Nuclear relaxometry helps designing systems for solution DNP on proteins

The measurement of water proton longitudinal relaxation rates in aqueous solutions of paramagnetic compounds (radicals, paramagnetic metal ions) as a function of magnetic field (relaxometry) provides two important pieces of information for predicting the efficacy of dynamic nuclear polarization (DNP) to be ultimately achieved on protein nuclei: the correlation time for the electron-nuclear interaction and the actual water proton relaxation rate at any magnetic field of interest. Likewise, relaxometry measurements on protein protons themselves, which can be performed in D2O solutions, provide information on the polarization loss occurring on protein protons in DNP-shuttling experiments during the shuttling of the sample from the electron paramagnetic resonance to the nuclear magnetic resonance field. A collection of water proton and protein proton relaxometry profiles, recently obtained in the course of the DNP Design Study project, is here reported, and the relevant parameters are discussed.