Spin-lattice Relaxation Rate Research Articles

1H, 31P NMR, Raman and FTIR spectroscopies for investigating phosphoric acid dissociation to understand phosphate ion kinetics in body fluids

The study investigates the formation and transportation of ionic charge carriers in phosphoric acid-water system. This investigation encompasses an analysis of 1H and 31P NMR chemical shifts, self-diffusion coefficients, spin-lattice relaxation rates, spin-spin relaxation rates, activation energies, dissociation constants, electrical conductivity, and Raman shifts, along with FTIR spectra across various water concentrations. Significantly, the maxima observed in these curves at around 0.8 water molar fraction predominantly from the unique molecular arrangement between phosphoric acid and water molecules, influenced by a hydrogen bonding network. These findings yield valuable insights into phosphate ion kinetics within body fluids, covering essential aspects like hydrogen bonding networks, ionization processes, and the energy kinetics of phosphoric dissociation. A customized semiempirical model is applied to calculate dissociated species (water, phosphoric acid, and hydronium ion) at different water contents within a wide range of water mole fraction. Furthermore, this investigation extends to the dissociation of phosphoric acid in DMEM cell culture media, offering a more precise model for phosphate ionic kinetics within body fluids, especially at nominal phosphate concentrations of approximately 1:700μL.

Reduction of the Spin–Phonon Coupling of Quadrupole Nuclei in NaF Crystals under Magnetic Saturation

The rate of nuclear spin-lattice relaxation is determined by the efficiency of interaction between thermal phonons and nuclear spins. The results on reducing the efficiency of spin–phonon coupling by suppressing the contribution from paramagnetic centers to quadrupole nucleus relaxation are presented. The suppression has been performed by stationary magnetic effect at the Larmor frequency. It is shown that, as in the presence of an acoustic field, the rate of spin-lattice relaxation of 23Na nuclei in a sodium fluoride crystal at magnetic saturation of the NMR signal does not change in the region of a negative average spin temperature.In the region of positive spin temperature, the rate of relaxation of 23Na spins significantly decreases and nuclear magnetization recovery with time is described by the sum of two exponentials. The contribution from nuclear spins with a lower efficiency of spin–phonon coupling, corresponding to the exponential with a long relaxation time, increases with increasing saturating field intensity. It is demonstrated that the efficiency of spin–phonon coupling for 19F nuclei without quadrupole moment does not change under the saturation conditions.The results obtained can be used for analyzing the structure of real crystals.

Depth-resolved measurement of the Meissner screening profile in a niobium thin film from spin-lattice relaxation of the implanted β-emitter 8Li

We report measurements of the Meissner screening profile in a Nb(300 nm)/Al2O3 thin film using 8Liβ-detected nuclear magnetic resonance (β-NMR). The NMR probe 8Li was ion-implanted into the Nb film at energies ≤ 20 keV, corresponding to mean stopping depths comparable to Nb’s magnetic penetration depth λ. 8Li’s strong dipole–dipole coupling with the host 93Nb nuclei provided a “cross-relaxation” channel that dominated in low magnetic fields, which conferred indirect sensitivity to the local magnetic field via the spin-lattice relaxation (SLR) rate 1/T1. From a fit of the 1/T1 data to a model accounting for its dependence on temperature, magnetic field, and 8Li+ implantation energy, we obtained a magnetic penetration depth λ0= 51.5(22) nm, consistent with a relatively short carrier mean-free-path ℓ= 18.7(29) nm typical of similarly prepared Nb films. The results presented here constitute an important step toward using 8Liβ-NMR to characterize bulk Nb samples with engineered surfaces, which are often used in the fabrication of particle accelerators.

Water Dynamics in Dextran-Based Hydrogel Micro/Nanoparticles Studied by NMR Diffusometry and Relaxometry.

Water dynamics in mesoporous dextran hydrogel micro/nanoparticles was investigated by means of nuclear magnetic resonance (NMR) techniques. High-resolution 1H NMR spectra and pulsed field gradient (PFG) NMR diffusometry measurements obtained on swollen state dextran micro/nanogel revealed the existence of different fractions of water molecules based on their interaction with the gel matrix. In addition to the translational diffusion of bulk water, two more diffusion processes characterized with self-diffusion coefficients 1 and 2 orders of magnitude smaller than that of bulk water were identified. 1H spin-lattice relaxation dispersion profiles obtained for a broad range of Larmor frequencies using fast field cycling (FFC) and conventional NMR relaxometry techniques allowed us to further clarify the mechanisms of molecular motion. According to the water proton pool fractions and associated self-diffusion coefficients, it is shown that the relaxation contribution associated with reorientation-mediated translational motions (RMTDs) dominates the relaxation dispersion observed at intermediate frequencies. At very low frequencies, the spin-lattice relaxation rate is dominated by the slow solid-gel dynamics probed by the water molecules interacting with the pores' surface hydroxyl groups due to the rapid chemical exchange between surface hydroxyl groups and free water. The correlation time for the thumbling-like motion of the dextran gel was found to be in the submillisecond range. The values of the self-diffusion and coherence lengths associated with motion of water molecules interacting with the solid-gel particles are consistent with the particle size and pore size distributions obtained for the studied dextran gels.

Chiral distinction between hydroxychloroquine enantiomers in binding to angiotensin-converting enzyme 2, the forward receptor of SARS-CoV-2

Soon after the outset of the Coronavirus Disease 2019 (COVID-19) pandemic (March-April 2020), formulations of the old antimalarial racemic drug hydroxychloroquine (HCQ) sulfate were authorized by the U.S. Food and Drug Administration (FDA) for emergency treatment of hospitalized patients with COVID-19. A call for the chiral switch of HCQ to the single enantiomer (S)-(+)-HCQ for treating the disease followed. The above authorizations were later withdrawn. Angiotensin-converting enzyme 2 (ACE2) has been recognized to be the forward receptor of SARS-CoV-2, the virus responsible for COVID-19. The objective of the present study was to evaluate the chiral distinction in the potential preferential binding of the HCQ enantiomers to ACE2, as a basis for its future drug repurposing, using high-field solution Nuclear Magnetic Resonance (NMR) spectroscopy. Proton selective spin-lattice relaxation rates were measured for selected diagnostic nuclei; in particular, protons belonging to the quinoline ring proved to be the most affected by the presence of the protein, for both (S)-(+)-HCQ and (R)-(−)-HCQ enantiomers. An increase in mono-selective relaxation rates was observed for both enantiomers. A significant difference in the magnitude of the increase was detected for all protons investigated, up to a 5-fold and an 8-fold increase in the case of (R)-(−)-HCQ and (S)-(+)-HCQ, respectively. Furthermore, comparison between the normalized mono-selective relaxation rates of the two HCQ enantiomers in their binary mixtures with ACE2 pointed out a certain preference for the (S)-(+)-HCQ enantiomer over (R)-(−)-HCQ in the interaction with ACE2. The findings form the basis for a future application of the drug repurposing/chiral-switch combination strategy to racemic HCQ in previously reported indications for hydroxychloroquine treatment and in the search for new indications in which ACE2 receptors are involved.

In vitro oxygen imaging of acellular and cell-loaded beta cell replacement devices

Type 1 diabetes (T1D) is an autoimmune disease that leads to the loss of insulin-producing beta cells. Bioartificial pancreas (BAP) or beta cell replacement strategies have shown promise in curing T1D and providing long-term insulin independence. Hypoxia (low oxygen concentration) that may occur in the BAP devices due to cell oxygen consumption at the early stages after implantation damages the cells, in addition to imposing limitations to device dimensions when translating promising results from rodents to humans. Finding ways to provide cells with sufficient oxygenation remains the major challenge in realizing BAP devices’ full potential. Therefore, in vitro oxygen imaging assessment of BAP devices is crucial for predicting the devices’ in vivo efficiency. Electron paramagnetic resonance oxygen imaging (EPROI, also known as electron MRI or eMRI) is a unique imaging technique that delivers absolute partial pressure of oxygen (pO2) maps and has been used for cancer hypoxia research for decades. However, its applicability for assessing BAP devices has not been explored. EPROI utilizes low magnetic fields in the mT range, static gradients, and the linear relationship between the spin–lattice relaxation rate (R1) of oxygen-sensitive spin probes such as trityl OX071 and pO2 to generate oxygen maps in tissues. With the support of the Juvenile Diabetes Research Foundation (JDRF), an academic-industry partnership consortium, the “Oxygen Measurement Core” was established at O2M to perform oxygen imaging assessment of BAP devices originated from core members’ laboratories. This article aims to establish the protocols and demonstrate a few examples of in vitro oxygen imaging of BAP devices using EPROI. All pO2 measurements were performed using a recently introduced 720 MHz/25 mT preclinical oxygen imager instrument, JIVA-25™. We began by performing pO2 calibration of the biomaterials used in BAPs at 25 mT magnetic field since no such data exist. We compared the EPROI pO2 measurement with a single-point probe for a few selected materials. We also performed trityl OX071 toxicity studies with fibroblasts, as well as insulin-producing cells (beta TC6, MIN6, and human islet cells). Finally, we performed proof-of-concept in vitro pO2 imaging of five BAP devices that varied in size, shape, and biomaterials. We demonstrated that EPROI is compatible with commonly used biomaterials and that trityl OX071 is nontoxic to cells. A comparison of the EPROI with a fluorescent-based point oxygen probe in selected biomaterials showed higher accuracy of EPROI. The imaging of typically heterogenous BAP devices demonstrated the utility of obtaining oxygen maps over single-point measurements. In summary, we present EPROI as a quality control tool for developing efficient cell transplantation devices and artificial tissue grafts. Although the focus of this work is encapsulation systems for diabetes, the techniques developed in this project are easily transferable to other biomaterials, tissue grafts, and cell therapy devices used in the field of tissue engineering and regenerative medicine (TERM). In summary, EPROI is a unique noninvasive tool to experimentally study oxygen distribution in cell transplantation devices and artificial tissues, which can revolutionize the treatment of degenerative diseases like T1D.

Anion and cation dynamics in a carbon-substituted cesium closo-hydroborate CsCB11H12: 1H and 133Cs NMR studies

Cesium monocarba-hydroborate CsCB11H12 belongs to the family of alkali-metal closo- hydroborate salts which are considered as promising solid electrolytes for battery applications. To study the dynamical properties of this compound at the microscopic level, we have measured the 1H and 133Cs nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates over the temperature range of 80–434 K. Our measurements have revealed a coexistence of two types of reorientational jump processes of the complex [CB11H12]− anions. For the faster of these processes characterized by the average activation energy of 310 meV, the anion reorientations are not “frozen” at the NMR frequency scale down to 120 K. The behavior of the 133Cs NMR line width in CsCB11H12 is consistent with the onset of long-range diffusive motion of Cs+ ions at the frequency scale of ∼104 s−1 above 340 K, and the distinct effects of the diffusive Cs+ jumps on both the 1H and 133Cs spin-lattice relaxation rates become observable above 380 K.

Localization of the ibuprofen molecule in model lipid membranes revealed by spin-label-enhanced NMR relaxation

Non-steroidal anti-inflammatory drugs (NSAIDs) have antipyretic, anti-inflammatory and analgesic effects, and can be used in the treatment of various diseases. These drugs have also a number of side effects, which may be related to their interaction with lipid membranes. In this study, we use the spin-labeled NSAID ibuprofen (ibuprofen-SL) as a relaxation enhancer to study its interaction with model lipid membranes employing liquid-state 1H NMR at 500 MHz. The high magnetic moment of unpaired electron in the spin label made it possible to reduce the concentration of the studied drug in the membrane to tenths of a mole percent. As model membranes, unilamellar POPC liposomes and bicelles consisting of a 2:1 mixture of DHPC:DMPC or DHPC:POPC lipids were used. An increase in the rate of proton spin-lattice relaxation, T1−1, selectively detected for protons at different positions in the lipid molecule, showed that ibuprofen-SL is localized in the hydrophobic part of the lipid bilayer. As the concentration of ibuprofen-SL increases to 0.5 mol%, the distribution of positions of ibuprofen-SL across the bilayer becomes wider. In the presence of 20 mol% of cholesterol, ibuprofen-SL is displaced from the core of the membrane to a region closer to the head group of the bilayer. This displacement was also confirmed by the NMR NOESY experiment conducted with unlabeled ibuprofen. For bilayers containing unsaturated POPC lipids, the distribution of ibuprofen positions across the bilayer becomes narrower compared to the presence of saturated DMPC lipids.

Gapless fermionic excitation in the antiferromagnetic state of ytterbium zigzag chain

The emergence of charge-neutral fermionic excitations in magnetic systems is one of the unresolved issues in recent condensed matter physics. This type of excitations has been observed in various systems, such as low-dimensional quantum spin liquids, Kondo insulators, and antiferromagnetic insulators. Here, we report the presence of a pronounced gapless spin excitation in the low-temperature antiferromagnetic state of YbCuS2 semiconductor, where trivalent ytterbium atoms form a zigzag chain structure. We confirm the presence of this gapless excitations by a combination of experimental probes, namely 63/65Cu-nuclear magnetic resonance and nuclear quadrupole resonance, as well as specific heat measurements, revealing a linear low-temperature behavior of both the nuclear spin-lattice relaxation rate 1/T1 and the specific heat. This system provides a platform to investigate the origin of gapless excitations in spin chains and the relationship between emergent fermionic excitations and frustration.

Antiferromagnetic Spin Fluctuations and Unconventional Superconductivity in Topological Superconductor Candidate YPtBi Revealed by ^{195}Pt-NMR.

We report ^{195}Pt nuclear magnetic resonance (NMR) measurements on topological superconductor candidate YPtBi, which has broken inversion symmetry and topological nontrivial band structures due to the strong spin-orbit coupling. In the normal state, we find that Knight shift K is field- and temperature independent, suggesting that the contribution from the topological bands is very small at low temperatures. However, the spin-lattice relaxation rate 1/T_{1} divided by temperature (T), 1/T_{1}T, increases with decreasing T, implying the existence of antiferromagnetic spin fluctuations. In the superconducting state, no Hebel-Slichter coherence peak is seen below T_{c} and 1/T_{1} follows T^{3} variation, indicating the unconventional superconductivity. The finite spin susceptibility at zero-temperature limit and the anomalous increase of the NMR linewidth below T_{c} point to a mixed state of spin-singlet and spin-triplet (or spin-septet) pairing.

Nuclear Magnetic Resonance Studies of Bicontinuous Liquid Crystalline Phases of Cubic Symmetry: Transport Properties from 2H Nuclear Magnetic Resonance Relaxation Rates.

The ternary system didodecyltrimethylammonium bromide, 1-decanol, and water forms an extended reversed continuous phase of cubic symmetry at 25 °C. The cubic phase belongs to the space group Im3m, as shown by small-angle X-ray experiments. We present extensive deuterium NMR relaxation data from this cubic phase for 1-decanol, deuterated at the carbon adjacent to the hydroxyl carbon position. 2H spin-lattice (R1) and spin-spin (R2) relaxation rates were measured over the existence region of the cubic phase, which extends from 0.2 to 0.6 in volume fraction of the dividing bilayer surface of the cubic phase. The data are interpreted with an existing theoretical framework for NMR spin relaxation in bicontinuous cubic phases, which takes its starting point in the description of bicontinuous phases using periodic minimal surfaces. Specifically, we obtain the self-diffusion coefficient over the minimal surface in one unit cell for 1-decanol. We also present pulsed field gradient NMR-derived self-diffusion data for didodecyltrimethylammonium bromide and compare the two sets of data. The diffusion data for both components show a mild, if any, dependence on the volume fraction of the bilayer surface. Furthermore, we present diffusion data for the water component in the cubic phase. Finally, we discuss the influences of the choice of the value of the product of the deuterium quadrupole constant and the order parameter S. Within the framework of the model used to analyze the relaxation data, a value for this parameter is required. As an initial value, we rely on measurements of deuterium quadrupolar splittings from deuterated decanol in an anisotropic phase.

Anomalous spin dynamics of the S=32 kagome ferromagnet Li9Cr3(P2O7)3(PO4)2

We report the dynamical magnetic properties of the $s=\frac{3}{2}$ kagome ferromagnet ${\mathrm{Li}}_{9}{\mathrm{Cr}}_{3}{({\mathrm{P}}_{2}{\mathrm{O}}_{7})}_{3}{({\mathrm{PO}}_{4})}_{2}$ using magnetic susceptibility, specific heat, electron spin resonance (ESR), and nuclear magnetic resonance techniques. The magnetic susceptibility and specific heat show ferromagnetic ordering at ${T}_{\mathrm{C}}=2.7$ K. The in-plane ferromagnetic correlation length ${\ensuremath{\xi}}_{ab}$ deduced from the magnetization curves reveals that ${\ensuremath{\xi}}_{ab}$ increases steeply from 18.2 to 41.3 \AA{} as the temperature is lowered from 50 K to ${T}_{\mathrm{C}}$, indicative of the persistence of two-dimensional magnetic correlations up to temperatures as high as $\ensuremath{\sim}18\phantom{\rule{0.16em}{0ex}}{T}_{\mathrm{C}}$. The ESR linewidth $\mathrm{\ensuremath{\Delta}}{H}_{\mathrm{pp}}$ and magnetic specific heat data indicate a change in spin dynamics at about ${T}^{*}\ensuremath{\approx}4.4{T}_{\mathrm{C}}$. Intriguingly, for temperatures below 50 K, we observe an unexpected suppression of the $^{7}\mathrm{Li}$ spin-lattice relaxation rate $1/{T}_{1}$. The decrease in $1/{T}_{1}$, which occurs well above ${T}_{\mathrm{C}}$, is at odds with the critical slowing down of magnetic correlations typically observed in conventional ferromagnets. This anomalous thermal evolution of spin fluctuations is interpreted in terms of the concerted interplay of flat and Dirac magnon bands inherent in kagome ferromagnets.

A direct view on Ni substitution in Sc2Ir6-xNixB as probed by NMR

Intermetallic borides, such as the Sc2Ir6-xNixB (x = 0–6) series, exhibit technologically relevant physicochemical properties, that are all associated to boron ordering. X-ray diffraction has not yet fully investigated the effect of Ir substitution by Ni on the boron arrangement of these borides, primarily due to its weak scattering factor. Herein we employ 11B-NMR to probe the boron ordering of Sc2Ir6-xNixB series at atomic scale, overcoming limitations on low-Z materials. Across the Ni-substitution range, the spectral parameters and spin-lattice relaxation rates (T1−1) were responsive to changes in crystal symmetry and valence charges. The spectral analysis and T1−1 vs. x reveal that Ni substitution causes phase separation at the nanoscale between ordered and disordered borides. For x = 3, T1−1 is T-independent, indicating a relaxation mechanism caused by paramagnetic centers whereas x = 5 follows a T2-dependence that is consistent with band structure predictions of a reduction in the density of states, correlated with sharp features near the Fermi level.

Magnetic resonance study of novel detonation nanodiamonds originated from non-conventional explosives

Detonation nanodiamonds (DND) are conventionally synthesized from a mixture of trinitrotoluene (TNT) and hexogen (RDX) explosives. It is generally believed that the internal structure of DNDs, which consists of a diamond core covered with a partially disordered shell, is very similar in all DNDs. Variations of the properties of DNDs are usually associated with the surface functionalization of the hydrogen-terminated surface with various chemicals to obtain a family of different nanodiamonds. In this paper, we report on the synthesis, electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) study of novel DNDs synthesized from a few non-conventional explosives, which have not previously been used for this purpose. We show that DNDs synthesized from different explosive precursors display different ratios of carbon atoms in the core and shell. The variation of this ratio correlates well with variations in the content of paramagnetic defects and 13C nuclear spin-lattice relaxation rates. The presented data advances new opportunities for controllable synthesis of DNDs with engineered properties.

Nuclear spin relaxation in aqueous paramagnetic ion solutions.

A Brownian shell model describing the random rotational motion of a spherical shell of uniform particle density is presented and validated by molecular dynamics simulations. The model is applied to proton spin rotation in aqueous paramagnetic ion complexes to yield an expression for the Larmor-frequency-dependent nuclear magnetic resonance spin-lattice relaxation rate T_{1}^{-1}(ω) describing the dipolar coupling of the nuclear spin of the proton with the electronic spin of the ion. The Brownian shell model provides a significant enhancement to existing particle-particle dipolar models without added complexity, allowing fits to experimental T_{1}^{-1}(ω) dispersion curves without arbitrary scaling parameters. The model is successfully applied to measurements of T_{1}^{-1}(ω) from aqueous manganese(II), iron(III), and copper(II) systems where the scalar coupling contribution is known to be small. Appropriate combinations of Brownian shell and translational diffusion models, representing the inner and outer sphere relaxation contributions, respectively, are shown to provide excellent fits. Quantitative fits are obtained to the full dispersion curve of each aquoion with just five fit parameters, with the distance and time parameters each taking a physically justifiable numerical value.

Calorimetric measurement of nuclear spin-lattice relaxation rate in metals

The quasiparticle density of states in correlated and quantum-critical metals directly probes the effect of electronic correlations on the Fermi surface. Measurements of the nuclear spin-lattice relaxation rate provide one such experimental probe of quasiparticle mass through the electronic density of states. By far the most common way of accessing the spin-lattice relaxation rate is via nuclear magnetic resonance and nuclear quadrupole resonance experiments, which require resonant excitation of nuclear spin transitions. Here we report non-resonant access to spin-lattice relaxation dynamics in AC-calorimetric measurements. The nuclear spin-lattice relaxation rate is inferred in our measurements from its effect on the frequency dispersion of the thermal response of the calorimeter-sample assembly. We use fast, lithographically-defined nanocalorimeters to access the nuclear spin-lattice relaxation times in metallic indium from 0.3~K to 7~K and in magnetic fields up to 35~T.

Low-Temperature Magnetic Fluctuations Investigated by 125Te-NMR on the Uranium-Based Superconductor UTe2

To investigate the static and dynamic magnetic properties on the uranium-based superconductor UTe2, we measured the NMR Knight shift K and the nuclear spin–lattice relaxation rate 1/T1 in H || a by 125Te-NMR on a 125Te-enriched single-crystal sample. 1/T1T in H || a is much smaller than 1/T1T in H || b and c, and magnetic fluctuations along each axis are derived from the 1/T1T measured in H parallel to all three crystalline axes. The magnetic fluctuations are almost identical at two Te sites and isotropic at high temperatures, but become anisotropic below 40 K, where heavy-fermion state is formed. The character of magnetic fluctuations in UTe2 is discussed with the comparison to its static susceptibility and the results on other U-based superconductors. It is considered that the magnetic fluctuations probed with the NMR measurements are determined by the magnetic properties inside the two-leg ladder formed by U atoms, which are dominated by the qa = 0 ferromagnetic fluctuations.

Dynamical properties of the magnesium borohydride – ethylenediamine compound Mg(en)1.2(BH4)2: A nuclear magnetic resonance study

The magnesium borohydride – ethylenediamine (en) compound Mg(en)1.2(BH4)2 exhibits high Mg-ion conductivity above room temperature. To study the dynamical properties of this compound and its partially deuterium-substituted counterpart, we have measured the 1H, 2D, and 11B nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates over a wide temperature range (6–324 K). Our measurements have revealed a coexistence of four types of BH4 reorientational jump processes with different activation energies. For the fastest of these processes characterized by the activation energy of 47(3) meV, the unusually high reorientational mobility is retained down to low temperatures, so that the corresponding H jump rate is of the order of 108 s−1 near 70 K. The presence of fast reorientational motion of [BH4]− anions in Mg(en)1.2(BH4)2 supports the idea that anion reorientations may facilitate the cation diffusion. In the studied temperature range, we have not found any distinct signs of localized motion of the ethylenediamine molecules or their fragments at the NMR frequency scale.

Microsecond Motion of the Bacterial Transporter EmrE in Lipid Bilayers.

The bacterial transporter EmrE is a homo-dimeric membrane protein that effluxes cationic polyaromatic substrates against the concentration gradient by coupling to proton transport. As the archetype of the small multidrug resistance family of transporters, EmrE structure and dynamics provide atomic insights into the mechanism of transport by this family of proteins. We recently determined high-resolution structures of EmrE in complex with a cationic substrate, tetra(4-fluorophenyl)phosphonium (F4-TPP+), using solid-state NMR spectroscopy and an S64V-EmrE mutant. The substrate-bound protein exhibits distinct structures at acidic and basic pH, reflecting changes upon binding or release of a proton from residue E14, respectively. To obtain insight into the protein dynamics that mediate substrate transport, here we measure 15N rotating-frame spin-lattice relaxation (R1ρ) rates of F4-TPP+-bound S64V-EmrE in lipid bilayers under magic-angle spinning (MAS). Using perdeuterated and back-exchanged protein and 1H-detected 15N spin-lock experiments under 55 kHz MAS, we measured 15N R1ρ rates site-specifically. Many residues show spin-lock field-dependent 15N R1ρ relaxation rates. This relaxation dispersion indicates the presence of backbone motions at a rate of about 6000 s-1 at 280 K for the protein at both acidic and basic pH. This motional rate is 3 orders of magnitude faster than the alternating access rate but is within the range estimated for substrate binding. We propose that these microsecond motions may allow EmrE to sample different conformations to facilitate substrate binding and release from the transport pore.

Survival of magnetic correlations above the ordering temperature in the ferromagnetically ordered classical kagome magnet Li9Cr3(P2O7)3(PO4)2

Motivated by the recent discovery of a semiclassical nematic spin liquid state in a Heisenberg kagom\'e antiferromagnet ${\mathrm{Li}}_{9}{\mathrm{Fe}}_{3}{({\mathrm{P}}_{2}{\mathrm{O}}_{7})}_{3}{({\mathrm{PO}}_{4})}_{2}$ (LFPO) with $S=5/2$ [Kermarrec $et\phantom{\rule{4pt}{0ex}}al$. Phys. Rev. Lett. 127, 157202 (2021)], we now investigate the impact of spin quantum number $S$ on the ground-state properties by studying the isostructural kagom\'e magnet ${\mathrm{Li}}_{9}{\mathrm{Cr}}_{3}{({\mathrm{P}}_{2}{\mathrm{O}}_{7})}_{3}{({\mathrm{PO}}_{4})}_{2}$ (LCPO) with active ${t}_{2g}$ orbitals and $S=3/2$. Thermodynamic measurements reveal that the ground-state properties of LCPO is dominated by the ferromagnetic interactions with a mean-field temperature $\mathrm{\ensuremath{\Theta}}\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}3\phantom{\rule{0.28em}{0ex}}\text{K}(J<1\phantom{\rule{0.28em}{0ex}}\text{K})$ and the ordering temperature, ${T}_{c}$ \ensuremath{\sim}2.7 K, and the size of the ordered moment \ensuremath{\sim}$1.05\phantom{\rule{0.28em}{0ex}}\ifmmode\pm\else\textpm\fi{}0.25\phantom{\rule{0.28em}{0ex}}{\textmu{}}_{\mathrm{B}}$ is significantly reduced from that of a fully ordered moment. The ab initio electronic structure calculations nicely corroborate the thermodynamic results and suggest the presence of additional in- and out-of-plane further-neighbor antiferromagnetic couplings, although significantly weaker in comparison to the dominant first-nearest-neighbor ferromagnetic coupling. The spin-lattice relaxation rate measured with fields larger than the saturation field shows a magnetic field induced gap $(\mathrm{\ensuremath{\Delta}}\ensuremath{\propto}$ B) in the excitation spectrum, and in $B\ensuremath{\rightarrow}0$ limit the gap has a finite intercept \ensuremath{\sim}3 K, equivalent to the mean-field scale. We interpret the origin of this gap is associated with the magnetic interactions inherent to the material. With our experimental results, we establish the stabilization of a ferromagnetic like ground state and the persistence of magnetic correlations above the ordering temperature in LCPO.