Mechanism-based quantitative structure-phytotoxicity relationships comparative inhibition of substituted phenols on...

Phytotoxicity of selected substituted phenols to Cucumis sativus (log1/RC50, root elongation half inhibition concentration [mol/L] in logarithmic form) was determined by using a root elongation inhibition method and quantitative structure-activity relationships (QSARs) were developed. Hydrophobicity, described by 1-octanol/water partition coefficient in logarithmic form (log Kow) and electronic effect (characterized by the energy of the lowest unoccupied orbital, Elumo) proved to contribute mainly to the phytotoxicity of phenols in this study. Log Kow was used as a probe to classify the tested chemicals into subsets according to the modes of toxic action. The result indicated that the substituted phenols fell into two classes in general: narcotic phenols and bioreactive phenols. Most phenols elicited their toxic response via a polar narcosis mechanism and an excellent log Kow-dependent QSAR was developed (log1/RC50 = 0.94 log Kow + 2.04, n = 22, r2adj = 0.89). Selected phenols with bioreactive substructures exhibited elevated bioreactive toxicity and a strong Elumo-dependent correlation was achieved (log1/RC50 = -0.91 Elumo + 2.73, n = 6, r2adj = 0.81). This implied that their reactive toxicity involved mainly their in vivo electrophilic reactions. In an effort to model all chemicals without regard to their mechanisms, a highly predictive response-surface was developed with the phytotoxicity, hydrophobicity, and electrophilic reactivity (log1/RC50 = 0.66 log Kow - 0.57 Elumo + 2.31, n = 28, r2adj = 0.88).