Predicting bioconcentration factors of highly hydrophobic chemicals.
Effects of molecular size*
S.D. Dimitrov 1, N.C. Dimitrova 1, J.D. Walker
2, G.D. Veith 3, and O.G. Mekenyan 1
1 Laboratory of Mathematical Chemistry, Bourgas University
As. Zlatarov,
8010 Bourgas, Bulgaria; 2 TSCA Interagency Testing Committee (ITC),
U.S. Environmental Protection Agency (7401M), 1200 Pennsylvania Ave.,
NW, Washington, DC 20460, USA; 3 U.S. Environmental Protection Agency,
Mid-Continent Ecology Division, 6201 Congdon Boulevard, Duluth, MN 55804,
USA
Abstract: The bioconcentration factor (BCF) is a parameter that
describes the ability of chemicals to concentrate in aquatic organisms.
Traditionally, it is modeled by the loglog quantitative structure
activity relationship (QSAR) between the BCF and the octanol
water partition coefficient (Kow). A significant scatter in the
parabolic log(BCF)/log(Kow) curve has been observed
for narcotics with log(Kow) greater than 5.5. This
study shows that the scatter in the log(BCF)/log(Kow)
relationship for highly hydrophobic chemicals can be explained by the
molecular size. The significance of the maximal cross-sectional diameter
on bioconcentration was compared with the traditionally accepted effective
diameter. A threshold value of about 1.5 nm for this parameter has been
found to discriminate chemicals with log(BCF) > 3.3 from those with
log(BCF) < 3.3. This critical value for the maximum diameter is comparable
with the architecture of the cell membrane. This threshold is half thickness
of leaflet constituting the lipid bilayer. The existence of a size threshold
governing bioconcentration is an indication of a possible switch in
the uptake mechanism from passive diffusion to facilitated diffusion
or active transport. The value of the transition point can be used as
an additional parameter to hydrophobicity for predicting BCF variation.
The effect of molecular size on bioconcentration has been studied by
accounting for conformational flexibility of molecules.
* Lectures presented at the 10th International Symposium
on Solubility Phenomena, Varna, Bulgaria, 22 -26 July 2002. Other
lectures are published in this issue, pp. 1785-1920.
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