Nuclear magnetic resonance (NMR) spectroscopy is a powerful and versatile tool for monitoring dynamic processes. Conformational changes and binding events can produce drastic effects on NMR spectra in response to variations in chemical exchange rate. Here, an approach based on increasing the exchange rate through anion-induced conformational flexibility is presented, which allows ultra-sensitive anion detection in NMR spectroscopy. Our novel approach is based on a small synthetic sensor containing anion binding thioureido moieties and displaying conformational rigidity due to intramolecular hydrogen bonding (HB). In the rigid sensor, peak splitting of geminal protons (-CH2-) occurs and slow chemical exchange rate is observed in NMR. Conformational flexibility is generated by exposing the sensor to anions such that intramolecular interactions are competitively disrupted by formation of intermolecular HBs between anions and the thioureido protons. Thus, detection of anion-induced conformational flexibility is achieved through an increase in exchange rate in NMR: the two well-separated germinal proton signals merge into one sharp peak upon anion addition. NMR titration experiments with different anions also confirmed predictable anion selectivity. Strong HB acceptors, such as chloride or bromide, induced significant conformational changes with only catalytic amounts of anions. We were therefore able to selectively detect anion species by NMR in the nM range, at a concentration more than 1000 folds lower than the sensor. Figure 1. 1H NMR titration of sensor with increasing anion (Cl-) concentrations.

Ultra-sensitive anion detection by NMR: A supramolecular approach based on anion-modulated increase in chemical exchange rate

247th National Meeting (2014)

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