Removing t1-noise in DNP-enhanced NMR at natural isotopic abundance using two-spin order filter

MAS-DNP has markedly improved the inherently low sensitivity of NMR, enabling advanced applications such as multidimensional correlation experiments (e.g., 13C -13C , 13C -15N ) at natural isotopic abundance (NA). However, these experiments are frequently hindered by 𝑡1-noise, an artifact resulting from experimental instabilities during the indirect evolution period. Due to its multiplicative nature, 𝑡1-noise presents a particular challenge for NA samples, as cross-peaks from correlated spins may be obscured by noise contributions from uncorrelated nuclei, leading to spectral ambiguities and compromised data interpretation. Here, we introduce a novel strategy to suppress 𝑡1-noise in 13C -13C DQ-SQ correlation spectroscopy acquired on samples at NA. The method relies on converting DQ coherences into longitudinal two-spin order (𝑧𝑧-terms), allowing the selective elimination of transverse magnetization from uncoupled 13C nuclei via a 𝑧𝑧-filter. We demonstrate the effectiveness of this approach in both J- and dipolar-based correlation experiments, achieving dramatic improvements in spectral clarity on both a commercial MAS-DNP system at 100 K and also at 30 K using a custom-built He-spinning MAS-DNP instrument. Notably, we show that the suppression of 𝑡1-noise reveals previously hidden cross-peaks corresponding to long-range correlations, with improvement in signal-to-noise ratio in the indirect dimension of up to a factor ∼6, enabling the first 13C -13C DQ-SQ spectrum at 30 K at NA.