Schematics of radio-frequency pulse sequences used in this study for 13C-detected (A) and 1H-detected (B) cross-polarization with a variable contact time (CPVC) 2D SLF experiment. The t1 incrementation was achieved by simultaneously increasing the spin-lock durations in the cross-polarization sequence. SPINAL-64 was used to decouple protons during signal acquisition for the 13C-detected CPVC experiment. The following phase cyclings were used in the experiment: ϕ1=0,2,0,2; ϕ2=1,1,1,1; ϕ3=0,0,1,1; ϕCrec=0,2,1,3; ϕHrec=0,2,0,2.