Electric dipole spin resonance

Résumé

Electric dipole spin resonance (EDSR) is a method to control the magnetic moments inside a material using quantum mechanical effects like the spin–orbit interaction. Mainly, EDSR allows to flip the orientation of the magnetic moments through the use of electromagnetic radiation at resonant frequencies. EDSR was first proposed by Emmanuel Rashba. Computer hardware employs the electron charge in transistors to process information and the electron magnetic moment or spin for magnetic storage devices. The emergent field of spintronics aims in unifying the operations of these subsystems. For achieving this goal, the electron spin should be operated by electric fields. EDSR allows to use the electric component of AC fields to manipulate both charge and spin. Introduction Free electrons possess electric charge e and magnetic moment \boldsymbol{\mu} whose absolute value is about one Bohr magneton \mu_{\rm B}. The standard electron spin resona

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https://en.wikipedia.org/wiki/Electric_dipole_spin_resonance

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Dissolution dynamic nuclear polarization (dDNP) is a powerful technique that enhances the magnetic resonance signal of nuclear spins by several orders of magnitude. DNP relies on the principle of cross-relaxation by electron spins driven out of equilibrium to enhance nuclear polarization. When coupled to magnetic resonance spectroscopy/imaging (MRS/I), infusion of 13C-labeled tracers hyperpolarized via DNP in vivo provides real-time estimates of metabolic fluxes, physiological parameters and/or tissue perfusion. The present work focuses on fundamental aspects of DNP, in particular how sample composition affects the DNP of low-gamma nuclei, as well as on in vivo dDNP applications targeting renal metabolism and physiology. The DNP properties of the SA-BDPA radical were studied in a sample of 13C-urea. SA-BDPA, a water-soluble derivative of BDPA, is characterized by a small g-anisotropy and unresolved hyperfine coupling to protons. As a result, at 6.7 T, 1.1 K its ESR linewidth is much smaller than the 13C Larmor frequency, which enabled the observation for the first time of 13C DNP via the solid effect and pure thermal mixing, the latter defined as the process where the electron non-Zeeman reservoir alone provides the energy required for triple-spin flips. The effect of solvent deuteration on 6Li DNP and radical ESR properties was studied in 6LiCl water:glycerol solutions doped with either the nitroxide radical TEMPOL or the trityl radical OX063. Unexpectedly, the TEMPOL-doped samples polarized better than the trityl-doped ones. Across all samples, the relationship between the degree of solvent deuteration with the buildup time constant and polarization level was notably different from what has been reported for 13C. This behavior is indicative of DNP via a combination of the cross effect and thermal mixing mechanisms. The uptake and metabolism of hyperpolarized L-[1-13C]alaninamide was investigated in the rat kidney in vivo. This probe of aminopeptidase N, which can play a role in tumor growth, was previously studied in vitro. Our study showed that alanine production from alaninamide also occurs in vivo, however, with spectral overlap of substrate and product. Alaninamide, having a pKa of 7.9, proved to be sensitive to local pH. Three spectral peaks, corresponding to at least three environments with different pH values, could be observed in the kidney. The two peaks at higher pH were assigned to the blood extra- and (partially) intracellular compartments, while the third one was mainly in the inner part of the kidney. Finally, alaninamide was shown to also be sensitive to dissolved CO2, with the rapid formation of a carbamate adduct following infusion. The renal metabolism of D-[1-13C]alanine by D-amino acid oxidase (DAO) was also studied. Conversion of hyperpolarized D-alanine to pyruvate and further metabolism to lactate and bicarbonate was observed in the kidney only when DAO was not inhibited. DAO activity could also be detected in blood, where leukocytes express the enzyme, but not in the brain and liver, in line with their lower DAO activity. Overall, this thesis provides additional insight into how the experimental conditions can favor a particular DNP mechanism over another. It also significantly expands the scope of in vivo dDNP applications, showing that additional enzyme-catalyzed processes can be detected, along with the potential of amino-acid based hyperpolarized 13C sensors for physiological studies.

EPFL2021

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Radical-free hyperpolarized MRI using endogenously occurring pyruvate analogues and UV-induced nonpersistent radicals

Lionel Pierre Arn, Josefina Adriana Maria Bastiaansen, Andrea Capozzi, Rolf Gruetter, Alice Radaelli, Hikari Ananda Infinity Yoshihara, Claudia Christina Zanella

It was recently demonstrated that nonpersistent radicals can be generated in frozen solutions of metabolites such as pyruvate by irradiation with UV light, enabling radical-free dissolution dynamic nuclear polarization. Although pyruvate is endogenous, the presence of pyruvate may interfere with metabolic processes or the detection of pyruvate as a metabolic product, making it potentially unsuitable as a polarizing agent. Therefore, the aim of the current study was to characterize solutions containing endogenously occurring alternatives to pyruvate as UV-induced nonpersistent radical precursors for in vivo hyperpolarized MRI. The metabolites alpha-ketovalerate (alpha kV) and alpha-ketobutyrate (alpha kB) are analogues of pyruvate and were chosen as potential radical precursors. Sample formulations containing alpha kV and alpha kB were studied with UV-visible spectroscopy, irradiated with UV light, and their nonpersistent radical yields were quantified with electron spin resonance and compared with pyruvate. The addition of C-13-labeled substrates to the sample matrix altered the radical yield of the precursors. Using alpha kB increased the C-13-labeled glucose liquid-state polarization to 16.3% +/- 1.3% compared with 13.3% +/- 1.5% obtained with pyruvate, and 8.9% +/- 2.1% with alpha kV. For [1-C-13]butyric acid, polarization levels of 12.1% +/- 1.1% for alpha kV, 12.9% +/- 1.7% for alpha kB, 1.5% +/- 0.2% for OX063 and 18.7% +/- 0.7% for Finland trityl, were achieved. Hyperpolarized [1-C-13]butyrate metabolism in the heart revealed label incorporation into [1-C-13]acetylcarnitine, [1-C-13]acetoacetate, [1-C-13]butyrylcarnitine, [5-C-13]glutamate and [5-C-13]citrate. This study demonstrates the potential of alpha kV and alpha kB as endogenous polarizing agents for in vivo radical-free hyperpolarized MRI. UV-induced, nonpersistent radicals generated in endogenous metabolites enable high polarization without requiring radical filtration, thus simplifying the quality-control tests in clinical applications.

WILEY2021

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Engineering the Eigenstates of Coupled Spin-1/2 Atoms on a Surface

Donat Fabian Natterer

Quantum spin networks having engineered geometries and interactions are eagerly pursued for quantum simulation and access to emergent quantum phenomena such as spin liquids. Spin-1/2 centers are particularly desirable, because they readily manifest coherent quantum fluctuations. Here we introduce a controllable spin-1/2 architecture consisting of titanium atoms on a magnesium oxide surface. We tailor the spin interactions by atomic-precision positioning using a scanning tunneling microscope (STM) and subsequently perform electron spin resonance on individual atoms to drive transitions into and out of quantum eigenstates of the coupled-spin system. Interactions between the atoms are mapped over a range of distances extending from highly anisotropic dipole coupling to strong exchange coupling. The local magnetic field of the magnetic STM tip serves to precisely tune the superposition states of a pair of spins. The precise control of the spin-spin interactions and ability to probe the states of the coupled-spin network by addressing individual spins will enable the exploration of quantum many-body systems based on networks of spin-1/2 atoms on surfaces.

American Physical Society2017

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EPFL2021

Radical-free hyperpolarized MRI using endogenously occurring pyruvate analogues and UV-induced nonpersistent radicals

Lionel Pierre Arn, Josefina Adriana Maria Bastiaansen, Andrea Capozzi, Rolf Gruetter, Alice Radaelli, Hikari Ananda Infinity Yoshihara, Claudia Christina Zanella

WILEY2021