Mechanism of dilute-spin-exchange in solid-state NMR

Effects of deuteration on solid-state NMR spectra of single peptide crystals and oriented protein samples

Extensive deuteration can be used to simplify NMR spectra by "diluting" and minimizing the effects of the abundant ¹H nuclei. In solution-state NMR and magic angle spinning solid-state NMR of proteins, perdeuteration has been widely applied and its effects are well understood. Oriented sample solid-state NMR of proteins, however, is at a much earlier stage of development. In spite of the promise of the approach, the effects of sample deuteration are largely unknown. Here we map out the effects of perdeuteration on solid-state NMR spectra of aligned samples by closely examining differences in results obtained on fully protiated and perdeuterated samples, where all of the carbon sites have either ¹H or ²H bonded to them, respectively. The ²H and ¹⁵N labeled samples are back-exchanged in ¹H₂O solution so that the amide ¹⁵N sites have a bonded ¹H. Line-widths in the ¹⁵N chemical shift, ¹H chemical shift, and ¹H-¹⁵N dipolar coupling frequency dimensions were compared for peptide single crystals as well as membrane proteins aligned along with the phospholipids in bilayers with their normals perpendicular to the direction of the magnetic field. Remarkably, line-width differences were not found between fully protiated and perdeuterated samples. However, in the absence of effective ¹H-¹H homonuclear decoupling, the line-widths in the ¹H-¹⁵N heteronuclear dipolar coupling frequency dimension were greatly narrowed in the perdeuterated samples. In proton-driven spin diffusion (PDSD) experiments, no effects of perdeuteration were observed. In contrast, in mismatched Hartmann-Hahn experiments, perdeuteration enhances cross-peak intensities by allowing more efficient spin-exchange with less polarization transfer back to the carbon-bound ¹H. Here we show that in oriented sample solid-state NMR, the effects of perdeuteration can be exploited in experiments where ¹H-¹H homonuclear decoupling cannot be applied. These data also provide evidence for the possible contribution of direct ¹⁵N-¹⁵N dilute-spin mixing mechanism in proton-driven spin diffusion experiments.