Influence of the energy relationships of organic compounds on toxicity to the cladoceran Daphnia magna and the fish Pimephales promelas.

A concern of ecotoxicology is to predict the toxicity of substances to living systems. Existing models of structure-activity relationships (SARs) are statistical and applicable within classes of substances only. Predictive models derived from first principles are wanting. Transformity, a measure of the relative amount of energy required to generate a component or a flow in a transformation process, may help predict toxicity. This notion derives from two concepts. First, common substances are more likely to be processed by the biosphere than are rare substances. Second, transformity expresses energy relationships between parts of a system. Substances that require more energy to form are also the more unusual and the more difficult to process. A correlation was hypothesized to occur between the rarity and complexity of a substance, and thus its transformity, and its toxicity. To search for general patterns that transcend individual studies, this hypothesis was tested by using data available for 79 compounds, including simple and chlorinated alkanes, alkenes, alcohols, benzenes, phenols, biphenyls, organic acids, and acetaldehyde. Published data on their Gibbs energy of formation were used as an estimate of transformity. These were compared to published data on their acute toxicity to the cladoceran Daphnia magna and the fish Pimephales promelas (measured as the 48- and 96-hr LC50 values, respectively). For both species there were significant positive correlations across compound classes between transformity and toxicity. In contrast, some correlations within classes were negative, with chemical reactivity and physicochemical properties presumably exerting the prevailing influences within these classes. This suggests that the general (across-classes) and smaller-scale (within-class) patterns are complementary perspectives. The functional relationship across classes was a monotonous increase in toxicity followed by a plateau, though the exact form could not be established with certainty. Gibbs energy of formation has limitations as an estimate of transformity and therefore these observations await confirmation. The correlation between transformity and toxicity may be an important generalization in ecotoxicology, because it may contribute a conceptual framework for making a cross-class comparisons of toxicity. Moreover, transformity may provide a unifying approach to the study of bioaccumulation, toxicity, and specificity.