Kinetics of ptaquiloside hydrolysis in aqueous solution

Ptaquiloside (PTA) is a well-known toxin produced by the bracken fern (Pteridium aquilinum (L.) Kuhn). It is proposed that PTA from bracken stands can leach through soil and sediments into drinking-water reservoirs, thus representing a concern for human health. To predict the persistence of the toxin, a full understanding of the PTA degradation in aqueous environments is important. The kinetics of PTA hydrolysis was examined at 22 C in aqueous buffered solutions (pH 2.88-8.93). The reaction was found to follow first-order kinetics with respect to PTA at all pH and temperature conditions. At pH lower than 4.43 (+/- 0.32), the reaction is acid-mediated, whereas the reaction is base-mediated at pH higher than 6.39 (+/- 0.28). The rate constants for the acid-catalyzed, base-catalyzed, and neutral hydrolysis are 25.70 (+/- 0.96), 4.83 (+/- 0.03) X 10(4), and 9.49 (+/- 6.02) X 10(-4) h(-1), respectively. The PTA hydrolysis at pH 4.46 is strongly dependent on temperature, with an activation energy of 74.4 (+/- 2.6) kJ mol(-1). Stoichiometric calculations, reaction kinetics, and ultraviolet-visible spectrophotometry strongly indicates the formation of an intermediary compound at pH 5.07 and 6.07 via a mechanism comprising two first-order consecutive reactions. Ptaquiloside has the lowest rate of hydrolysis at slightly acidic pH and low temperatures. Therefore, because PTA is not sorbed in soil, slightly acidic sandy soils in cold climates are most prone to PTA leaching to deeper soil layers and aquifers.