Structure-activity relationships of chlorobenzenes using DNA measurement as a toxicity parameter in algae

This study investigated the effects of chlorobenzenes on the diatom Cyclotella meneghiniana using DNA measurement as a toxicity parameter, and related toxicity to different physicochemical properties of chlorobenzenes.; The organism was exposed to several concentrations of each chlorobenzene for 48 hours and DNA content was measured by fluorometry to obtain EC{dollar}sb{lcub}50{rcub}{dollar} values (in terms of % DNA reduction). The toxicity (EC{dollar}sb{lcub}50{rcub}{dollar}) increased with increasing degree of chlorination, as follows: monochlorobenzene (235.74 mg/l), dichlorobenzenes (23.33-51.88 mg/l), trichlorobenzenes (0.59-6.42 mg/l), tetrachlorobenzenes (0.27-1.39 mg/l), pentachlorobenzene (0.008 mg/l), and hexachlorobenzene (0.002 mg/l).; Kinetic studies were also performed using disappearance rates of chlorobenzenes from water as an indirect measurement of algal uptake. An increase in disappearance rate was observed with increasing degree of chlorination, as follows: monochlorobenzene (0.13/hr), dichlorobenzenes (0.23-0.26/hr), trichlorobenzenes (0.31-0.33/hr), tetrachlorobenzenes (0.35-0.46/hr), pentachlorobenzene (0.52/hr), and hexachlorobenzene (0.57/hr). The rates were related to octanol/water partition coefficient (r{dollar}sp2{dollar} = 0.91), bioconcentration factor (r{dollar}sp2{dollar} = 0.91), and toxicity (r{dollar}sp2{dollar} = 0.94).; In addition, using monochlorobenzene as a model compound for metabolism studies, chlorophenols were identified as products, suggesting an oxidative pathway with the formation of epoxide intermediates.; Quantitative structure-activity relationships between the toxicity and molecular properties were developed showing high correlations, as follows: water solubility (r{dollar}sp2{dollar} = 0.96); molecular volume (r{dollar}sp2{dollar} = 0.92); zero-order connectivity index (r{dollar}sp2{dollar} = 0.92); second-order connectivity index (r{dollar}sp2{dollar} = 0.92); molecular surface area (r{dollar}sp2{dollar} = 0.92); octanol/water partition coefficient (r{dollar}sp2{dollar} = 0.86); bioconcentration factor (r{dollar}sp2{dollar} = 0.86); inorganic/organic character (r{dollar}sp2{dollar} = 0.83). A poor correlation, however, was obtained with the molecular negentropy index (r{dollar}sp2{dollar} = 0.25).; The chemical activity was related to the degree of saturation of chlorobenzenes; the lower the aqueous solubility the lower the effective concentration. The toxicity effect was explained in terms of a partitioning process in which the degree of solubility will determine the EC{dollar}sb{lcub}50{rcub}{dollar} value, which was found to occur at similar degrees of exposure saturation (66-79%).