NMR Sensing Strategies Based on Modulation of Chemical Exchange Rate With Small-Molecule Synthetic Sensors
Nuclear magnetic resonance (NMR) spectroscopy is a powerful and versatile tool for monitoring molecular interactions. Dynamic processes, such as conformational changes or binding events, can induce drastic effects on NMR spectra in response to variations in chemical exchange rate. Molecular strategies in which the chemical exchange rate is deliberately controlled can lead to attractive opportunities in NMR sensor design. This thesis explores two main strategies involving structurally similar sensors in which modifications of the chemical exchange rate have been utilized to sense anions or detect changes in pH. The first section of the thesis describes a novel and ultrasensitive approach to detect anions indirectly by NMR and is based on a small synthetic sensor (TUC) displaying conformational rigidity due to intramolecular hydrogen bonding. With an increase in chemical exchange rate, due to anion-induced conformational flexibility, detection and quantification of spherical and tetrahedral anions as low as 120 nM concentrations, which is more than 1000 fold lower than the detection limit of NMR, are possible. The next part describes a slow proton exchange strategy for pH sensing. Monitoring pH change accurately and non-invasively by NMR has important implications in the detection and treatment of pathologies such as cancer, ischemia or cystic fibrosis. Conventional NMR pH detection methods are mostly based on changes in chemical shift due to fast proton exchange between the different protonation states of the sensors. Slowing down proton exchange allows ratiometric pH measurement, which eliminates the errors from pH-independent factors such as artifacts caused by the chemical environment, ionic strength, etc. Therefore, pH can be measured by using the ratio of the different protonation states of the pH sensor. Detection of pH in vitro and in bacterial cultures was successfully demonstrated with the sensor SPE1 using this new method, and preliminary chemical exchange saturation transfer (CEST) experiments were carried out. Optimization of SPE1, through 13C-labeling (13C-SPE1) and new design to decrease the pKa and increase the pH window of the sensor (SPE2), was also studied. Overall, both strategies demonstrate how modulation of chemical exchange rate can provide a platform for the development of new NMR sensors with high sensitivity and accuracy.
anion, chemical exchange, hydrogen bonds, NMR, pH, sensing
Networked Digital Library of Theses and Dissertations