Sensitivity Enhancement by Progressive Saturation of the Proton Reservoir: A Solid-State NMR Analogue of Chemical Exchange Saturation Transfer

Abstract

Chemical exchange saturation transfer (CEST) enhances solution-state NMR signals of labile and otherwise invisible chemical sites, by indirectly detecting their signatures as a highly magnified saturation of an abundant resonancefor instance, the 1H resonance of water. Stimulated by this sensitivity magnification, this study presents PROgressive Saturation of the Proton Reservoir (PROSPR), a method for enhancing the NMR sensitivity of dilute heteronuclei in static solids. PROSPR aims at using these heteronuclei to progressively deplete the abundant 1H polarization found in most organic and several inorganic solids, and implements this 1H signal depletion in a manner that reflects the spectral intensities of the heteronuclei as a function of their chemical shifts or quadrupolar offsets. To achieve this, PROSPR uses a looped cross-polarization scheme that repeatedly depletes 1H–1H local dipolar order and then relays this saturation throughout the full 1H reservoir via spin-diffusion processes that act as analogues of chemical exchanges in the CEST experiment. Repeating this cross-polarization/spin-diffusion procedure multiple times results in an effective magnification of each heteronucleus’s response that, when repeated in a frequency-stepped fashion, indirectly maps their NMR spectrum as sizable attenuations of the abundant 1H NMR signal. Experimental PROSPR examples demonstrate that, in this fashion, faithful wideline NMR spectra can be obtained. These 1H-detected heteronuclear NMR spectra can have their sensitivity enhanced by orders of magnitude in comparison to optimized direct-detect experiments targeting unreceptive nuclei at low natural abundance, using modest hardware requirements and conventional NMR equipment at room temperature.

Molecular and Electronic Structure of Isolated Platinum Sites Enabled by the Expedient Measurement of195Pt Chemical Shift Anisotropy

“…By plotting the 1D NMR signal intensities as a function of the 195 Pt pulse offset, the MAS 195 Pt NMR spectrum can be reconstructed (Figures and S2–S4). This method is analogous to previous work that utilized indirect detection schemes such as TRAPDOR, , REAPDOR, RESPDOR, , or PROSPR where the signal dephasing of the detected nucleus at different transmitter offsets was used to reconstruct the NMR spectrum of the indirectly detected spin.…”

“…75 These multiple ). This method is analogous to previous work that utilized indirect detection schemes such as TRAPDOR, 76,77 REAPDOR, 78 RESPDOR, 79,80 or PROSPR 81 where the signal dephasing of the detected nucleus at different transmitter offsets was used to reconstruct the NMR spectrum of the indirectly detected spin.…”

Recent progress in solid-state NMR of spin-½ low-γ nuclei applied to inorganic materials

“…This has been termed PROgressive Saturation of the Proton Reservoir (PROSPR) and has been demonstrated on some low-γ quadrupolar nuclei. 44…”

“…This has been termed PROgressive Saturation of the Proton Reservoir (PROSPR) and has been demonstrated on some low-g quadrupolar nuclei. 44 2.1.3 Dynamic nuclear polarisation. The progress that different approaches have made can be gauged by the rate at which they appear in the literature and how their referencing in reviews change.…”

Chlorination of Hydrogenated Silicon Nanosheets Revealed by 1H{35Cl} and 29Si{35Cl} Solid-State NMR Spectroscopy

“…24 With the failure of the 35 Cl WURST-CPMG experiments on the Si-NS a different strategy was required to measure the 35 Cl CQ. Recently, we described techniques to indirectly map out wideline MAS 195 Pt NMR spectra through a spy nucleus such as 1 H. 51 This work was directly inspired by similar dipolar dephasing experiments that were used to map out the quadrupolar powder patterns of 27 Al and 14 N. [52][53][54][55][56] Motivated by these prior studies, we performed 1 H{ 35 Cl} DE-RESPDOR experiments on Si-NS where the offset of the 35 Cl saturation pulse offset was varied in steps of 500 kHz over a range of +3 to -4 MHz (Figure 2C). Plotting the signal dephasing as a function of the 35 Cl saturation pulse offset enables the reconstruction of the 35 Cl central transition (CT) quadrupolar powder pattern for the Si-NS (Figure 2C).…”