Preserving Proton Resolution Under Cryogenic Dynamic Nuclear Polarization for Biomolecular Solid-State NMR

The combination of proton detection with cryogenic dynamic nuclear polarization (DNP) promises to unite the intrinsic sensitivity and resolution of ¹H-detected magic-angle spinning (MAS) NMR with the large signal enhancements afforded by hyperpolarization, but has long been considered impractical because of cryogenic line broadening and the limited spinning frequencies available under DNP conditions. Here, we demonstrate that high-resolution proton-detected spectra of proteins can indeed be obtained under cryogenic DNP conditions. Using extensive deuteration, selective ¹³CHD₂ methyl labeling, fast MAS and high magnetic fields, we show that site-resolved proton detection is preserved in both microcrystalline proteins and amyloid fibrils. Quantitative linewidth analysis reveals that, despite broader resonances, proton coherence lifetimes remain remarkably long because high magnetic fields, fast MAS, and extensive proton dilution efficiently suppress the homogeneous contribution, leaving the observed linewidth dominated by refocusable inhomogeneous broadening. The resulting sensitivity enables structurally informative proton-detected ¹H-¹H correlation spectroscopy under DNP conditions. These results establish proton-detected DNP as a practical methodology for biomolecular MAS NMR and extend structural studies to challenging protein assemblies limited by sensitivity, spectral crowding, or sample availability.