Acrylamide-Linked Water-Soluble Nitroxide Biradicals with Enhanced Electron–Electron Couplings for Dynamic Nuclear Polarization at 14.1 T

Dynamic nuclear polarization (DNP) has emerged as a powerful technique to overcome the sensitivity limit of solid-state nuclear magnetic resonance (ssNMR) spectroscopy through polarizing agents (PAs) carrying unpaired electrons. Nitroxide biradical-based PAs have garnered great attention due to their superior DNP performance. Here, we report TJPols─water-soluble acrylamide-linked nitroxide biradicals featuring facile large-scale synthesis. These PAs yield fast polarization build-up and reduced depolarization effect in magic-angle spinning (MAS) DNP experiments at 14.1 T, compensating for the moderate DNP enhancements and thus achieving higher overall NMR sensitivity gains than AMUPol, the most widely utilized binitroxide PA. Electron paramagnetic resonance (EPR) spectroscopy and molecular dynamics (MD) simulations reveal strong electron-electron dipolar and exchange couplings, validating the strategy of introducing a double bond in the linker region to tune the DNP behaviors. Our work presents a new synthetically feasible and functionalizable scaffold for developing new generations of PAs for DNP ssNMR at medium-to-high magnetic fields.