2D 15N–15N isotropic chemical shift correlation established by 1H–1H dipolar coherence transfer in biological solids

A 2D 15N–15N chemical shift correlation method is proposed to assign resonances in non-selectively or uniformly 15N labeled biological solids. A reverse cross-polarization sequence is used to transfer 15N magnetization to the directly bonded amide-protons through the 1H–15N dipolar coupling and a longitudinal proton magnetization transfer via 1H–1H dipolar couplings is used to mix amide-1H resonances. It is shown that this procedure allows the observation of cross-peaks between 15N chemical shift resonances of adjacent amino acid residues with a proton mixing time as short as 10 μs in a polycrystalline sample of N-acetyl-L–15N-valyl-L–15N-leucine.