van der Heijden agree that PCB-52 is not a planar molecule; therefore the comparison with experimental results is inappropriate. Even more fundamentally, survey of the literature shows that the partition of nonpolar solutes into phospholipid bilayers is considerably more complex than can be explained by comparing with their partioning into bulk alkanes (4). It has also been reported (5) that the dipole interactions between solute and bilayer and the response of the bilayer to partitioning can influence interfacial stabilization and modify the overall thermodynamics of partitioning. For these reasons, we are not persuaded by the arguments proposed by Jonker and van der Heijden (1). A model will have to accommodate the differences in diffusion and phase transition observed between the two isomers considered. The text of our manuscript did discuss the possibility of alternative scenarios, in addition to the one that we deemed most likely based on the new evidence proposed in that paper.
Literature Cited (1) Jonker, M. T. O.; van der Heijden, S. A. Comment on ″PCB Association with Model Phospholipid Bilayers″. Environ. Sci. Technol. 2009, 43, doi: es803280h. (2) Pirard, C.; Focant, J.-F.; De Pauw, E. An improved clean-up strategy for simultaneous analysis of polychlorinated dibenzop-dioxins (PCDD), polychlorinated dibenzofurans (PCDF), and polychlorinated biphenyls (PCB) in fatty food samples. Anal. Bioanal. Chem. 2002, 372, 373. (3) Hawker, D. W.; Connell, D. W. Octanol-water partition coefficients of polychlorinated biphenyl cogeners. Environ. Sci. Technol. 1988, 22, 382. (4) De Young, L. R.; Dill, K. A. Partitioning of nonpolar solutes into bilayers and amorphous n-alkanes. J. Phys. Chem. 1990, 94, 801. (5) Wimley, W. C.; White, S. H. Membrane partitioning: Distinguishing bilayer effects from the hydrophobic effect. Biochemistry 1993, 32, 6307.
Andrew S. Campbell, Yan Yu, Steve Granick, and Andrew A. Gewirth Department of Chemistry and Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois ES9011959
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