Department of Plant and Environmental Sciences, University of Copenhagen, Thorvaldsensvej 40, 1871 Frederiksberg, Denmark.
Environ Sci Process Impacts. 2019 Jul 17;21(7):1204-1214. doi: 10.1039/c9em00012g.
Saponins form a group of plant-produced glycosides with potential as biopesticide ingredients. The environmental fate of saponins has never been fully investigated. In the present study, we use QS-18, a specific saponin from Quillaja saponaria as an example, to quantify hydrolysis under different conditions of pH, temperature and water chemical composition. Saponin hydrolysis in buffer solutions was base-catalyzed and followed first-order kinetics. Thus, hydrolysis was slow at pH 5.1 with a half-life of 330 ± 220 d (26 °C), which increases to 0.06 ± 0.01 d at pH 10.0. Hydrolysis rates were highly sensitive to temperature with an activation energy of 56.9 ± 14.2 kJ mol-1 at pH 7.2. In strong contrast, hydrolysis in lake waters (pH 6.4-8.2) produced different patterns with a fast initial dissipation of 25 to 60% of the added saponin within the first five hours, followed by an extremely slow reaction with 25 to 75% unreacted saponin left after reaction times longer than 120 h. The fast dissipation followed by slow hydrolysis in lake water was hypothesized to be attributed to sorption and/or flocculation of saponins by inorganic nanoparticles and/or solutes in the lake water followed by inactivation of hydrolysis due to the sorption/flocculation. The present study demonstrates that saponins may hydrolyze slowly under acidic and cold conditions. In addition, it demonstrates that dissipation kinetics in natural waters may deviate substantially from the kinetics predicted based on laboratory experiments with "clean" buffered solutions. This emphasizes the need for a deeper understanding of the processes affecting the dissipation kinetics of potential toxins under natural conditions, as fate models based on laboratory derived kinetic data may be seriously flawed.
皂苷是一组植物来源的糖苷，具有成为生物农药成分的潜力。皂苷的环境归宿从未被充分研究过。在本研究中，我们以 Quillaja saponaria 的特有皂苷 QS-18 为例，定量研究了不同 pH 值、温度和水化学成分条件下的水解情况。在缓冲溶液中，皂苷的水解为碱催化，遵循一级动力学。因此，在 pH 值为 5.1 时，水解速度较慢，半衰期为 330 ± 220 d（26 °C），而在 pH 值为 10.0 时，水解速度增加到 0.06 ± 0.01 d。水解速率对温度非常敏感，在 pH 值为 7.2 时，活化能为 56.9 ± 14.2 kJ mol-1。与此形成鲜明对比的是，在湖水（pH 值为 6.4-8.2）中，水解产生了不同的模式，在最初的 5 小时内，加入的皂苷中有 25%至 60%迅速消散，随后反应时间超过 120 h 时，反应速度极慢，剩余未反应的皂苷有 25%至 75%。推测在湖水中快速消散后缓慢水解是由于无机纳米粒子和/或湖水中的溶质对皂苷的吸附和/或絮凝，以及由于吸附/絮凝导致水解失活。本研究表明，在酸性和低温条件下，皂苷可能会缓慢水解。此外，本研究表明，天然水中的消散动力学可能与基于“清洁”缓冲溶液的实验室实验预测的动力学有很大偏差。这强调了需要更深入地了解潜在毒素在自然条件下的消散动力学的影响过程，因为基于实验室获得的动力学数据的命运模型可能存在严重缺陷。
