SRC's Pckoc Program
The Pckoc program
calculates the soil or sediment adsorption coefficient (Koc, the ratio
of the chemical adsorbed per unit weight of organic carbon in the soil
or sediment to the concentration of the chemical in solution at
equilibrium). SMILES notation structural input is used to calculate the
Koc from a correlation to the molecular connectivity indices and
correction factors for certain chemical classes.
The newest version of
Pckoc runs under Windows (3.1, 95, 98, NT) making the estimation of soil
adsorption coefficients more convenient and accurate. Methods for
chemical structure input now support SMILES notations as well as
chemical structures produced in popular chemical drawing programs.
Enhanced features of the Windows version include batch-mode data entry
and user functions.
Example
Entry / Results (for Chlorfenvinphos)
The method was
developed with EPA's Office of Pollution Prevention and Toxics using a
training set of 189 chemicals and evaluated with a validation set of 131
chemicals. The correlation coefficient for the validation set was 0.92
[Environmental Science and Technology, Volume 26, pp. 1560-67 (1992)].
The following is the article abstract:
"The first-order
molecular connectivity index (MCI) has been successfully used to predict
soil sorption coefficients (Koc) for nonpolar organics, but extension of
the model to polar compounds has been problematic. To address this, we
developed a new estimation method based on MCI and series of
statistically derived fragment contribution factors for polar compounds.
After developing an extensive database of measured Koc values, we
divided the dataset into a training set of 189 chemicals and an
independent validation set of 205 chemicals. Two linear regressions were
then performed. First, measured log Koc values for nonpolar compounds in
the training set were correlated with MCI. The second regression was
developed by using the deviations between measured log Koc and the log
Koc estimated with the nonpolar equation and the number of certain
structural fragments in the polar compounds. The final equation for
predicting log Koc accounts for 96% and 86% of the variation in the
measured values for the training and validation sets, respectively.
Results also show that the model outperforms and covers a wider range of
chemical structures than do models based on octanol-water partition
coefficients (Kow) or water solubility." [Meylan, W., P.H. Howard and
R.S. Boethling, "Molecular Topology/Fragment Contribution Method for
Predicting Soil Sorption Coefficients", Environ. Sci. Technol. 26:
1560-7 (1992).] |
