Nitroxide free radicals are the most commonly used source for dynamic nuclear polarization (DNP) enhanced nuclear magnetic resonance (NMR) experiments and are also exclusively employed as spin labels for electron spin resonance (ESR) spectroscopy of diamagnetic molecules and materials. Nitroxide free radicals have been shown to have strong dipolar coupling to (1)H in water, and thus result in large DNP enhancement of (1)H NMR signal via the well known Overhauser effect. The fundamental parameter in a DNP experiment is the coupling factor, since it ultimately determines the maximum NMR signal enhancements which can be achieved. Despite their widespread use, measurements of the coupling factor of nitroxide free radicals have been inconsistent, and current models have failed to successfully explain our experimental data. We found that the inconsistency in determining the coupling factor arises from not taking into account the characteristics of the ESR transitions, which are split into three (or two) lines due to the hyperfine coupling of the electron to the (14)N nuclei (or (15)N) of the nitric oxide radical. Both intermolecular Heisenberg spin exchange interactions as well as intramolecular nitrogen nuclear spin relaxation mix the three (or two) ESR transitions. However, neither effect has been taken into account in any experimental studies on utilizing or quantifying the Overhauser driven DNP effects. The expected effect of Heisenberg spin exchange on Overhauser enhancements has already been theoretically predicted and observed by Bates and Drozdoski [J. Chem. Phys. 67, 4038 (1977)]. Here, we present a new model for quantifying Overhauser enhancements through nitroxide free radicals that includes both effects on mixing the ESR hyperfine states. This model predicts the maximum saturation factor to be considerably higher by the effect of nitrogen nuclear spin relaxation. Because intramolecular nitrogen spin relaxation is independent of the nitroxide concentration, this effect is still significant at low radical concentrations where electron spin exchange is negligible. This implies that the only correct way to determine the coupling factor of nitroxide free radicals is to measure the maximum enhancement at different concentrations and extrapolate the results to infinite concentration. We verify our model with a series of DNP experimental studies on (1)H NMR signal enhancement of water by means of (14)N as well as (15)N isotope enriched nitroxide radicals.

Surface and internal water dynamics of molecules and soft matter are of great relevance to their structure and function, yet the experimental determination under ambient and steady-state conditions is challenging. One of the most powerful approaches to measure local water dynamics within 5 A distances is to utilize the modulation of the nuclear spin relaxation rate of water protons through their time-dependent dipolar coupling to paramagnetic probes, here nitroxide spin labels. We recently introduced a method to obtain local water dynamics through Overhauser dynamic nuclear polarization (DNP). This has a unique advantage over other related techniques available in that a highly amplified proton nuclear magnetic resonance signal carries the information, allowing the use of minute microliter sample volumes and 100 muM sample concentrations. The outcome of our approach is the quantitative determination of the key DNP parameter known as the coupling factor, which provides local translational diffusion dynamics of the solvent within 5 A of the spin label. In contrast to recent reports that the coupling factor for nitroxide radicals cannot be quantified due to the difficulty in determining the saturation factor for the spin label, we show the saturation factor can be accurately determined and for the first time present agreement between measurements and theory. We discuss the discrepancy between the related field cycling relaxometery technique and DNP in determining the coupling factor and present arguments in support of the DNP-determined value. DNP measurements of local hydration dynamics around nitroxides in bulk water and on the surface of proteins are presented.

Water-protein interactions play a direct role in protein folding. The chain collapse that accompanies protein folding involves extrusion of water from the nonpolar core. For many proteins, including apomyoglobin (apoMb), hydrophobic interactions drive an initial collapse to an intermediate state before folding to the final structure. However, the debate continues as to whether the core of the collapsed intermediate state is hydrated and, if so, what the dynamic nature of this water is. A key challenge is that protein hydration dynamics is significantly heterogeneous, yet suitable experimental techniques for measuring hydration dynamics with site-specificity are lacking. Here, we introduce Overhauser dynamic nuclear polarization at 0.35 T via site-specific nitroxide spin labels as a unique tool to probe internal and surface protein hydration dynamics with site-specific resolution in the molten globular, native, and unfolded protein states. The 1H NMR signal enhancement of water carries information about the local dynamics of the solvent within ~10 Å of a spin label. EPR is used synergistically to gain insights on local polarity and mobility of the spin-labeled protein. Several buried and solvent-exposed sites of apoMb are examined, each bearing a covalently bound nitroxide spin label. We find that the hydrophobic core of the apoMb molten globule is hydrated with water bearing significant translational dynamics, only 4–6-fold slower than that of bulk water. The hydration dynamics of the native state is heterogeneous, while the acid-unfolded state bears fast-diffusing hydration water. This study provides a high-resolution glimpse at the folding-dependent nature of protein hydration dynamics.

Purpose As the premiere modality for brain imaging, MRI could find wider applicability if lightweight, portable systems were available for siting in unconventional locations such as Intensive Care Units, physician offices, surgical suites, ambulances, emergency rooms, sports facilities, or rural healthcare sites. Methods We construct and validate a truly portable (<100kg) and silent proof-of-concept MRI scanner which replaces conventional gradient encoding with a rotating lightweight cryogen-free, low-field magnet. When rotated about the object, the inhomogeneous field pattern is used as a rotating Spatial Encoding Magnetic field (rSEM) to create generalized projections which encode the iteratively reconstructed 2D image. Multiple receive channels are used to disambiguate the non-bijective encoding field. Results The system is validated with experimental images of 2D test phantoms. Similar to other non-linear field encoding schemes, the spatial resolution is position dependent with blurring in the center, but is shown to be likely sufficient for many medical applications. Conclusion The presented MRI scanner demonstrates the potential for portability by simultaneously relaxing the magnet homogeneity criteria and eliminating the gradient coil. This new architecture and encoding scheme shows convincing proof of concept images that are expected to be further improved with refinement of the calibration and methodology.

We report coherent operation of a singlet-triplet qubit controlled by the spatial arrangement of two confined electrons in an adjacent double quantum dot that is electrostatically coupled to the qubit. This four-dot system is the specific device geometry needed for two-qubit operations of a two-electron spin qubit. We extract the strength of the capacitive coupling between qubit and adjacent double quantum dot and show that the present geometry allows fast conditional gate operation, opening pathways toward implementation of a universal set of gates for singlet-triplet spin qubits. DOI: 10.1103/PhysRevLett.107.030506 PACS numbers: 03.67.Lx, 73.21.La Advances in control of single electrons in quantum dots [1] have led to the prospect of using electron spin as a quantum bit (qubit) in quantum computation [2]. One formulation of the qubit uses singlet jSi ¼ 1 ffiffi 2 p ðj"#iÀ j"#iÞ and triplet jT 0 i ¼ 1 ffiffi 2 p ðj"#iþ j"#iÞ states [3] of two electrons in a double quantum dot (DQD) [ Fig. 1(a)]. Most requirements for quantum computing [4] with this qubit have been met [5][6][7][8], including all electrical full single-qubit control [9]. Rotation about the z axis of the Bloch sphere [ Fig. 1(a)] is governed by the exchange interaction between two spins, which can be controlled electrostatically near degeneracies of the charge arrangement of the two electrons. Rotation about the x axis is mediated by gradients of the Zeeman field, produced either by nuclear gradients [9] or by permanent magnets [10].The electrostatic interaction between DQDs was identified theoretically to lead to a two-qubit interaction sufficient for universal quantum computation [11]. In this scheme, the control (C) DQD is configured to allow its spin configuration (S or T 0 ) to determine its charge state via Pauli blockade [12] of the charge transition from the singly occupied ð1; 1Þ to the doubly occupied ð0; 2Þ [or ð2; 0Þ] configuration, where ðN L ; N R Þ are the absolute electron occupancies of the left and right QD. That is, rapid relaxation into the symmetric orbital ground state of ð0; 2Þ occurs only for the spin-antisymmetric singlet (S) state, while the spin-symmetric triplet (T 0 ) remains trapped in the ð1; 1Þ charge configuration. The resulting charge state of the control DQD in turn influences the rate of coherent state evolution in the target (T) DQD through the dependence of the exchange interaction on electrostatic tuning. The two-qubit operation is thus mediated by the charge configuration of the control DQD [ Fig. 1(b)].While electrostatically coupled proximal electron pairs constitute the main candidate for two-qubit operations for the singlet-triplet qubits, this system has not been realized or assessed experimentally to date. The present study realizes the relevant four-dot system and provides key parameters of the capacitive interaction. We further demonstrate controlled coherent operation of one DQD, operating as a singlet-triplet qubit, using the two-electron charge configuration of the other DQD. We find that the...

We show that annealing of stoichiometric SrRuO 3 perovskites in high-pressure oxygen of 600 atm near 1100°C produces SrRu 1−v O 3 compounds with vacancies on the Ru-sites. The creation of Ru vacancies rapidly suppresses the ferromagnetic ordering temperature, T C , from 163 K to 45 K with an increase of v Ϸ 0.09. Subtle structural changes that accompany the creation of Ru-site vacancies are different from the typical properties of transition metal perovskites, for which an increased formal oxidation state of the B-site cations normally leads to decreased B-O interatomic distances and contraction of the unit cell volume. The reduced charge screening caused by the Ru-vacancies offsets an expected decrease of the average interatomic distance Ru-O and rotation of the RuO 6 octahedra as Sr atoms relax toward Ru-vacancies increases the observed volume.

Pure water in a highly 1 H spin-polarized state is proposed as a contrast-agent-free contrast agent to visualize its macroscopic evolution in aqueous media by MRI. Remotely enhanced liquids for image contrast (RELIC) utilizes a 1 H signal of water that is enhanced outside the sample in continuous-flow mode and immediately delivered to the sample to obtain maximum contrast between entering and bulk fluids. Hyperpolarization suggests an ideal contrast mechanism to highlight the ubiquitous and specific function of water in physiology, biology, and materials because the physiological, chemical, and macroscopic function of water is not altered by the degree of magnetization. We present an approach that is capable of instantaneously enhancing the 1 H MRI signal by up to 2 orders of magnitude through the Overhauser effect under ambient conditions at 0.35 tesla by using highly spin-polarized unpaired electrons that are covalently immobilized onto a porous, water-saturated gel matrix. The continuous polarization of radicalfree flowing water allowed us to distinctively visualize vortices in model reactors and dispersion patterns through porous media. A 1 H signal enhancement of water by a factor of ؊10 and ؊100 provides for an observation time of >4 and 7 s, respectively, upon its injection into fluids with a T1 relaxation time of >1.5 s. The implications for chemical engineering or biomedical applications of using hyperpolarized solvents or physiological fluids to visualize mass transport and perfusion with high and authentic MRI contrast originating from water itself, and not from foreign contrast agents, are immediate.Water is the driver of nature.Leonardo da Vinci I n a world where water is so ubiquitous and vital, the exchange and transport characteristics of water are fundamental for the function of an endless range of biological and industrial processes: blood physiology, protein folding, plant metabolism, biomaterial function, and oil recovery from reservoir rocks are only drops in the bucket. However, there exists a paucity of analytical tools capable of directly tracing and quantifying the transport and function of water through these already-watersaturated materials in a chemically selective and noninvasive manner. Although NMR and MRI are the best tools for this purpose, they face two main challenges. One is the lack of sensitivity inherent to all NMR experiments, especially for in vivo NMR studies of transport in biological and biomedical samples. A general approach to this sensitivity issue is the employment of high magnetic fields and cryoprobes, which is not only expensive technology but also is limited to Ͻ1 order of magnitude improvement in sensitivity. The other challenge is the lack of contrast, e.g., between the flowing water molecules being traced and the bulk water or water contained in the specimen. Current perfusion MRI techniques that address this contrast issue are dynamic susceptibility contrast-enhanced imaging (1, 2) and proton electron double-resonance imaging (PEDRI), also known as Overhaus...

We present our experimental setup for both dynamic nuclear polarization (DNP) and electron paramagnetic resonance (EPR) detection at 7 T using a quasi-optical bridge for propagation of the 200 GHz beam and our initial results obtained at 4 K. Our quasi-optical bridge allows the polarization of the microwave beam to be changed from linear to circular. Only the handedness of circular polarization in the direction of the Larmor precession is absorbed by the electron spins, so a gain in effective microwave power of two is expected for circular vs. linear polarization. Our results show an increase in DNP signal enhancement of 28% when using circularly vs. linearly polarized radiation. We measured a maximum signal enhancement of 65 times that of thermal polarization for a (13)C labeled urea sample corresponding to 3% nuclear spin polarization. Since the time constant for nuclear spin polarization buildup during microwave irradiation is 10 times faster than the (13)C nuclear spin T(1), the actual gain in detection sensitivity with DNP is much greater.