Figure 7.

Experimental build-up curves for polarization transfer in U-¹³C, ¹⁵N alanine: (a) C_α-C_β, and (b) C_α-C′, by R2₁², R2₁¹, R2₂¹, and R2₂² symmetry-based sequences. The sample was spun at a MAS frequency of 40 kHz. The cross-peak intensities in each spectrum were normalized to those of the corresponding diagonal peaks in the following way: $I_{norm}=\frac{2I_{AB}}{I_{AA}+I_{BB}}$, where I_norm is the normalized cross peak intensity, I_AB is the non-normalized cross peak intensity, I_AA and I_BB are the non-normalized intensities of the corresponding diagonal peaks.