Leibniz-Institut f. Analytische Wissenschaften - ISAS, Dortmund, Germany.
Environmental Geochemistry, Centre for Microbiology and Environmental Systems Science, University of Vienna, Vienna, Austria.
Biometals. 2020 Dec;33(6):305-321. doi: 10.1007/s10534-020-00248-y. Epub 2020 Oct 5.
Due to the low iron solubility in alkaline soils, plants have evolved different iron acquisition strategies, which are either based on ferric iron reduction (strategy I) or complexation by phytosiderophores (strategy II). Recently, a prominent role of coumarins for iron acquisition has been discovered, but details of the respective mechanism remain unclear. Since coumarins may act as iron-binding ligands but also as reductants, various reaction sequences are possible, resulting in different iron species and oxidized coumarins. In this context, it is often overlooked that oxidized coumarins are not just byproducts of iron(III) reduction, but may be actively involved in further steps of iron mobilization. In order to verify this active role of oxidized coumarins in Fe(hydr)oxide dissolution, we complemented iron dissolution data with data of single coumarins (esculetin, scopoletin, fraxetin) and their oxidation products, as a function of time, pH, and mineral (goethite, lepidocrocite). Our results demonstrate that there are four different routes for coumarin oxidation, leading to quinones, dimers, hydroxylated coumarins, demethylated coumarins, and combinations of these. The time-dependent species pattern differs with respect to mineral, pH, and coumarin molecule. Oxidized coumarins are often more reactive than the original coumarins, explaining unexpected iron mobilization by scopoletin, which is demethylated to esculetin. Also oxidative hydroxylation and dimerization increase the number of phenolic groups and yield new chelating properties. Several iron-species are identified for the three coumarins. Since oxidation reactions are initiated directly at mineral surfaces, they are often very effective-but this does not always result in more iron mobilization.
由于碱性土壤中铁的溶解度较低，植物已经进化出不同的铁获取策略，这些策略要么基于三价铁的还原（策略 I），要么基于植物螯合肽的络合（策略 II）。最近，香豆素在铁获取中的重要作用已经被发现，但具体的机制仍不清楚。由于香豆素可能作为铁结合配体，也可能作为还原剂，因此存在多种反应序列，从而产生不同的铁物种和氧化香豆素。在这种情况下，人们常常忽略了氧化香豆素不仅仅是三价铁还原的副产物，它们可能会主动参与进一步的铁动员步骤。为了验证氧化香豆素在 Fe（hydr）氧化物溶解中的这种主动作用，我们将铁溶解数据与单一香豆素（esculetin、scopoletin、fraxetin）及其氧化产物的数据进行了补充，这些数据是随时间、pH 值和矿物（针铁矿、纤铁矿）变化的。我们的结果表明，香豆素氧化有四种不同的途径，分别生成醌、二聚体、羟基化香豆素、去甲基香豆素以及这些物质的组合。随着时间的推移，不同矿物、pH 值和香豆素分子的物种模式也会发生变化。氧化香豆素通常比原始香豆素更具反应性，这解释了 scopoletin 出乎意料的铁动员作用，它被 demethylated 为 esculetin。此外，氧化羟化和二聚化增加了酚基团的数量，并产生了新的螯合特性。三种香豆素都鉴定出了几种铁物种。由于氧化反应直接在矿物表面引发，因此它们通常非常有效——但这并不总是导致更多的铁动员。
