Beyond spectral dispersion: proton line narrowing at ultra-high magnetic fields

Ultra-high magnetic field NMR spectrometers operating at proton frequencies approaching and exceeding 1.2 GHz are opening new opportunities for biomolecular solid-state NMR. While higher magnetic fields are expected to improve spectral resolution through increased chemical-shift dispersion, the extent to which they continue to improve proton linewidths under fast magic-angle spinning (MAS) conditions has remained unclear. Addressing this question is particularly important as increasingly fast MAS and ever higher magnetic fields become available. Here, we systematically investigate proton linewidths and coherence lifetimes across proton Larmor frequencies from 500 to 1200 MHz under fast MAS conditions. Using microcrystalline GB1 as a benchmark system, we combine bulk and site-specific analyses of proton linewidths and transverse coherence lifetimes (T₂') to characterize linewidths as a function of magnetic field strength. Increasing field strengths systematically improve proton spectral resolution and prolong proton coherence lifetimes, yielding gains that exceed those expected from increased chemical-shift dispersion alone. Linewidth analysis reveals a progressive reduction of non-refocusable broadening at high fields, consistent with suppression of coherent proton spin dynamics. The resulting benefits are particularly pronounced for aliphatic side-chain resonances and remain clearly observable in membrane proteins reconstituted in lipid bilayers, where spectral crowding often limits analysis. These results provide a systematic view of linewidth evolution in proton-detected solid-state NMR and support continuing developments toward higher magnetic fields and faster MAS frequencies for challenging biomolecular systems.