¹⁷O quadrupolar chemical exchange saturation transfer (Q-CEST) NMR for investigations of molecular dynamics in solids

We introduce quadrupolar chemical exchange saturation transfer (Q-CEST) for half-integer quadrupolar nuclei such as oxygen-17 as a complementary NMR tool for studies of molecular dynamics in solids. Experiments on the model compounds NaNO₃ and hydration water in fibrils formed by pyro-glutamate E3 Amyloid-β protein are combined with simulations and theoretical approaches to obtain parameters of molecular motions. We determine the rate constants for 3-site jumps of oxygen atoms in NaNO₃ and rate constants and populations of tetrahedral jumps of hydration water in the proximity to protein surface below the bulk freezing point. The detection is focused on the central transition (CT). However, during the saturation the transmitter is swept across a wide range of frequencies reaching the first satellite transitions. A detailed analysis is provided for CT Q-CEST profiles in the presence of molecular dynamics covering two dynamical transitions occurring approximately in the microsecond and nanosecond time scale ranges and paying special attention to correct inclusion of the dynamics averaging of the second order quadrupolar interaction.