Bacillus subtilis I50
The acute aquatic toxicity model predicts the concentration of chemicals required for 50% inhibiton (I50) of the Bacillus subtilis spore germination.
The training set consists of I50 values for 19 chemicals . All tested chemicals belong to the class of narcotic toxicants.
The organism response to the presence of toxicant in the environment is considered as a consequence of the combined influence of two different processes: uptake of the chemical into the biophase and interaction with the site of action . In the present model, the uptake is modeled by maximum potential of the toxicant to bioconcentrate in the fish, while the interaction of chemicals is explained by descriptors assessing the electrophilic character of the molecule . Such descriptors could include the energy of the lowest unoccupied molecular orbital, electronegativity, average or maximum superdelocalizability, maximum charge at non-hydrogen atom, etc. The following model was developed based on regression analysis of the data:
log 1/ I50 = 0.93(±0.10)+1.05(±0.07)logBCFMax
where BCFMax is the maximum bioconcentration factor .
The stepwise approach  was used to define the applicability domain of the model. It consists of the following sub-domain levels:
- General parametric requirements - includes ranges of variation of log KOW and MW,
- Structural domain - based on atom-centered fragments (ACFs).
A chemical is considered In Domain if its log KOW and MW are within the specified ranges and its ACFs are presented in the training chemicals. The information implemented in the applicability domain is extracted from the correctly predicted training chemicals used to build the model and in this respect the applicability domain determines practically the interpolation space of the model.
The precision of the regression model is characterized by the following estimates:
- 95% Confidence intervals of model parameters,
- Coefficient of determination R2 = 0.93,
- Mean squared error (estimate of error variance) s2 = 0.07,
- F value = 232,
- Number of chemicals, n=19.
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