Artifact-free ultralow-temperature DNP-enhanced NMR of molecular assemblies at natural isotopic abundance

Magic-angle spinning dynamic nuclear polarization (MAS-DNP) has greatly increased solid-state NMR sensitivity, enabling multidimensional correlation experiments (e.g., ¹³C-¹³C and ¹³C-¹⁵N) at natural isotopic abundance. Yet, these experiments often suffer from t₁-noise, an artifact that obscures weak cross-peaks, especially in natural abundance samples. Here, we present a method to suppress t₁-noise in ¹³C-¹³C double-quantum-single-quantum (DQ-SQ) spectra by converting DQ coherences into longitudinal two-spin order (zz-terms), enabling selective removal of uncoupled ¹³C 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 ¹³C-¹³C DQ-SQ spectrum at 30 K, expanding the analytical reach of MAS-DNP NMR for characterizing molecular assemblies at natural abundance.