Hybridizing cross-polarization with NOE or refocused-INEPT enhances the sensitivity of MAS NMR spectroscopy

Abstract

Heteronuclear cross polarization (CP) has been commonly used to enhance the sensitivity of dilute low-γ nuclei in almost all solid-state NMR experiments. However, CP relies on heteronuclear dipolar couplings, and therefore the magnetization transfer efficiency becomes inefficient when the dipolar couplings are weak, as is often the case for mobile components in solids. Here, we demonstrate methods that combine CP with heteronuclear Overhauser effect (referred to as CP-NOE) or with refocused-INEPT (referred to as CP-RINEPT) to overcome the efficiency limitation of CP and enhance the signal-to-noise ratio (S/N) for mobile components. Our experimental results reveal that, compared to the conventional CP, significant S/N ratio enhancement can be achieved for resonances originating from mobile components, whereas the resonance signals associated with rigid groups are not significantly affected due to their long spin-lattice relaxation times. In fact, the S/N enhancement factor is also dependent on the temperature, CP contact time as well as on the system under investigation. Furthermore, we also demonstrate that CP-RINEPT experiment can be successfully employed to independently detect mobile and rigid signals in a single experiment without affecting the data collection time. However, the resolution of CP spectrum obtained from the CP-RINEPT experiment could be slightly compromised by the mandatory use of continuous wave (CW) decoupling during the acquisition of signals from rigid components. In addition, CP-RINEPT experiment can be used for spectral editing utilizing the difference in dynamics of different regions of a molecule and/or different components present in the sample, and could also be useful for the assignment of resonances from mobile components in poorly resolved spectra. Therefore, we believe that the proposed approaches are beneficial for the structural characterization of multiphase and heterogeneous systems, and could be used as a building block in multidimensional solid-state NMR experiments.

Engineering l-asparaginase for spontaneous formation of calcium phosphate bioinspired microreactors

“…One needs to consider the hypothesis of a substantial water-protein magnetization transfer (either through intermolecular NOE, which is prevailing at high spinning rates and low temperature, or through chemical exchange followed by spin diffusion, which is most probably dominant under the present experimental conditions 62 ) at the time of the NOE mixing, plus the contribution due to direct 13 C excitations which is intrinsic for the pulse sequence applied. 61 Either of the two mechanisms does not detract from the idea, but rather corroborates it, that a large share of the protein is sizably exposed to water.…”

“…8, middle trace) that are both increased when ramp-CP is hybridized with the nuclear Overhauser effect (NOE, Fig. 8, top trace): 61 this sequence results in the appearance of a larger number of signals. Most of these additional peaks appear in the aliphatic region (0-75 ppm) and some peaks from aromatic side chains also appear (110-140 ppm).…”

Exploiting Heterogeneous Time Scale of Dynamics to Enhance 2D HETCOR Solid-State NMR Sensitivity

“…PMMA is a semi-crystalline polymer with glass transition (Tg) and melting (Tm) temperatures both above 100 o C, whereas PB is a viscous liquid with a Tg of ~ -90 o C. Therefore, in the PMMA/PB blend, PMMA is considered as the rigid and PB is the mobile components. It was previously shown that in such a polymer blend system with a significant mobility contrast for the components, the 13 C signals of PB cannot be detected by CP, [55] because the 13 C-1 H dipolar couplings have been completely averaged out by the fast molecular motion of PB at room temperature. However, the 13 In the 1 H-detected 2D Mobile-HETCOR spectra (Figure 6b) obtained using the 1 H-detected HETCOR pulse sequence (Figure 2b), signals from the minor components in PB (vinyl-1,2-units, end-groups, etc.)…”

1 H-detected MAS solid-state NMR experiments enable the simultaneous mapping of rigid and dynamic domains of membrane proteins

“…[23, 41, 42] Recently, Ramamoorthy and coworkers have utilized the hybridization of CP and INEPT pulse sequences for sensitivity enhancement of oriented as well as MAS samples. [43, 44]…”