TmDOTP: An NMR-based thermometer for magic angle spinning NMR experiments

Knowing the actual sample temperature in a solid-state NMR experiment is crucial in many ways. Many different approaches exist from measuring the chemical shift difference in spectra of ethylene glycol (solution-state NMR spectroscopy) to measuring the peak position in lead nitrate or the T1 relaxation times of KBr (solid-state NMR spectroscopy). All of these methods have their pros and cons. This approach using TmDOTP, having a temperature coefficient of 1ppm/K and being inert to biopolymers is a valuable addition to the ssNMR toolbox.

Zhang, Dongyu, Boris Itin, and Ann E. McDermott. “TmDOTP: An NMR-Based Thermometer for Magic Angle Spinning NMR Experiments.” Journal of Magnetic Resonance 308 (November 1, 2019): 106574.

Solid state NMR is a powerful tool to probe membrane protein structure and dynamics in native lipid membranes. Sample heating during solid state NMR experiments can be caused by magic angle spinning and radio frequency irradiation such heating produces uncertainties in the sample temperature and temperature distribution, which can in turn lead to line broadening and sample deterioration. To measure sample temperatures in real time and to quantify thermal gradients and their dependence on radio frequency irradiation or spinning frequency, we use the chemical shift thermometer TmDOTP, a lanthanide complex. The H6 TmDOTP proton NMR peak has a large chemical shift (−176.3 ppm at 275 K) and it is well resolved from the protein and lipid proton spectrum. Compared to other NMR thermometers (e.g., the proton NMR signal of water), the proton spectrum of TmDOTP, particularly the H6 proton line, exhibits very high thermal sensitivity and resolution. In MAS studies of proteoliposomes we identify two populations of TmDOTP with differing temperatures and dependency on the radio frequency irradiation power. We interpret these populations as arising from the supernatant and the pellet, which is sedimented during sample spinning. In this study, we demonstrate that TmDOTP is an excellent internal standard for monitoring real-time temperatures of biopolymers without changing their properties or obscuring their spectra. Real time temperature calibration is expected to be important for the interpretation of dynamics and other properties of biopolymers.

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