Efficient band-selective homonuclear CO-CA cross-polarization in protonated proteins

Previously introduced for highly deuterated proteins, band-selective magnetization transfer between CO and CA spins by dipolar-based homonuclear cross polarization is applied here to a protonated protein. Robust and efficient recoupling is achieved when the sum of effective radio-frequency fields on CO and CA resonances equals two times the spinning rate, yielding up to 33 % of magnetization transfer efficiency in protonated ubiquitin. The approach is designed for moderate magic-angle spinning rates and high external magnetic fields when the isotropic chemical shift difference of CO and CA considerably exceeds the spinning rate. This method has been implemented in N(i)CO(i-1)CA(i-1) and CA(i)(N-i)CO(i-1)CA(i-1) two-dimensional interresidual correlation experiments for fast and efficient resonance assignment of ubiquitin by solid-state NMR spectroscopy.