Deuteron Quadrupolar Chemical Exchange Saturation Transfer (Q‐CEST) Solid‐State NMR for Static Powder Samples: Approach and Applications to Amyloid‐β Fibrils

Abstract

We provide an experimental and computational framework for 2 H quadrupolar chemical exchange saturation transfer NMR experiments (Q-CEST) under static solid-state conditions for the quantification of dynamics on μs-ms timescales. Simulations using simple 2-site exchange models provide insights into the relation between spin dynamics and motions. Biological applications focus on two sites of amyloid-β fibrils in the 3-fold symmetric polymorph. The first site, the methyl group of A2 of the disordered N-terminal domain, undergoes diffusive motions and conformational exchange due to transient interactions.Earlier 2 H rotating frame relaxation and quadrupolar CPMG measurements are combined with the Q-CEST approach to characterize the multiple conformational states of the domain. The second site, the methyl group of M35, spans the wateraccessible cavity inside the fibrils' core and undergoes extensive rotameric exchange. Q-CEST permits us to refine the rotameric exchange model for this site and allows the more precise determination of populations and rotameric exchange rate constants than line shape analysis.[a] Prof. L. Vugmeyster

Deuteron Chemical Exchange Saturation Transfer for the Detection of Slow Motions in Rotating Solids

“…As demonstrated in Supplementray Figure S11, the diffusion mode alone can match the width of the narrow central region in the −6-+6 kHz range. However, it not only misses the overall outer width and shape of the pattern [which was also observed for the static data, see Supplementray Figure S5 of prior work (Vugmeyster et al, 2020)], but also broadens any traces of the coherent resonances. Supplementray Figure S12 demonstrates the alternative fits for the 10 kHz MAS rate, T 20 ms profile using three (D, k ex ) pairs, including the best-fit profile of Figure 12.…”

“…The best-fit k flip value for the 76°C data was between 9,000 and 10,000 s −1 , whereas it was 2,100 s −1 for the 55°C data (shown in Figure 8 as solid lines). These are in the range found by other techniques, particularly static 2 H CEST (Au et al, 2019;Vugmeyster et al, 2020). All the simulations of the profiles included the effect of the RF inhomogeneity of the coil with the profiles of Figure 1B the CEST profiles simulated without the effect of RF inhomogeneity overestimated the saturation for offsets for which the saturation extent was significant, thus also affecting the overall shape of the profile, not only the resulting fitted rate constant.…”

“…These discrepancies may reflect the existence of more complex ensembles of free and bound states of the N-terminal subdomain of the fibrils. A previous combined analysis of static 2 H NMR CEST, rotating frame R 1ρ rates, and QCPMG data of the N-terminal subdomain indicated a complex conformational space, corresponding to an ensemble of conformations for the free state in exchange with a single bound state (Au et al, 2019;Vugmeyster et al, 2020). The ensemble is characterized by clusters of D values around 1-3•10 6 rad 2 /s, and 1•10 8 rad 2 /s, with corresponding k ex values clustered in 0.1-1•10 5 and 1-3•10 6 s −1 .…”

“…We recently demonstrated the effectiveness of the technique for fluctuations of the anisotropic quadrupolar tensor of 2 H nuclei under static conditions in the solid state ( Vugmeyster et al, 2020 ). The goal of this work is to expand the methodology for magic angle spinning (MAS) conditions.…”

“…In particular, we apply the experiment to the model compound dimethyl-sulfone (DMS) deuterated at its two methyl groups that undergo 2-site rotameric exchange ( Frydman et al, 1994 ; Brown et al, 1996 ; Gerardy-Montouillout et al, 1996 ; Favre et al, 1998 ; Quinn and McDermott, 2012 ) and to amyloid-β fibrils (Aβ 1-40 ) with the deuterium label at a mobile methyl group of the A2 residue belonging to the disordered N-terminal domain, for which we previously determined the dynamics using static 2 H solid-state NMR techniques ( Au et al, 2019 ; Vugmeyster et al, 2019 ; Vugmeyster et al, 2020 ). The experimental work is complemented with theoretical considerations using the Liouvillian formalism ( Bain and Berno, 2011 ) and insights into the main features of the CEST profiles resulting from simple 2-site exchange simulations.…”