Nuclear magnetic resonance studies of DNP-ready trehalose obtained by solid state mechanochemical amorphization

Abstract

(1)H nuclear spin-lattice relaxation and Dynamic Nuclear Polarization (DNP) have been studied in amorphous samples of trehalose sugar doped with TEMPO radicals by means of mechanical milling, in the 1.6-4.2 K temperature range. The radical concentration was varied between 0.34 and 0.81%. The highest polarization of 15% at 1.6 K, observed in the sample with concentration 0.50%, is of the same order of magnitude of that reported in standard frozen solutions with TEMPO. The temperature and concentration dependence of the spin-lattice relaxation rate 1/T1, dominated by the coupling with the electron spins, were found to follow power laws with an exponent close to 3 in all samples. The observed proportionality between 1/T1 and the polarization rate 1/Tpol, with a coefficient related to the electron polarization, is consistent with the presence of Thermal Mixing (TM) and a good contact between the nuclear and the electron spins. At high electron concentration additional relaxation channels causing a decrease in the nuclear polarization must be considered. These results provide further support for a more extensive use of amorphous DNP-ready samples, obtained by means of comilling, in dissolution DNP experiments and possibly for in vivo metabolic imaging.

Electron decoupling with cross polarization and dynamic nuclear polarization below 6 K

“…Remarkably, the polarization build-up time (T 1DNP , 1 H) was only 1.3 s (Figure 2c), making the optimal recycle delay to maximize signal-to-noise per unit time only 1.6 s (1.26*T 1 ). [26] Polarizing agents also yield short polarization build-up times at 90 K (T 1DNP = 1.3 s), [13,27,28] but here we demonstrate similar build-up times below 6 K. While T 1DNP time constants in static samples at 1.2 K can be several minutes to hours, [29,30] these short recovery time constants are comparable to those observed at room temperature. [31] Therefore, MAS DNP below 6 K delivers unprecedented NMR sensitivity per unit time.…”

Dynamic Nuclear Polarization of β-Cyclodextrin Macromolecules

“…On the one hand, the similar exponents derived for 1/ T 1n and 1/ T POL are consistent with the thermal mixing model. It is remarked that, the values of exponent b for both rates are different from those found in other DNP substrates, where the nuclear spin-lattice relaxation rate scaled as C 3 . To justify the values of b , one can refer to eq and notice that C appears twice on the right side of the equation, one time as N e / N n and the other time hidden in 1/ T 1e .…”

Proton and Carbon-13 Dynamic Nuclear Polarization of Methylated β-Cyclodextrins

“…On the other hand, molecular imaging aims at detecting the signal arising from selected molecules, such as the metabolites, whose concentration in the human body is 10 –4 times lower than that of water. Accordingly, one can track the real time evolution of these molecules during physiological processes. − Although it is possible to efficiently hyperpolarize hydrogen in a wide range of organic molecules, − 1 H molecular imaging is often hindered by the very short nuclear spin–lattice relaxation times (<1 s) that prevent protons from retaining the hyperpolarization at room temperature. Hence, great attention has been devoted to the possibility of hyperpolarizing 13 C nuclei, whose concentration in organic molecules can be increased through isotopic substitution.…”