3D ¹⁵N−¹³C−¹³C Chemical Shift Correlation Spectroscopy in Rotating Solids

Abstract

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 uniformly-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.