Effect of water/glycerol polymorphism on dynamic nuclear polarization

Abstract

A paramount feature of robust experimental methods is acquiring consistent data. However, in dynamic nuclear polarization (DNP), it has been observed that the DNP-induced NMR signal enhancement of nominally the same sample can vary between different experimental sessions. We investigated the impact of various freezing conditions on the DNP results for a standard sample, a 50/40/10 by volume d8-glycerol/D2O/H2O solution of 40 mM 4-amino TEMPO, and found that annealing the samples 10 K above the glass transition temperature (Tg) causes significant changes to the DNP profiles and enhancements compared to that in rapidly frozen samples. When varying the glycerol composition to yield a solution of 60/30/10 d8-glycerol/D2O/H2O, the DNP performance became markedly more consistent, even for samples prepared under vastly different sample freezing methods, in stark contrast with that of the 50/40/10 solution. The EPR lineshapes, Tm, and glass transition temperature, Tg, were measured under the same sample and experimental conditions as used for the DNP experiments to support the conclusion that different freezing methods change the distribution of 4-amino TEMPO radials in the 50/40/10 solution due to the formation of different polymorphs of the glass, which is mitigated in the 60/30/10 solution and is consistent with the water/glycerol vitrification literature.

Dynamic Nuclear Polarization / solid-state NMR of membranes. Thermal effects and sample geometry

“…However, it is possible that membrane phase transitions such as fluid-to-gel or other changes at low temperatures play an analogous role as the waterice transition in isotropic solvent systems. In such cases the speed of the transition can have an effect how the biradicals redistribute within the membrane interface [35,43] similar to recently published observations on water/glycerol mixtures used for DNP [66].…”

Viscosity effects on optically generated electron and nuclear spin hyperpolarization

“…28,29 Slower spin relaxation in glycerol containing solutions (up to 69% added glycerol) was previously shown to arise from an increase in the tumbling correlation time of the radical, which affects both the spin rotation and modulation of g-and hyperfine anisotropies. 29 While the reproducibility of solid-state DNP studies has been reported to be affected by polymorphism of water-glycerol solutions upon freezing to below 150 K, 30 to our knowledge there are no reports of polymorphism influencing relaxation properties at ambient temperatures. As our measurements are carried out exclusively at room temperature (B295 K), over 100 K above the glass transition temperature of the waterglycerol mixtures used and in a composition region not subject to reported liquid-liquid transitions, 31 polymorphism is not expected to affect our study.…”

Trehalose as an alternative to glycerol as a glassing agent for in vivo DNP MRI

“…In this case, longer relaxation times were associated with faster buildup kinetics, as expected for narrow linewidth radicals like OX063, where rapid spin diffusion exists and the solid effect is an important component in DNP transfer. Although not tested here, annealing may prove an efficient method for reducing inhomogeneities during the freezing process 7,30‐32 …”