Cross polarization in the tilted frame: assignment and spectral simplification in heteronuclear spin systems

A frequency selective heteronuclear polarization transfer technique is introduced for rotating solids. In this method, radiofrequency fields comparable with the frequency offsets are applied to establish Hartmann-Hahn cross polarization that therefore depends explicitly on the resonance offset of the nuclei involved. Under these conditions, spectrally induced filtering in combination with cross polarization (SPECIFIC CP) can be achieved and is demonstrated to be useful for spectral simplification or assignment in heteronuclear spin pairs. The design principles are outlined and demonstrated experimentally on 13 C, 15 N labelled amino acids.