Artifact-free ultralow-temperature DNP-enhanced NMR of molecular assemblies at natural isotopic abundance
2025-12-19
Magic-angle spinning dynamic nuclear polarization (MAS-DNP) has greatly increased solid-state NMR sensitivity, enabling multidimensional correlation experiments (e.g., 13 C– 13 C and 13 C– 15 N) at natural isotopic abundance. Yet, these experiments often suffer from t 1 -noise, an artifact that obscures weak cross-peaks, especially in natural abundance samples. Here, we present a method to suppress t 1 -noise in 13 C– 13 C double-quantum–single-quantum (DQ-SQ) spectra by converting DQ coherences into longitudinal two-spin order ( zz -terms), enabling selective removal of uncoupled 13 C magnetization. This zz -filter, compatible with both J - and dipolar-based sequences, markedly improves the spectral quality on both commercial (100 K) and custom-built helium-spinning (30 K) MAS-DNP setups. Up to a fivefold increase in signal-to-noise ratio in the indirect dimension allows detection of previously hidden long-range correlations, including intermolecular contacts. This method yields the first artifact-free 13 C– 13 C DQ-SQ spectrum at 30 K, expanding the analytical reach of MAS-DNP NMR for characterizing molecular assemblies at natural abundance.