Deuterium solid-state NMR quadrupolar order rotating frame relaxation with applications to amyloid-β fibrils

We describe a new method for measuring molecular dynamics based on the deuterium solid-state nuclear magnetic resonance (NMR) quadrupolar order rotating frame relaxation rate R_1ρ,Q under static conditions. The observed quadrupolar order coherence is created using the broad-band Jeener-Broekaert excitation and is locked with a weak radio frequency (RF) field. We describe the experimental and theoretical approaches and show applications to a selectively deuterated valine side chain of the phosphorylated amyloid-β (1-40) fibrils phosphorylated at the serine-8 position. The R_1ρ,Q rate is sensitive to the rotameric exchange mode. For biological samples, the low spin-lock field in the 5- to 10-kHz range has the advantage of avoiding sample heating and dehydration. Thus, it provides an alternative to approaches based on single-quantum coherence, which require larger spin-lock fields.