Ionic liquids (ILs) are perceived as a promising group of chemicals within the realm of green chemistry. They are increasingly of interest for open environmental applications. Rules for the design of biodegradable and preferably completely mineralizing ILs after their introduction into the environment are highly desirable. In this study, the impact of the length of the alkyl chain and of the cationic head group on the environmental biodegradability of L-phenylalanine ester (PheC_n) derived ILs using the pyridinium (PyPheC_n), imidazolium (ImPheC_n) and cholinium (CholPheC_n) head groups was systematically studied. This included high-resolution mass spectrometry monitoring of formed products of incomplete mineralization. Completely biodegradable ILs were only identified in the PyPheC_n series. PyPheC₂ and PyPheC₄ were fully mineralizable within 42 days with no detectable transformation products (TPs), whereas mineralization of longer chain length PyPheC_n ILs required more time. Our results show biodegradation by microorganisms was affected by an alkyl chain length of at least 10 carbon atoms. Biodegradation of the ILs started with biotic ester hydrolysis independent of the alkyl chain length or core scaffold. In contrast, the second step of microbial degradation was an amide bond cleavage which was only shown for ILs with a pyridinium or imidazolium core, even after a test prolongation up to 42 days. Hence, the nature of the cationic head group and the chain length impacted on the mineralization and biodegradability of the ILs. For ILs with an imidazolium core, amide bond cleavage resulted in the formation of recalcitrant 1-carboxymethylimidazolium (ImAc). Whereas for most ILs with a pyridinium core, amide bond cleavage resulted in 1-carboxymethylpyridinium (PyAc) which was mineralizable, although not readily biodegradable according to the test guideline 301D. In the case of cholinium ILs, no further degradation of cholinium phenylalanine amide TP was observed after the hydrolysis of the ester to the alkyl alcohol. The biodegradability of ILs decreased in the order PyPheC_n > ImPheC_n > CholPheC_n, but a key finding is that ImPhe is degraded to ImAc but not further (cf. favourable PyAc mineralisation). These results can be used to design ILs with the property to be effectively biodegraded and mineralized in the environment.

中文翻译：

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苯丙氨酸烷基酯键合离子液体的环境生物降解设计规则

离子液体（ILs）被认为是绿色化学领域内有希望的一组化学物质。它们对于开放环境应用越来越感兴趣。引入环境后，设计可生物降解的IL（最好是完全矿化的IL）的规则非常必要。在这项研究中，烷基链的长度和阳离子头基对使用吡啶鎓（PyPheC _n），咪唑鎓（ImPheC _n）和胆碱（PyChe C _n）的L-苯丙氨酸酯（PheC _n）衍生的ILs的环境生物降解性的影响。CholPheC _ñ）对头组进行了系统的研究。这包括对不完全矿化的成形产品进行高分辨率质谱监测。完全可生物降解的IL仅在PyPheC _n系列中鉴定。PyPheC ₂和PyPheC ₄在42天内完全矿化，没有可检测到的转化产物（TPs），而更长链长的PyPheC _n矿化IL需要更多的时间。我们的结果表明，微生物的生物降解受到至少10个碳原子的烷基链长度的影响。IL的生物降解始于与烷基链长或核心骨架无关的生物酯水解。相比之下，微生物降解的第二步是酰胺键裂解，仅在具有吡啶鎓或咪唑鎓核心的IL上显示，即使经过长达42天的测试也是如此。因此，阳离子头基的性质和链长影响IL的矿化和生物降解性。对于具有咪唑鎓核心的IL，酰胺键断裂导致形成顽固的1-羧甲基咪唑鎓（ImAc）。而对于大多数具有吡啶鎓核心的离子液体，酰胺键裂解会产生可矿化的1-羧甲基吡啶鎓（PyAc），尽管根据测试指南301D不易生物降解。在胆碱ILs的情况下，在酯水解成烷基醇后，未观察到胆碱苯丙氨酸酰胺TP的进一步降解。IL的生物降解能力按PyPheC顺序降低_n > ImPheC _n > CholPheC _n，但是一个关键的发现是ImPhe被降解为ImAc，但不会进一步降解（参见有利的PyAc矿化作用）。这些结果可用于设计具有在环境中被有效生物降解和矿化的性质的IL。