3D ¹⁵N-¹³C-¹³C chemical shift correlation spectroscopy in rotating solids

We describe 3D solid state MAS NMR experiments which separate chemical shifts in one ¹⁵N and two ¹³C dimensions. The polarization transfer from ¹H to ¹⁵N and subsequently to ¹³C, is accomplished with recently developed techniques which overcome the shortcomings of constant amplitude Hartmann-Hahn cross polarization. Homonuclear (¹³C-¹³C) recoupling is performed with the MELODRAMA sequence. In the 2D ¹³C-¹³C spectra of adenosine monophosphate (AMP), successive MELODRAMA transfers propagate ¹³C magnetization around the five ¹³C's of the ribose ring, and the cross-peaks from successive transfer steps exhibit the negative/positive/negative... pattern expected from the double quantum polarization transfer process. The 3D ¹⁵N-¹³C-¹³C spectrum of unifonnly-¹⁵N,¹³C-labeled histidine consists of ¹³C-¹³C planes separated by ¹⁵N chemical shifts. The 2D planes are asymmetric since in many cases the polarization giving rise to each cross-peak originates on only one nuclear spin of the coupled pair. 3D ¹⁵N-¹³C-¹³C spectra of this type should be useful for dispersing overlapping spectra of peptides, proteins, and nucleic acids.