Tailored Microstructured Hyperpolarizing Matrices for Optimal Magnetic Resonance Imaging

Rapid and Simple 13C-Hyperpolarization by 1H Dissolution Dynamic Nuclear Polarization Followed by an Inline Magnetic Field Inversion

Dissolution dynamic nuclear polarization (dDNP) is a method of choice for preparing hyperpolarized 13C metabolites such as 1-13C-pyruvate used for in vivo applications, including the real-time monitoring of cancer cell metabolism in human patients. The approach consists of transferring the high polarization of electron spins to nuclear spins via microwave irradiation at low temperatures (1.0-1.5 K) and moderate magnetic fields (3.3-7 T). The solid sample is then dissolved and transferred to an NMR spectrometer or MRI scanner for detection in the liquid state. Common dDNP protocols use direct hyperpolarization of 13C spins reaching polarizations of >50% in ∼1-2 h. Alternatively, 1H spins are polarized before transferring their polarization to 13C spins using cross-polarization, reaching polarization levels similar to those of direct DNP in only ∼20 min. However, it relies on more complex instrumentation, requiring highly skilled personnel. Here, we explore an alternative route using 1H dDNP followed by inline adiabatic magnetic field inversion in the liquid state during the transfer. 1H polarizations of >70% in the solid state are obtained in ∼5-10 min. As the hyperpolarized sample travels from the dDNP polarizer to the NMR spectrometer, it goes through a field inversion chamber, which causes the 1H → 13C polarization transfer. This transfer is made possible by the J-coupling between the heteronuclei, which mixes the Zeeman states at zero-field and causes an antilevel crossing. We report liquid-state 13C polarization up to ∼17% for 3-13C-pyruvate and 13C-formate. The instrumentation needed to perform this experiment in addition to a conventional dDNP polarizer is simple and readily assembled.

Fine optimization of a dissolution dynamic nuclear polarization experimental setting for 13C NMR of metabolic samples

NMR-based analysis of metabolite mixtures provides crucial information on biological systems but mostly relies on 1D 1 H experiments for maximizing sensitivity. However, strong peak overlap of 1 H spectra often is a limitation for the analysis of inherently complex biological mixtures. Dissolution dynamic nuclear polarization (d-DNP) improves NMR sensitivity by several orders of magnitude, which enables 13 C NMR-based analysis of metabolites at natural abundance. We have recently demonstrated the successful introduction of d-DNP into a full untargeted metabolomics workflow applied to the study of plant metabolism. Here we describe the systematic optimization of d-DNP experimental settings for experiments at natural 13 C abundance and show how the resolution, sensitivity, and ultimately the number of detectable signals improve as a result. We have systematically optimized the parameters involved (in a semi-automated prototype d-DNP system, from sample preparation to signal detection, aiming at providing an optimization guide for potential users of such a system, who may not be experts in instrumental development). The optimization procedure makes it possible to detect previously inaccessible protonated 13 C signals of metabolites at natural abundance with at least 4 times improved line shape and a high repeatability compared to a previously reported d-DNP-enhanced untargeted metabolomic study. This extends the application scope of hyperpolarized 13 C NMR at natural abundance and paves the way to a more general use of DNP-hyperpolarized NMR in metabolomics studies.

Triplet Dynamic Nuclear Polarization of Guest Molecules through Induced Fit in a Flexible Metal-Organic Framework

Dynamic nuclear polarization utilizing photoexcited triplet electrons (triplet-DNP) has great potential for room-temperature hyperpolarization of nuclear spins. However, the polarization transfer to molecules of interest remains a challenge due to the fast spin relaxation and weak interaction with target molecules at room temperature in conventional host materials. Here, we demonstrate the first example of DNP of guest molecules in a porous material at around room temperature by utilizing the induced-fit-type structural transformation of a crystalline yet flexible metal-organic framework (MOF). In contrast to the usual hosts, 1 H spin-lattice relaxation time becomes longer by accommodating a pharmaceutical model target 5-fluorouracil as the flexible MOF changes its structure upon guest accommodation to maximize the host-guest interactions. Combined with triplet-DNP and cross-polarization (CP), this system realizes an enhanced 19 F-NMR signal of guest target molecules.

Protonation tuned dipolar order mediated 1H→13C cross-polarization for dissolution-dynamic nuclear polarization experiments

A strategy of dipolar order mediated nuclear spin polarization transfer has recently been combined with dissolution-dynamic nuclear polarization (dDNP) and improved by employing optimized shaped radiofrequency pulses and suitable molecular modifications. In the context of dDNP experiments, this offers a promising means of transferring polarization from high-gamma ¹H spins to insensitive ¹³C spins with lower peak power and lower energy compared with state-of-the-art cross-polarization schemes. The role of local molecular groups and the glassing matrix protonation level are both postulated to play a key role in the polarization transfer pathway via an intermediary reservoir of dipolar spin order. To gain appreciation of the mechanisms involved in the dipolar order mediated polarization transfer under dDNP conditions, we investigate herein the influence of the pivotal characteristics of the sample makeup: (i) revising the protonation level for the constituents of the DNP glass; and (ii) utilizing deuterated molecular derivatives. Experimental demonstrations are presented for the case of [1-¹³C]sodium acetate. We find that the proton sample molarity has a large impact on both the optimal parameters and the performance of the dipolar order mediated cross-polarization sequence, with the ¹³C signal build-up time drastically shortened in the case of high solvent protonation levels. In the case of a deuterated molecular derivative, we observe that the nearby ²H substituted methyl group is deleterious to the ¹H→¹³C transfer phenomenon (particularly at low levels of sample protonation). Overall, increased solvent protonation makes the dipolar order governed polarization transfer significantly faster and more efficient. This study sheds light on the influential sample formulation traits which govern the dipolar order-controlled transfer of polarization and indicates that the polarization transfer efficiencies of deuterated molecules can be boosted and reach high performances simply by adequate solvent protonation.

Porous functionalized polymers enable generating and transporting hyperpolarized mixtures of metabolites

Hyperpolarization by dissolution dynamic nuclear polarization (dDNP) has enabled promising applications in spectroscopy and imaging, but remains poorly widespread due to experimental complexity. Broad democratization of dDNP could be realized by remote preparation and distribution of hyperpolarized samples from dedicated facilities. Here we show the synthesis of hyperpolarizing polymers (HYPOPs) that can generate radical- and contaminant-free hyperpolarized samples within minutes with lifetimes exceeding hours in the solid state. HYPOPs feature tunable macroporous porosity, with porous volumes up to 80% and concentration of nitroxide radicals grafted in the bulk matrix up to 285 μmol g-1. Analytes can be efficiently impregnated as aqueous/alcoholic solutions and hyperpolarized up to P(13C) = 25% within 8 min, through the combination of 1H spin diffusion and 1H → 13C cross polarization. Solutions of 13C-analytes of biological interest hyperpolarized in HYPOPs display a very long solid-state 13C relaxation times of 5.7 h at 3.8 K, thus prefiguring transportation over long distances.

Pulse sequence and sample formulation optimization for dipolar order mediated 1H→13C cross-polarization

We have recently demonstrated the use of contactless radiofrequency pulse sequences under dissolution-dynamic nuclear polarization conditions as an attractive way of transferring polarization from sensitive 1H spins to insensitive 13C spins with low peak radiofrequency pulse powers and energies via a reservoir of dipolar order. However, many factors remain to be investigated and optimized to enable the full potential of this polarization transfer process. We demonstrate herein the optimization of several key factors by: (i) implementing more efficient shaped radiofrequency pulses; (ii) adapting 13C spin labelling; and (iii) avoiding methyl group relaxation sinks. Experimental demonstrations are presented for the case of [1-13C]sodium acetate and other relevant molecular candidates. By employing the range of approaches set out above, polarization transfer using the dipolar order mediated cross-polarization radiofrequency pulse sequence is improved by factors approaching ∼1.65 compared with previous results. Dipolar order mediated 1H→13C polarization transfer efficiencies reaching ∼76% were achieved using significantly reduced peak radiofrequency pulse powers relative to the performance of highly sophisticated state-of-the-art cross-polarization methods, indicating 13C nuclear spin polarization levels on the order of ∼32.1% after 10 minutes of 1H DNP. The approach does not require extensive pulse sequence optimization procedures and can easily accommodate high concentrations of 13C-labelled molecules.

Efficient CO2 Hydrogenation to Formate with Immobilized Ir-Catalysts Based on Mesoporous Silica Beads

The Nozaki Ir-based CO₂ hydrogenation catalyst was successfully immobilized on post-functionalized silica beads (d=200 μm) through click chemistry. This material hydrogenates CO₂ into formic acid with turnover numbers reaching 2.8×10⁴ in a batch reactor within 24 hours, paving the way towards the design of efficient heterogeneous catalysts for this transformation.