Quantification of thiols free and complexed with trace metals in oxic soils: a modified qBBr-labeling method

A key property of thiols (R-SH) is their higher affinity for soft metals over less soft metals. They can therefore act as a filter in plants to separate the micronutrient zinc (Zn, less soft metal) from the ubiquitous pollutant cadmium (Cd, soft metal). Thiols are also present in oxic soils where they may play an important role in trace metal speciation. However, the role of thiols in oxic soils on the trace metal phytoavailability remains largely unexplored. We hypothesize that thiols in soils, similar as in plants, could act as filters to separate Zn from Cd.

The main reason for this knowledge gap is the lack of data on the thiol content in soils. This data paucity is caused by the analytical challenges associated with thiol quantification, including: (i) the sensitivity of thiols to oxidation, (ii) the structural heterogeneity of thiol molecules, and (iii) their lack of distinctive spectral characteristics. Thiols have previously been quantified in natural water using a monobromo(trimethylammonio)bimane -labeling protocol (qBBr). This molecule can bind thiols with high affinity and selectivity for them. The quantification of the non-reacted qBBr by combined chromatography-mass spectroscopy (LC-ESI-MS/MS) methods after derivatization time allows to overcome the aforementioned difficulties and to quantify thiol concentration in environmental samples, including soil pore water samples. However, this method, as it has been implemented until now, presents some limitations. Firstly, the qBBr-labeling was performed at near-neutral pH where most of the thiols are protonated, thereby limiting the qBBr-derivation yield. Secondly, the protocol allowed only for quantification of free thiols although a significant fraction of thiols may be complexed with metals in soils. These limitations likely resulted in an underestimation of thiols. Finally, the matrix of the natural samples may interfere with the quantification procedure (i.e. the binding of qBBr to thiols), a concern that is particularly relevant in soil samples due to the complex matrix.

In this study, we seek to refine the qBBr-labeling protocol to quantify for the first time free and complexed thiols in soil water extracts. We will test the effects of pH on the capacity of qBBr to bind thiols in order to maximize the derivation yield. Additionally, we will use the synthetic chelator EDTA to decomplex metals that are bound to thiols for the quantification of thiols that are complexed with metals. Finally, we will minimize matrix interferences using a control sample with the same matrix and cadmium-saturated thiol binding sites. Developing a thiol quantification method for soils is essential to improve our understanding of the role of thiols on the phytoavailability of essential and non-essential trace metals.