Cryogenic and Dissolution DNP NMR on γ-Irradiated Organic Molecules.

Nuclear magnetic resonance (NMR) plays a central role in the elucidation of chemical structures but is often limited by low sensitivity. Dissolution dynamic nuclear polarization (dDNP) emerges as a transformative methodology for both solution-state NMR and metabolic NMR imaging, which could overcome this limitation. Typically, dDNP relies on combining a stable radical with the analyte within a uniform glass under cryogenic conditions. The electron polarization is then transferred through microwave irradiation to the nuclei. The present study explores the use of radicals introduced via γ-irradiation, as bearers of the electron spins that will enhance 1H or 13C nuclides. 1H solid-state NMR spectra of γ-irradiated powders at 1-5 K revealed, upon microwave irradiation, signal enhancements that, in general, were higher than those achieved through conventional glass-based DNP. Transfer of these samples to a solution-state NMR spectrometer via a rapid dissolution driven by a superheated water provided significant enhancements of solution-state 1H NMR signals. Enhancements of 13C signals in the γ-irradiated solids were more modest, as a combined consequence of a low radical concentration and of the dilute concentration of 13C in the natural abundant samples examined. Nevertheless, ca. 700-800-fold enhancements in 13C solution NMR spectra of certain sites recorded at 11.7 T could still be achieved. A total disappearance of the radicals upon performing a dDNP-like aqueous dissolution and a high stability of the samples were found. Overall, the study showcases the advantages and limitations of γ-irradiated radicals as candidates for advancing spectroscopic dDNP-enhanced NMR.

New and rapid pulse sequences for two-dimensional D-T1 correlation measurements.

Longitudinal relaxation time (T1), transverse relaxation time (T2) and diffusion coefficient (D) values have been widely used for the characterizations of materials using low field Time Domain Nuclear Magnetic Resonance (TD-NMR). Each parameter can be determined using one-dimensional techniques or their values and correlations by multi-dimensional experiments such as T1-T2, D-T2, and T1-D-T2. In this work, we studied four D-T1 sequences for TD-NMR combining Stejskal-Tanner Pulse Gradient Spin Echo (PGSE) diffusion measurement with Inversion-Recovery (IR), Saturation-Recovery (SR), Small-Angle Continuous Wave Free Precession (CWFP-T1) and Small-Angle Flip-Flop (SAFF) for T1 measurement. The results show that rapid D-T1 measurements can be obtained with single shot CWFP-T1 and SAFF sequences. The two sequences were two and eight time fast than sequences based on SR and IR, respectively. Although the two fast sequences yield low signal-to-noise ratio signal, they can be as fast as the traditional D-T2 experiment, or even faster, because it is not necessary to wait a recycle delay of 5 T1. Another advantage of the CWFP-T1 and SAFF methods, when compared to the one based on SR or CPMG (for D-T2) are the low specific absorption rate (SAR) of these sequences due the low flip angles in the sequences, that reduces the sample heating problem. These sequences were initially studied using phantom samples. They also were used to study plant tissues to observe the anisotropic diffusion in asparagus. Therefore, they can be useful methods for practical application in TD-NMR.

Using T1 as a direct detection dimension in two-dimensional time-domain NMR experiments using CWFP regime.

The transverse relaxation time (T2), measured with Carr-Purcell-Meiboom-Gill (CPMG) sequence, has been widely used to obtain the direct dimension data in two-dimension time domain NMR (2D TD-NMR). In this paper we are demonstrating that Continuous Wave Free Precession sequence, with low flip angle (CWFP-T1), can be an alternative to CPMG as direct detection dimension. CWFP-T1 is a fast single shot sequence, like CPMG, and yields an exponential signal governed predominantly by the longitudinal (T1) relaxation time. To obtain the correlations between T1 and T2 (T1-T2 maps) we are proposing the use of CPMG-CWFP-T1 pulse sequence. In this sequence CPMG encodes T2 information (indirect dimension) that modulates the CWFP-T1 (direct dimension) signal amplitudes. CPMG-CWFP-T1 experiments were compared with classical 2D sequences such as Saturation-Recovery-CPMG (SR-CPMG) and Inversion-Recovery-CPMG (IR-CPMG) sequence and yields similar results in phantom sample. The experimental time for the 2D sequences, using single scan, shows that SR-CPMG ≤ CPMG-CWFP-T1 < IR-CPMG. Experimental and simulated results demonstrated that 2D-CPMG-CWFP-T1 maps have higher resolution in T1 dimension than the techniques that uses CPMG as direct dimension. CPMG-CWFP-T1 sequence was also applied to study beef samples, and 2D maps showed higher resolution in the two fat signals than the classical IR-CPMG method.

T2-Filtered T2 - T2 Exchange NMR.

This work introduces an alternative way to perform the T2 - T2 Exchange NMR experiment. Rather than varying the number of π pulses in the first CPMG cycle of the T2 - T2 Exchange NMR pulse sequence, as used to obtain the 2D correlation maps, it is fixed and small enough to act as a short T2-filter. By varying the storage time, a set of 1D measurements of T2 distributions can be obtained to reveal the effects of the migration dynamics combined with relaxation effects. This significantly reduces the required time to perform the experiment, allowing a more in-depth study of exchange dynamics and relaxation processes with improved signal-to-noise ratio. These aspects stand as basis of this novel experiment, T2-Filtered T2 - T2 Exchange NMR or simply T2 F-TREx.