Fig. 6. Overview of f-MLF spectra acquired over a range of MAS frequencies.

(Top) ¹⁵N ramp-CP (linear rf ramp 70 to 100% on ¹H channel) spectra of approximately equal intensity over the whole range of MAS rates. (Middle and Bottom) ¹³C spectra of the Cα and C′ spectral region, respectively, acquired using NCA and NCO experiments with ¹H-¹⁵N and ¹⁵N-¹³C magnetization transfers. While the ¹H-¹⁵N step was carried out using the same pulse sequence element, ramp-CP (linear rf ramp 90 to 100% on ¹³C channel), tm-SPICE-16.5, and tm-SPICE-20 schemes were used for the ¹⁵N-¹³C step, resulting in spectra plotted in red, green, and blue, respectively. The optimal control tm-SPICE set of pulses consistently yields the highest signal intensity, which is about 1.5 times the intensity of the ramp-CP experiments. All NCA and NCO spectra are displayed with a common scale on the y axis, with gray grid lines facilitating visual comparison.