Measuring long-range contacts in a fully protonated protein at 105 kHz magic angle spinning.

The use of ¹H detection, made possible by very fast magic-angle spinning (MAS), has revolutionized the field of biomolecular solid-state NMR. In the past, ¹H detection was often paired with deuteration schemes to achieve the highest possible resolution needed for protein structural characterization. However, with modern probes capable of MAS rates over 100 kHz, deuteration is no longer required, resulting in a need to measure long-range distances in fully protonated systems. In this study, we evaluate the potential of two 3D pulse sequences, (H)NCOH and (H)NCAH, to measure long-range C-H correlations in a fully protonated protein sample at a MAS rate of 105 kHz. Our results show that the (H)NCOH spectrum contains multiple sequential and structurally relevant long-range CO-H contacts for each residue, capturing H^N contacts up to 6 Å despite transfers to side chain protons. Conversely, the (H)NCAH spectrum yields fewer Cα-H^N correlations, with those present mostly from intraresidue aliphatic proton contacts. Therefore, in protonated proteins, the extensive ¹H network leads to dipolar truncation in the Cα-H experiment, while the CO-H correlations observed are comparable to those in deuterated samples. These findings highlight the feasibility of conducting distance measurements based on long-range cross polarization, on more accessible and affordable samples, expanding the scope of proton detection for systems where deuteration and back-exchange are not possible.