Structural requirements for anaerobic biodegradation of organic chemicals: A fragment model analysis

A computer-automated structure evaluation program (MCASE) has been used to analyze rates of aquatic anaerobic biodegradation of a set of diverse organic compounds, and a predictive model for this endpoint has been developed. The model was evaluated in terms of possible anaerobic metabolic steps. The most important fragments linked to biodegradability (biophores) were aromatic and aliphatic thiol, methoxy, and alcohol groups and the carboxylic ester group. Fragments that can possibly inhibit anaerobic biodegradation were also identified but were not significant. A metabolic transformation step is proposed for all identified fragments. Our results are therefore such that the most likely transformation under anaerobic conditions is anticipated when biophores are present in the molecule, while nonbiodegradability is assumed when none of these fragments are present. The number of correct classifications by the model is over 93%. When predictions for all compounds were generated using cross-validation of the model, 84% of the overall predictions were correct. However, when only the predictions of possible biodegradability under anaerobic conditions are considered, the number of correct predictions is over 89%, whereas the percentage of correct predictions for nonbiodegradability is only 64%. It is therefore concluded that the analysis given in this contribution yields a model that can predict anaerobic biodegradability, but the assumption that the absence of a biophore leads to nonbiodegradability is not valid. Predictions of nonbiodegradability of compounds are therefore unreliable, probably due to the limited amount of data on anaerobic biodegradation available for use in this study.