Category Archives: Long Lived Spin States

Long live the singlet state! #DNPNMR

Levitt, Malcolm H. “Long Live the Singlet State!” Journal of Magnetic Resonance 306 (September 2019): 69–74.

https://doi.org/10.1016/j.jmr.2019.07.029

The field of long-lived states in NMR is reviewed. The relationship of long-lived-state phenomena to those associated with spin isomerism is discussed. A brief overview is given of key developments in the field of long-lived states, including chemical symmetry-switching, the role of magnetic equivalence and magnetic inequivalence, long-lived coherences, hyperpolarized NMR involving long-lived states, quantumrotor-induced polarization, and parahydrogen-induced hyperpolarization. Current application areas of long-lived states are reviewed, and a peer into the crystal ball reveals future developments in the field. Ó 2019 Published by Elsevier Inc.

Proton relaxometry of long-lived spin order

Kiryutin, Alexey S., Mikhail S. Panov, Alexandra V. Yurkovskaya, Konstantin L. Ivanov, and Geoffrey Bodenhausen. “Proton Relaxometry of Long-Lived Spin Order.” ChemPhysChem, January 2, 2019.

https://doi.org/10.1002/cphc.201800960

A study of long-lived spin order in chlorothiophene carboxylates at both high and low magnetic fields is described. Careful sample preparation (removal of dissolved oxygen in solution, chelating of paramagnetic impurities, reduction of convection) allows one to obtain very long-lived singlet order of the two coupled protons in chlorothiophene derivatives, having lifetimes TLLS of about 130 s in D2O and 240 s in deuterated methanol, which are much longer than the T1-relaxation times (18 and 30 s, respectively, at a field B0 = 9.4 T). In protonated solvents the relaxation times become shorter, but TLLS is still substantially longer than T1 . In addition, long-lived coherences are shown to have lifetimes TLLS as long as 30 s. Thiophene derivatives can be used as molecular tags to study slow transport, slow dynamics and slow chemical processes, as some of us have shown in recent years.

Unlocking a diazirine long-lived nuclear singlet state via photochemistry: NMR detection and lifetime of an unstabilized diazo-compound

Procacci, Barbara, Soumya S Roy, Philip Norcott, Norman Turner, and Simon B Duckett. “Unlocking a Diazirine Long-Lived Nuclear Singlet State via Photochemistry: NMR Detection and Lifetime of an Unstabilized Diazo-Compound.” Journal of the American Chemical Society, 2018, 19.

https://doi.org/10.1021/jacs.8b10923.

Diazirines are important for photoaffinity labeling, and their photoisomerization is relatively well-known. This work shows how hyperpolarized NMR spectroscopy can be used to characterize an unstable diazo-compound formed via photoisomerization of a 15N2-labeled silyl-ether-substituted diazirine. This diazirine is prepared in a nuclear spin singlet state via catalytic transfer of spin order from para-hydrogen. The active hyperpolarization catalyst is characterized to provide insight into the mechanism. The photochemical isomerization of the diazirine into the diazo-analogue allows the NMR invisible nuclear singlet state of the parent compound to be probed. The identity of the diazo-species is confirmed by trapping with N-phenyl maleimide via a cycloaddition reaction to afford bicyclic pyrazolines that also show singlet state character. The presence of singlet states in the diazirine and the diazo-compound is validated by comparison of experimental nutation behavior with theoretical simulation. The magnetic state lifetime of the diazo-compound is determined as 12 ± 1 s in CD3OD solution at room temperature, whereas its chemical lifetime is measured as 100 ± 5 s by related hyperpolarized NMR studies. Indirect evidence for the generation of the photoproduct para-N2 is presented.

Dynamic Nuclear Polarization of Long-Lived Nuclear Spin States in Methyl Groups

Dumez, J.-N., et al., Dynamic Nuclear Polarization of Long-Lived Nuclear Spin States in Methyl Groups. The Journal of Physical Chemistry Letters, 2017. 8(15): p. 3549-3555.

http://dx.doi.org/10.1021/acs.jpclett.7b01512

We have induced hyperpolarized long-lived states in compounds containing 13C-bearing methyl groups by dynamic nuclear polarization (DNP) at cryogenic temperatures, followed by dissolution with a warm solvent. The hyperpolarized methyl long-lived states give rise to enhanced antiphase 13C NMR signals in solution, which often persist for times much longer than the 13C and 1H spin–lattice relaxation times under the same conditions. The DNP-induced effects are similar to quantum-rotor-induced polarization (QRIP) but are observed in a wider range of compounds because they do not depend critically on the height of the rotational barrier. We interpret our observations with a model in which nuclear Zeeman and methyl tunnelling reservoirs adopt an approximately uniform temperature, under DNP conditions. The generation of hyperpolarized NMR signals that persist for relatively long times in a range of methyl-bearing substances may be important for applications such as investigations of metabolism, enzymatic reactions, protein–ligand binding, drug screening, and molecular imaging.

Communication: Dissolution DNP reveals a long-lived deuterium spin state imbalance in methyl groups #DNPNMR

Jhajharia, A., et al., Communication: Dissolution DNP reveals a long-lived deuterium spin state imbalance in methyl groups. J Chem Phys, 2017. 146(4): p. 041101.

https://www.ncbi.nlm.nih.gov/pubmed/28147551

We report the generation and observation of long-lived spin states in deuterated methyl groups by dissolution DNP. These states are based on population imbalances between manifolds of spin states corresponding to irreducible representations of the C3v point group and feature strongly dampened quadrupolar relaxation. Their lifetime depends on the activation energies of methyl group rotation. With dissolution DNP, we can reduce the deuterium relaxation rate by a factor up to 20, thereby extending the experimentally available time window. The intrinsic limitation of NMR spectroscopy of quadrupolar spins by short relaxation times can thus be alleviated.

High-Resolution Two-Field Nuclear Magnetic Resonance Spectroscopy

Cousin, S.F., et al., High-Resolution Two-Field Nuclear Magnetic Resonance Spectroscopy. Phys. Chem. Chem. Phys., 2016.

http://dx.doi.org/10.1039/C6CP05422F

Nuclear Magnetic Resonance (NMR) is a ubiquitous branch of spectroscopy that can explore matter on the scale of the atom. Significant improvements in sensitivity and resolution have been driven by a steady increase of static magnetic field strengths. However, some properties of nuclei may be more favourable at low magnetic fields. For example, line-broadening due to chemical shift anisotropy increases sharply at higher magnetic fields. Here, we present a two-field NMR spectrometer that permits the application of rf-pulses and acquisition of NMR signals in two magnetic centres. Our prototype operates at 14.1 T and 0.33 T. The main features of this system are demonstrated by novel NMR experiments that correlate zero-quantum coherences at low magnetic field with single quantum coherences at high magnetic field, so that high resolution can be achieved in both dimensions, despite a ca. 10 ppm inhomogeneity of the low field centre. Two-field NMR spectroscopy offers the possibility to circumvent the limits of high magnetic fields, while benefiting from their exceptional sensitivity and resolution. This approach opens new avenues for NMR above 1 GHz.

Long-lived states to sustain SABRE hyperpolarised magnetisation

Roy, S.S., et al., Long-lived states to sustain SABRE hyperpolarised magnetisation. Phys Chem Chem Phys, 2016. 18(36): p. 24905-24911.

https://www.ncbi.nlm.nih.gov/pubmed/27711398

The applicability of the magnetic resonance (MR) technique in the liquid phase is limited by poor sensitivity and short nuclear spin coherence times which are insufficient for many potential applications. Here we illustrate how it is possible to address both of these issues simultaneously by harnessing long-lived hyperpolarised spin states that are formed by adapting the Signal Amplification by Reversible Exchange (SABRE) technique. We achieve more than 4% net 1H-polarisation in a long-lived form that remains detectable for over ninety seconds by reference to proton pairs in the biologically important molecule nicotinamide and a pyrazine derivative whose in vivo imaging will offer a new route to probe disease in the future.

Magnetic resonance imaging of (1)H long lived states derived from parahydrogen induced polarization in a clinical system #DNPNMR

Graafen, D., et al., Magnetic resonance imaging of (1)H long lived states derived from parahydrogen induced polarization in a clinical system. J Magn Reson, 2016. 262: p. 68-72.

http://www.ncbi.nlm.nih.gov/pubmed/26754064

Hyperpolarization is a powerful tool to overcome the low sensitivity of nuclear magnetic resonance (NMR). However, applications are limited due to the short lifetime of this non equilibrium spin state caused by relaxation processes. This issue can be addressed by storing hyperpolarization in slowly decaying singlet spin states which was so far mostly demonstrated for non-proton spin pairs, e.g. (13)C-(13)C. Protons hyperpolarized by parahydrogen induced polarization (PHIP) in symmetrical molecules, are very well suited for this strategy because they naturally exhibit a long-lived singlet state. The conversion of the NMR silent singlet spin state to observable magnetization can be achieved by making use of singlet-triplet level anticrossings. In this study, a low-power radiofrequency pulse sequence is used for this purpose, which allows multiple successive singlet-triplet conversions. The generated magnetization is used to record proton images in a clinical magnetic resonance imaging (MRI) system, after 3min waiting time. Our results may open unprecedented opportunities to use the standard MRI nucleus (1)H for e.g. metabolic imaging in the future.

Symmetry constraints on spin dynamics: Application to hyperpolarized NMR

Levitt, M.H., Symmetry constraints on spin dynamics: Application to hyperpolarized NMR. J Magn Reson, 2016. 262: p. 91-9.

http://www.ncbi.nlm.nih.gov/pubmed/26462592

Spin dynamical evolution is constrained by the symmetries of the spin Hamiltonians that generate the quantum dynamics. The consequences of symmetry-induced constraints are examined for some common hyperpolarized NMR experiments, including the excitation of singlet order in spin-pair systems, and the transfer of parahydrogen-induced hyperpolarized singlet order to magnetization in systems displaying chemical and magnetic equivalence.

Long-lived spin states as a source of contrast in magnetic resonance spectroscopy and imaging

Kiryutin, A.S., et al., Long-lived spin states as a source of contrast in magnetic resonance spectroscopy and imaging. J Magn Reson, 2015. 261: p. 64-72.

http://www.ncbi.nlm.nih.gov/pubmed/26529204

A method is proposed to create Long-Lived spin States (LLSs) from longitudinal spin magnetization, which is based on adiabatic switching of a Radio-Frequency (RF) field with proper frequency. The technique is simple to implement with standard Nuclear Magnetic Resonance (NMR) equipment, providing an excellent conversion of population from the triplet T+ (or T-) state to the singlet state of a pair of spins and back. The method has been tested for the amino acid tyrosine and its partially deuterated isotopomer; for the deuterated compound, we have achieved a LLS lifetime, which exceeds the longitudinal relaxation time by a factor of 21. Furthermore, by slightly modifying the method, an enhanced contrast with respect to LLSs in NMR spectra is achieved; contrast enhancements of more than 1200 are feasible. This enables efficient suppression of longitudinal spin magnetization in NMR allowing one to look selectively at LLSs. Using this method we have demonstrated that not only spectral but also spatial contrast can be achieved: we have obtained spatial NMR images with strongly improved contrast originating from the difference of LLS lifetimes at different positions in the sample.

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