Broad-band homo-nuclear correlations assisted by ¹H irradiation for bio-molecules in very high magnetic field at fast and ultra-fast MAS frequencies

We propose a new broadband second-order proton-assisted ¹³C- ¹³C correlation experiment, SHANGHAI. The ¹³C- ¹³C magnetization transfer is promoted by ¹H irradiation with interspersed four phases super-cycling. This through-space homo-nuclear sequence only irradiates on the proton channel during the mixing time. SHANGHAI benefits from a large number of modulation sidebands, hence leading to a large robustness with respect to chemical shift differences, which permits its use in a broad MAS frequency range. At ultra-fast MAS (ν _R ≥ 60 kHz), SHANGHAI is only efficient when the amplitude of ¹H recoupling rf-field is close to half the spinning speed (ν ₁ ≈ ν _R/2). However, at moderate to fast MAS (ν _R = 20-35 kHz), SHANGHAI is efficient at any rf-power level larger than ν ₁ ≈ 10 kHz, which simultaneously permits avoiding excessive heating of bio-molecules, and using large sample volumes. We show that SHANGHAI can be employed at the very high magnetic field of 23.5 T and then allows the observation of correlation between ¹³C nuclei, even if their resonance frequencies differ by more than 38 kHz.