N-13 C - correlation experiment shown Figure S7 was performed using a 1 H-15 N CP step of 1 ms and a SPECIFIC - CP [ 10 ] 15 N-13 C step of 2.5 ms . The center frequencies were 50 ppm and 120 ppm for 15 N and 13 C dimensions , respectively . Acquisition times were 10 ms ( 800 data points used-497 ppm spectral width ) and 3.4 ms ( 25 data points-90 ppm spectral width ) for the direct and indirect dimensions , respectively . 384 scans were acquired . the 2D spectra were processed using a 0.33 π shifted sine squared window function in both dimensions .