15N-Azides as practical and effective tags for developing long-lived hyperpolarized agents

Azide moieties, unique linear species containing three nitrogen atoms, represent an attractive class of molecular tag for hyperpolarized magnetic resonance imaging (HP-MRI). Here we demonstrate (¹⁵N)₃-azide-containing molecules exhibit long-lasting hyperpolarization lifetimes up to 9.8 min at 1 T with remarkably high polarization levels up to 11.6% in water, thus establishing (¹⁵N)₃-azide as a powerful spin storage for hyperpolarization. A single (¹⁵N)-labeled azide has also been examined as an effective alternative tag with long-lived hyperpolarization. A variety of biologically important molecules are studied in this work, including choline, glucose, amino acid, and drug derivatives, demonstrating great potential of ¹⁵N-labeled azides as universal hyperpolarized tags for nuclear magnetic resonance imaging applications.

This work demonstrates that ¹⁵N-labeled azides are practical and effective tags for developing long-lived hyperpolarized MRI agents and can offer hyperpolarization lifetimes up to 9.8 min at 1 T and high polarization levels up to 11.6% in water.     

A Nuclear Singlet Lifetime of More than One Hour in Room‐Temperature Solution

Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) are supremely important techniques with numerous applications in almost all branches of science. However, until recently, NMR methodology was limited by the time constant T₁ for the decay of nuclear spin magnetization through contact with the thermal molecular environment. Long‐lived states, which are correlated quantum states of multiple nuclei, have decay time constants that may exceed T₁ by large factors. Here we demonstrate a nuclear long‐lived state comprising two ¹³C nuclei with a lifetime exceeding one hour in room‐temperature solution, which is around 50 times longer than T₁. This behavior is well‐predicted by a combination of quantum theory, molecular dynamics, and quantum chemistry. Such ultra‐long‐lived states are expected to be useful for the transport and application of nuclear hyperpolarization, which leads to NMR and MRI signals enhanced by up to five orders of magnitude.     

Diazirines as Potential Molecular Imaging Tags: Probing the Requirements for Efficient and Long‐Lived SABRE‐Induced Hyperpolarization

Diazirines are an attractive class of potential molecular tags for magnetic resonance imaging owing to their biocompatibility and ease of incorporation into a large variety of molecules. As recently reported, ¹⁵N₂‐diazirine can be hyperpolarized by the SABRE‐SHEATH method, sustaining both singlet and magnetization states, thus offering a path to long‐lived polarization storage. Herein, we show the generality of this approach by illustrating that the diazirine tag alone is sufficient for achieving excellent signal enhancements with long‐lasting polarization. Our investigations reveal the critical role of Lewis basic additives, including water, on achieving SABRE‐promoted hyperpolarization. The application of this strategy to a ¹⁵N₂‐diazirine‐containing choline derivative demonstrates the potential of ¹⁵N₂‐diazirines as molecular imaging tags for biomedical applications.     

Terminal Diazirines Enable Reverse Polarization Transfer from 15N2 Singlets

Diazirine moieties are chemically stable and have been incorporated into biomolecules without impediment of biological activity. The ¹⁵N₂ labeled diazirines are appealing motifs for hyperpolarization supporting relaxation protected states with long‐lived lifetimes. The (‐CH¹⁵N₂) diazirine groups investigated here are analogues to methyl groups, which provides the opportunity to transfer polarization stored on a relaxation protected (‐CH¹⁵N₂) moiety to ¹H, thus combining the advantages of long lifetimes of ¹⁵N polarization with superior sensitivity of ¹H detection. Despite the proximity of ¹H to ¹⁵N nuclei in the diazirine moiety, ¹⁵N T₁ times of up to (4.6±0.4) min and singlet lifetimes T_s of up to (17.5±3.8) min are observed. Furthermore, we found terminal diazirines to support hyperpolarized ¹H₂ singlet states in CH₂ groups of chiral molecules. The singlet lifetime of ¹H singlets is up to (9.2±1.8) min, thus exceeding ¹H T₁ relaxation time (at 8.45 T) by a factor of ≈100.     

Long‐Lived States of Magnetically Equivalent Spins Populated by Dissolution‐DNP and Revealed by Enzymatic Reactions

Hyperpolarization by dissolution dynamic nuclear polarization (D ‐DNP) offers a way of enhancing NMR signals by up to five orders of magnitude in metabolites and other small molecules. Nevertheless, the lifetime of hyperpolarization is inexorably limited, as it decays toward thermal equilibrium with the nuclear spin‐lattice relaxation time. This lifetime can be extended by storing the hyperpolarization in the form of long‐lived states (LLS) that are immune to most dominant relaxation mechanisms. Levitt and co‐workers have shown how LLS can be prepared for a pair of inequivalent spins by D ‐DNP. Here, we demonstrate that this approach can also be applied to magnetically equivalent pairs of spins such as the two protons of fumarate, which can have very long LLS lifetimes. As in the case of para‐hydrogen, these hyperpolarized equivalent LLS (HELLS) are not magnetically active. However, a chemical reaction such as the enzymatic conversion of fumarate into malate can break the magnetic equivalence and reveal intense NMR signals.     

Signal Amplification by Reversible Exchange (SABRE): From Discovery to Diagnosis

Signal amplification by reversible exchange (SABRE) turns typically weak magnetic resonance responses into strong signals making previously impractical measurements possible. This technique has gained significant popularity because of its speed and simplicity. This Minireview tracks the development of SABRE from the initial hyperpolarization of pyridine in 2009 to the point in which 50 % ¹H polarization levels have been achieved in a di‐deuterio‐nicotinate, a key step in the pathway to potential clinical use. Simple routes to highly efficient ¹⁵N hyperpolarization and the creation of hyperpolarized long‐lived magnetic states are illustrated. To conclude, we describe how the recently reported SABRE‐RELAY approach offers a route for parahydrogen to hyperpolarize a much wider array of molecular scaffolds, such as amides, alcohols, carboxylic acids, and phosphates, than was previously thought possible. We predict that collectively these developments ensure that SABRE will significantly impact on both chemical analysis and the diagnosis of disease in the future.     

Continuous Re‐hyperpolarization of Nuclear Spins Using Parahydrogen: Theory and Experiment

The continuous re‐hyperpolarization of nuclear spins in the liquid state by means of parahydrogen (para‐H₂) and chemical exchange at low magnetic fields was recently discovered and offers intriguing perspectives for many varieties of magnetic resonance. In this contribution, we provide a theoretical assessment of this effect and compare the results to experimental data. A distinct distribution of polarization is found, which shares some features with experimental data and, interestingly, does not directly correspond to the loss of the singlet order of para‐H₂. We derived expressions for the magnetic field and para‐H₂–substrate interaction time, for which the polarization transfer is maximal. This work sheds light onto the effect of continuous hyperpolarization and elucidates the underlying mechanism, which may facilitate the development of an optimized catalyst. As an application, continuous hyperpolarization may enable highly sensitive nuclear magnetic resonance at very low magnetic fields, for example, for the cost‐efficient screening of drugs.     

Direct Monitoring of γ‐Glutamyl Transpeptidase Activity In Vivo Using a Hyperpolarized 13C‐Labeled Molecular Probe

The γ‐glutamyl transpeptidase (GGT) enzyme plays a central role in glutathione homeostasis. Direct detection of GGT activity could provide critical information for the diagnosis of several pathologies. We propose a new molecular probe, γ‐Glu‐[1‐¹³C]Gly, for monitoring GGT activity in vivo by hyperpolarized (HP) ¹³C magnetic resonance (MR). The properties of γ‐Glu‐[1‐¹³C]Gly are suitable for in vivo HP ¹³C metabolic analysis since the chemical shift between γ‐Glu‐[1‐¹³C]Gly and its metabolic product, [1‐¹³C]Gly, is large (4.3 ppm) and the T₁ of both compounds is relatively long (30 s and 45 s, respectively, in H₂O at 9.4 T). We also demonstrate that γ‐Glu‐[1‐¹³C]Gly is highly sensitive to in vivo modulation of GGT activity induced by the inhibitor acivicin.     

In Situ and Ex Situ Low‐Field NMR Spectroscopy and MRI Endowed by SABRE Hyperpolarization

By using 5.75 and 47.5 mT nuclear magnetic resonance (NMR) spectroscopy, up to 10⁵‐fold sensitivity enhancement through signal amplification by reversible exchange (SABRE) was enabled, and subsecond temporal resolution was used to monitor an exchange reaction that resulted in the buildup and decay of hyperpolarized species after parahydrogen bubbling. We demonstrated the high‐resolution low‐field proton magnetic resonance imaging (MRI) of pyridine in a 47.5 mT magnetic field endowed by SABRE. Molecular imaging (i.e. imaging of dilute hyperpolarized substances rather than the bulk medium) was conducted in two regimes: in situ real‐time MRI of the reaction mixture (in which pyridine was hyperpolarized), and ex situ MRI (in which hyperpolarization decays) of the liquid hyperpolarized product. Low‐field (milli‐Tesla range, e.g. 5.75 and 47.5 mT used in this study) parahydrogen‐enhanced NMR and MRI, which are free from the limitations of high‐field magnetic resonance (including susceptibility‐induced gradients of the static magnetic field at phase interfaces), potentially enables new imaging applications as well as differentiation of hyperpolarized chemical species on demand by exploiting spin manipulations with static and alternating magnetic fields.     

Luminescence of Eu3+ and Tb3+ Complexes of Two Macrobicyclic Ligands Derived from a Tetralactam Ring and a Chromophoric Antenna

Two macrobicyclic ligands derived from an 18‐membered tetralactam ring and 2,2′‐bipyridine or 2,6‐bis(pyrazol‐1‐yl)pyridine moieties, 1 and 2 , respectively, form stable complexes with Gd^III, Eu^III, and Tb^III ions in aqueous solution. The ligand‐based luminescence is retained in the Gd^III cryptates, whereas this radiative deactivation is quenched in the Eu^III and Tb^III cryptates by ligand‐to‐metal energy transfer, resulting in the usual metal‐centered emission spectra. Singlet‐ and triplet‐state energies, emission‐decay lifetimes, and luminescence yields were measured. [Tb⊂ 1 ]³⁺ cryptate shows a long luminescence lifetime (τ=1.12 ms) and a very high metal luminescence quantum yield (Φ=0.25) in comparison with those reported in the literature for Tb³⁺ complexes sensitized by a bipyridine chromophore. By comparison to [Ln⊂ 1 ]³⁺, [Ln⊂ 2 ]³⁺ presents markedly lower luminescence properties, due to worse interaction between the 2,6‐bis(pyrazol‐1‐yl)pyridine unit and the metal ion. Moreover, the luminescent metal and the triplet ligand energy levels of [Eu⊂ 2 ]³⁺ do not match. The effects of H₂O molecules coordinated to the metal centre and of thermally activated decay processes on nonradiative deactivation to the ground‐state are also reported.     

Iridium α-Carboxyimine Complexes Hyperpolarized with para-Hydrogen Exist in Nuclear Singlet States before Conversion into Iridium Carbonates

The formation and hyperpolarization of an [Ir(H)₂(amine)(IMes)(η²-imine)]Cl complex that can be created in a hyperpolarized nuclear singlet state is reported. These complexes are formed when an equilibrium mixture of pyruvate, amine (benzylamine or phenylethylamine), and the corresponding imine condensation product, react with preformed [Ir(H)₂(amine)₃(IMes)]Cl. These iridium α-carboxyimine complexes exist as two regioisomers differentiated by the position of amine. When examined with para-hydrogen the hydride resonances of the isomer with amine trans to hydride become strongly hyperpolarized. The initial hydride singlet states readily transfer to the corresponding ¹³C₂ state in the labelled imine and exhibit magnetic state lifetimes of up to 11 seconds. Their ¹³C signals have been detected with up to 420 fold signal gains at 9.4 T. On a longer timescale, and in the absence of H₂, further reaction leads to the formation of neutral carbonate containing [Ir(amine)(η²-CO₃)(IMes)(η²-imine)]. Complexes are characterized by, IR, MS, NMR and X-ray diffraction.     

On The Potential of Dynamic Nuclear Polarization Enhanced Diamonds in Solid‐State and Dissolution 13C NMR Spectroscopy

Dynamic nuclear polarization (DNP) is a versatile option to improve the sensitivity of NMR and MRI. This versatility has elicited interest for overcoming potential limitations of these techniques, including the achievement of solid‐state polarization enhancement at ambient conditions, and the maximization of ¹³C signal lifetimes for performing in vivo MRI scans. This study explores whether diamond's ¹³C behavior in nano‐ and micro‐particles could be used to achieve these ends. The characteristics of diamond's DNP enhancement were analyzed for different magnetic fields, grain sizes, and sample environments ranging from cryogenic to ambient temperatures, in both solution and solid‐state experiments. It was found that ¹³C NMR signals could be boosted by orders of magnitude in either low‐ or room‐temperature solid‐state DNP experiments by utilizing naturally occurring paramagnetic P₁ substitutional nitrogen defects. We attribute this behavior to the unusually long electronic/nuclear spin‐lattice relaxation times characteristic of diamond, coupled with a time‐independent cross‐effect‐like polarization transfer mechanism facilitated by a matching of the nitrogen‐related hyperfine coupling and the ¹³C Zeeman splitting. The efficiency of this solid‐state polarization process, however, is harder to exploit in dissolution DNP‐enhanced MRI contexts. The prospects for utilizing polarized diamond approaching nanoscale dimensions for both solid and solution applications are briefly discussed.     

Indene Formation under Single‐Collision Conditions from the Reaction of Phenyl Radicals with Allene and Methylacetylene—A Crossed Molecular Beam and Ab Initio Study

Polycyclic aromatic hydrocarbons (PAHs) are regarded as key intermediates in the molecular growth process that forms soot from incomplete fossil fuel combustion. Although heavily researched, the reaction mechanisms for PAH formation have only been investigated through bulk experiments; therefore, current models remain conjectural. We report the first observation of a directed synthesis of a PAH under single‐collision conditions. By using a crossed‐molecular‐beam apparatus, phenyl radicals react with C₃H₄ isomers, methylacetylene and allene, to form indene at collision energies of 45 kJ mol^?1. The reaction dynamics supported by theoretical calculations show that both isomers decay through the same collision complex, are indirect, have long lifetimes, and form indene in high yields. Through the use of deuterium‐substituted reactants, we were able to identify the reaction pathway to indene.     

Singlet Oxygen Formation during the Charging Process of an Aprotic Lithium–Oxygen Battery

Aprotic lithium–oxygen (Li–O₂) batteries have attracted considerable attention in recent years owing to their outstanding theoretical energy density. A major challenge is their poor reversibility caused by degradation reactions, which mainly occur during battery charge and are still poorly understood. Herein, we show that singlet oxygen (¹Δ_g) is formed upon Li₂O₂ oxidation at potentials above 3.5 V. Singlet oxygen was detected through a reaction with a spin trap to form a stable radical that was observed by time‐ and voltage‐resolved in operando EPR spectroscopy in a purpose‐built spectroelectrochemical cell. According to our estimate, a lower limit of approximately 0.5 % of the evolved oxygen is singlet oxygen. The occurrence of highly reactive singlet oxygen might be the long‐overlooked missing link in the understanding of the electrolyte degradation and carbon corrosion reactions that occur during the charging of Li–O₂ cells.     

Para‐hydrogen induced polarization of Si‐29 NMR resonances as a potentially useful tool for analytical applications

Para‐hydrogen–induced polarization effects have been observed in the ²⁹Si NMR spectra of trimethylsilyl para‐hydrogenated molecules. The high signal enhancements and the long T₁ values observed for the ²⁹Si hyperpolarized resonances point toward the possibility of using ²⁹Si for hyperpolarization applications. A method for the discrimination of multiple compounds and/or complex mixtures of hydroxylic compounds (such as steroids), consisting of the silylization of alcoholic functionalities with an unsaturated silylalkyl moiety and subsequent reaction with para‐H₂, is proposed. Copyright © 2012 John Wiley & Sons, Ltd.     

Long‐Lived,Directional Photoinduced Charge Separation in RuII Complexes Bearing Laminate Polypyridyl Ligands

Ru^II complexes incorporating both amide‐linked bithiophene donor ancillary ligands and laminate acceptor ligands; dipyrido[3,2‐a:2′,3′‐c]phenazine (dppz), tetrapyrido[3,2‐a:2′,3′‐c:3′′,2′′‐h:2′′′,3′′′‐j]phenazine (tpphz), and 9,11,20,22‐tetraazatetrapyrido[3,2‐a:2′,3′‐c:3′′,2′′‐l:2′′′,3′′′]‐pentacene (tatpp) exhibit long‐lived charge separated (CS) states, which have been analyzed using time‐resolved transient absorption (TA), fluorescence, and electronic absorption spectroscopy in addition to ground state electrochemical and spectroelectrochemical measurements. These complexes possess two electronically relevant ³MLCT states related to electron occupation of MOs localized predominantly on the proximal “bpy‐like” portion and central (or distal) “phenazine‐like” portion of the acceptor ligand as well as energetically similar ³LC and ³ILCT states. The unusually long excited state lifetimes (τ up to 7 μs) observed in these complexes reflect an equilibration of the ³MLCT_prox or ³MLCT_dist states with additional triplet states, including a ³LC state and a ³ILCT state that formally localizes a hole on the bithiophene moiety and an electron on the laminate acceptor ligand. Coordination of a Zn^II ion to the open coordination site of the laminate acceptor ligand is observed to significantly lower the energy of the ³MLCT_dist state by decreasing the magnitude of the excited state dipole and resulting in much shorter excited state lifetimes. The presence of the bithiophene donor group is reported to substantially extend the lifetime of these Zn adducts via formation of a ³ILCT state that can equilibrate with the ³MLCT_dist state. In tpphz complexes, Zn^II coordination can reorder the energy of the ³MLCT_prox and ³MLCT_dist states such that there is a distinct switch from one state to the other. The net result is a series of complexes that are capable of forming CS states with electron–hole spatial separation of up to 14 Å and possess exceptionally long lifetimes by equilibration with other triplet states.     

Kinetics from indirectly detected hyperpolarized NMR spectroscopy by using spatially selective coherence transfers

An important recent development in NMR spectroscopy is the advent of ex situ dynamic nuclear polarization (DNP) approaches, which are capable of yielding liquid‐state sensitivities that exceed considerably those afforded by the highest‐field spectrometers. This increase in sensitivity has triggered new research avenues, particularly concerning the in vivo monitoring of metabolism and disease by NMR spectroscopy. So far such gains have mainly materialized for experiments that focus on nonprotonated, low‐γ nuclei; targets favored by relatively long relaxation times T₁, which enable them to withstand the transfer from the cryogenic hyperpolarizer to the reacting centers of interest. Recent studies have also shown that transferring this hyperpolarization to protons by indirectly detected methods could successfully give rise to ¹H NMR spectra of hyperpolarized compounds with a high sensitivity. The present study demonstrates that, when merged with spatially encoded methods, indirectly detected ¹H NMR spectroscopy can also be exploited as time‐resolved hyperpolarized spectroscopy. A methodology is thus introduced that can successfully deliver a series of hyperpolarized ¹H NMR spectra over a minutes‐long timescale. The principles and opportunities presented by this approach are exemplified by following the in vitro phosphorylation of choline by choline kinase, a potential metabolic marker of cancer; and by tracking acetylcholine’s hydrolysis by acetylcholine esterase, an important enzyme partaking in synaptic transmission and neuronal degradation.     

Synthesis,functionalization and crosslinking reactions of poly(silylenemethylene)s

Novel poly(silylenemethylene)s have been prepared by the ring‐opening polymerization of 1,3‐disilacyclobutanes followed by a protodesilylation reaction with triflic acid. The silicon–aryl bond cleavage could be controlled by using different leaving groups, for instance phenyl‐ and para‐anisyl substituents. The reactions of the triflate derivatives with organomagnesium compounds, LiAlH₄, amines or alcohols gave functional substituted poly(silylenemethylene)s. Hydrosilylation reactions or reductive coupling with potassium–graphite led to organosilicon network‐polymers, which may serve as suitable precursors for silicon carbide and Si/C/N‐based materials. The structures of the polymers were identified by nuclear magnetic resonance spectroscopy (²⁹Si, ¹³C, ¹H). Copyright © 1999 John Wiley & Sons, Ltd.     

Cholesterol Peroxidation as a Special Type of Lipid Oxidation in Photodynamic Systems

Like other unsaturated lipids in cell membranes and lipoproteins, cholesterol (Ch) is susceptible to oxidative modification, including photodynamic oxidation. There is a sustained interest in the pathogenic properties of Ch oxides such as those generated by photooxidation. Singlet oxygen (¹O₂)‐mediated Ch photooxidation (Type II mechanism) gives rise to three hydroperoxide (ChOOH) isomers: 5α‐OOH, 6α‐OOH and 6β‐OOH, the 5α‐OOH yield far exceeding that of the others. 5α‐OOH detection is relatively straightforward and serves as a definitive indicator of ¹O₂ involvement in a reaction, photochemical or otherwise. Like all lipid hydroperoxides (LOOHs), ChOOHs can disrupt membrane or lipoprotein structure/function on their own, but subsequent light‐independent reactions may either intensify or attenuate such effects. Such reactions include (1) one‐electron reduction to redox‐active free radical intermediates, (2) two‐electron reduction to redox‐silent alcohols and (3) translocation to other lipid compartments, where (1) or (2) may take place. In addition to these effects, ChOOHs may act as signaling molecules in reactions that affect cell fates. Although processes a‐c have been well studied for ChOOHs, signaling activity is still poorly understood compared with that of hydrogen peroxide. This review focuses on these various aspects Ch photoperoxidation and its biological consequences.     

Tetraphenylporphyrin‐based dual‐functional medical agent for magnetic resonance and fluorescence imaging

A bimodal magnetic resonance imaging contrast agent, TPP‐M‐Gd, was developed by modifying tetraphenylporphyrin (TPP) with a small dendritic molecule as a ligand (M) to chelate gadolinium (Gd) ions. The ligand featured four carboxylate groups, which contributed to good water solubility and a strong combination with metal ions. The longitudinal relaxivity (R₁) of the resulting agent was calculated to be 12.45 mM^?1 s^?1, which is much higher than that of DTPA‐Gd (4.49 mM^?1 s^?1). The magnetic resonance imaging experiments showed that the newly synthesized contrast agent could enhance T₁‐weighted magnetic resonance imaging quality both in vitro and in vivo. In addition, TPP‐M‐Gd exhibited good fluorescent property as shown in cell imaging experiments. The cytotoxicity of TPP‐M‐Gd was even better than that of clinically approved DTPA‐Gd, which makes it a promising dual‐functional medical imaging agent to provide more detailed information about biological and disease‐related events.