Sensitivity enhancement in 2D Double Cross Polarization experiments under fast MAS by recycling unused protons

We demonstrate sensitivity enhancement via recycling of proton magnetization in 2D Double Cross Polarization (Double CP) experiments performed on fully protonated and uniformly labeled (¹³C, ¹⁵N) samples at a magic angle spinning rate of 60 kHz. Unused proton magnetization is preserved during t₁ evolution either by locking it with CW irradiation or by employing rotor-synchronized pi pulses. A flip-back pulse together with a modified second CP block preserves unused proton magnetization resulting in enhanced sensitivity. We have achieved sensitivity enhancements of 15-20% and 25-28% in ¹H-¹³C and ¹H-¹⁵N 2D Double CP experiments respectively. At shorter recycle delays (∼0.25T₁), relative sensitivity enhancements of 40-45% and 55% were obtained in ¹H-¹³C and ¹H-¹⁵N 2D Double CP experiments respectively. An analysis of the sensitivity enhancements and theoretical estimation of lineshapes in indirect dimension in the presence of proton recycling is provided.