International audienceHyperpolarization by Dissolution Dynamic Nuclear Polarization is usually achieved by monochromatic microwave irradiation of the ESR spectrum of free radicals embedded in glasses at 1.2 K and 3.35 T. Hovav et al. (2014) have recently shown that by using frequency-modulated (rather than monochromatic) microwave irradiation one can improve DNP at 3.35 T in the temperature range 10-50 K. We show in this Letter that this is also true under Dissolution-DNP conditions at 1.2 K and 6.7 T. We demonstrate the many virtues of using frequency-modulated microwave irradiation: higher polarizations, faster buildup rates, lower radical concentrations, less paramagnetic broadening, more efficient cross-polarization, and less critical frequency adjustments
Hyperpolarization of substrates for magnetic resonance spectroscopy (MRS) and imaging (MRI) by dissolution dynamic nuclear polarization (D-DNP) usually involves saturating the ESR transitions of polarizing agents (PAs; e.g., persistent radicals embedded in frozen glassy matrices). This approach has shown enormous potential to achieve greatly enhanced nuclear spin polarization, but the presence of PAs and/or glassing agents in the sample after dissolution can raise concerns for in vivo MRI applications, such as perturbing molecular interactions, and may induce the erosion of hyperpolarization in spectroscopy and MRI. We show that D-DNP can be performed efficiently with hybrid polarizing solids (HYPSOs) with 2,2,6,6-tetramethyl-piperidine-1-oxyl radicals incorporated in a mesostructured silica material and homogeneously distributed along its pore channels. The powder is wetted with a solution containing molecules of interest (for example, metabolites for MRS or MRI) to fill the pore channels (incipient wetness impregnation), and DNP is performed at low temperatures in a very efficient manner. This approach allows high polarization without the need for glass-forming agents and is applicable to a broad range of substrates, including peptides and metabolites. During dissolution, HYPSO is physically retained by simple filtration in the cryostat of the DNP polarizer, and a pure hyperpolarized solution is collected within a few seconds. The resulting solution contains the pure substrate, is free from any paramagnetic or other pollutants, and is ready for in vivo infusion.D-DNP | NMR signal enhancement | molecular imaging | mesostructured hybrid silica | porous materials D issolution dynamic nuclear polarization (D-DNP) (1, 2) usually requires freezing molecules of interest, such as metabolites, together with persistent free radicals often called polarizing agents (PA) in a glassy matrix at very low temperatures (1 < T < 4 K), so that their nuclear spin polarization can be enhanced by up to four to five orders of magnitude. Such enhancements are achieved by saturating the ESR transitions of the PAs. D-DNP is generally performed in moderate magnetic fields (B 0 = 3.35 or in this study, 6.7 T) and followed by rapid dissolution of the frozen sample with a burst of superheated water to give highly polarized solutions. Applications include detection of intermediates in chemical reactions (3-5), protein folding in real time (6), and detection of cancer by monitoring abnormal rates of metabolic reactions in humans (7). PAs with narrow EPR lines, such as trityl radicals, are usually used for the direct polarization of 13 C nuclei (2). In practice, polarizations P( 13 C) of 20% or higher can be obtained after dissolution. We have recently shown that DNP of 13 C can be significantly accelerated by combining increased magnetic fields with polarization of 1 H rather than 13 C [using nitroxide radicals, such as 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO), with broader ESR lines than trityl radicals] followed by Hartmann-Hahn C. In ...
Hyperpolarization is generated by dissolution dynamic nuclear polarization (d-DNP) using a polymer-based polarizing agent dubbed FLAP (filterable labeled agents for polarization). It consists of a thermo-responsive poly(N-isopropylacrylamide), also known as pNiPAM-COOH, labeled with nitroxide radicals. The polymer powder is impregnated with an arbitrary solution of interest and frozen as is. Dissolution is followed by a simple filtration, leading to hyperpolarized solutions free from any contaminants. We demonstrated the use of FLAP to hyperpolarize partially deuterated water up to P((1) H)=6 % with a long relaxation T1 >36 s characteristic of high purity. Water hyperpolarization can be transferred to drugs, metabolites, or proteins that are waiting in an NMR spectrometer, either by exchange of labile protons or through intermolecular Overhauser effects. We also show that FLAPs are suitable polarizing agents for (13) C-labeled metabolites such as pyruvate, acetate, and alanine.
Theory of long-lived nuclear spin states in methyl groups and quantum-rotor induced polarisation
Nuclear spin hyperpolarization of 13C-labelled metabolites by dissolution dynamic nuclear polarization can enhance the NMR signals of metabolites by several orders of magnitude, which has enabled in vivo metabolic imaging by MRI. However, because of the short lifetime of the hyperpolarized magnetization (typically <1 min), the polarization process must be carried out close to the point of use. Here we introduce a concept that markedly extends hyperpolarization lifetimes and enables the transportation of hyperpolarized metabolites. The hyperpolarized sample can thus be removed from the polarizer and stored or transported for use at remote MRI or NMR sites. We show that hyperpolarization in alanine and glycine survives 16 h storage and transport, maintaining overall polarization enhancements of up to three orders of magnitude.
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