Field-cycling NMR Relaxometry Research Articles

Trapping of Gd(III) Ions by Keplerate Polyanionic Nanocapsules in Water: A 1H Fast Field Cycling NMR Relaxometry Study

Polyoxomolybdates of the type [{MoVIMoVI5O21(H2O)6}12{MoV2O4(L)}30](12+30n)–, also referred to as keplerates (Kps), are polyanionic nanocapsules bearing 20 {Mo9O9} pores connected to a central cavity by 20 channels. In solution, they can entrap cations in different sites of their porous structure (in the internal cavity, in the pores, or on the capsule surface) in equilibrium with cations free in solution. The porosity of Kps was previously exploited to prepare nanostructured potential contrast agents through the self-assembly of KpOAc or KpHPO4 (i.e., Kp with L = acetate or hydrogen phosphate) macroanions and Gd(III) cations in water, accompanied by stabilization with F-127 Pluronics. The strongly enhanced water proton relaxivity observed at 20 MHz was postulated to derive from the trapping of Gd(III) aqua ions by the nanocapsules. In the present work, 1H fast field cycling NMR relaxometry allowed this hypothesis to be verified on aqueous suspensions containing either KpOAc or KpHPO4 and Gd(III) in diffe...

Exploring the tumour extracellular matrix by in vivo Fast Field Cycling relaxometry after the administration of a Gadolinium-based MRI contrast agent.

1 H Fast Field Cycling NMR (FFC-NMR) relaxometry is proposed as a powerful method to investigate tumour stroma in vivo upon the administration of a Gd-based contrast agent. To perform this study, an FFC-NMR equipment endowed with a wide bore magnet was used for the acquisition of Nuclear Magnetic Resonance Dispersion profiles on healthy muscle and tumour tissue in living mice. At magnetic field strengths < of ca. 1MHz, the differences in the relaxation rates of the intra and extracellular compartment become of the same order of magnitude of the exchange rate across the cellular membranes. Under this condition, the water exchange rate between the two compartments yields to a biexponential magnetization recovery that can be analysed by fitting the experimental data with the two-Site eXchange (2SX) model. Using this model, it was possible to obtain, for the two compartments, both relaxation properties and water kinetic constants for water exchange across cell membranes. The method allowed us to determine the effect of the "matrix" on the water proton relaxation times and, in turn, to get some insights of the composition of this compartment, till now, largely unknown.

Monitoring lubricant oil degradation using field-cycling NMR relaxometry

ABSTRACTProton fast field-cycling nuclear magnetic resonance (1HFFC-NMR) was used to study the effect of thermal stress in mineral base oil. The base oil underwent controlled thermal degradation and its relaxometric response was evaluated and interpreted in terms of self-diffusion and molecular rotations. Our results show slight changes in the relaxometric behaviour corresponding to samples exposed at typical engine working temperature (), whereas extreme heating treatment () have produced degradation that is clearly evidenced in the profiles, particularly at low Larmor frequencies. Additionally, diffusion coefficients were calculated using a simplified method, showing the expected physical tendency with temperature and degradation level.

Field-cycling NMR relaxometry: the benefit of constructing master curves

ABSTRACTThe currently available field-cycling NMR relaxometers still suffer from a rather narrow frequency range; they facilitate measurements of the 1H spin–lattice relaxation rate in the frequency of 10 kHz–30 MHz. This limit may be overcome by constructing master curves via exploiting frequency-temperature superposition, the latter being an intrinsic feature of the collective dynamics in many soft matter systems. It states that the shape of the motional time-correlation function is essentially temperature independent over a large temperature interval. As will be demonstrated, master curves may be built in the susceptibility or the spectral density representation of spin–lattice relaxation data. As a result, the effective frequency range is expanded up to ten decades, and the applied shift factors provide the temperature dependence of the corresponding correlation time. Three examples are presented: cyano adamantane in its plastically crystalline phase, the liquid glycerol, and the melt of poly(ethylene propylene). Advantages and limitations of the approach are discussed.

Search and Subvert: Minimalist Bacterial Phosphatidylinositol-Specific Phospholipase C Enzymes.

Phosphatidylinositol-specific phospholipase C (PI-PLC) enzymes from Gram-positive bacteria are secreted virulence factors that aid in downregulating host immunity. These PI-PLCs are minimalist peripheral membrane enzymes with a distorted (βα)8 TIM barrel fold offering a conserved and stable scaffold for the conserved catalytic amino acids while membrane recognition is achieved mostly through variable loops. Decades of experimental and computational research on these enzymes have revealed the subtle interplay between molecular mechanisms of catalysis and membrane binding, leading to a semiquantitative model for how they find, bind, and cleave their respective substrates on host cell membranes. Variations in sequence and structure of their membrane binding sites may correlate with how enzymes from different Gram-positive bacteria search for their particular targets on the membrane. Detailed molecular characterization of protein-lipid interactions have been aided by cutting-edge methods ranging from 31P field-cycling NMR relaxometry to monitor protein-induced changes in phospholipid dynamics to molecular dynamics simulations to elucidate the roles of electrostatic and cation-π interactions in lipid binding to single molecule fluorescence measurements of dynamic interactions between PI-PLCs and vesicles. This toolkit is readily applicable to other peripheral membrane proteins including orthologues in Gram-negative bacteria and more recently discovered eukaryotic minimalist PI-PLCs.

Rouse dynamics in PEO-PPO-PEO block-copolymers in aqueous solution as observed through fast field-cycling NMR relaxometry

We present a proton fast field-cycling (FFC) NMR relaxometry study of the molecular dynamics in three different deuterated water-dispersed triblock copolymers of ethylene oxide (EO) and propylene oxide (PO):EO80PO27EO80(F68), EO141PO44EO141 (F108), and EO101PO56EO101(F127). Independently of the phase and molecular arrangement, bi-exponential decays of the magnetization during the spin-lattice relaxation process could be observed for F127, while mono-exponential decays were measured for F68 and F108. This fact has been attributed to the relative ratio of PEO and PPO protons for each case. In F127, each component of the magnetization decay could be associated with a particular block of the co-polymer. A direct consequence of this fact is the independent characterization of the molecular dynamics of each block. It was found that the dominant relaxation mechanism can be attributed to the Rouse model, and it seems to be independent on whether the molecules are incorporated into a micelle, or as individual unimers in the aqueous solution. The experimental results and the provided explanation are consistent with entanglement-free self-assembled structures, and a fast exchange of unimers between the micellar structure and the solvent. This particular feature was also investigated in F68 and F108, although for these cases a mono-exponential decay of the magnetization was observed. NMR relaxometry results are complemented with other relaxation experiments in the rotating frame, NMR spectroscopy and atomic-force microscopy.

Dynamic Characteristics of Guanosine-5'-monophosphate Reductase Complexes Revealed by High-Resolution 31P Field-Cycling NMR Relaxometry.

The ability of enzymes to modulate the dynamics of bound substrates and cofactors is a critical feature of catalysis, but the role of dynamics has largely been approached from the perspective of the protein. Here, we use an underappreciated NMR technique, subtesla high-resolution field-cycling 31P NMR relaxometry, to interrogate the dynamics of enzyme bound substrates and cofactors in guanosine-5'-monophosphate reductase (GMPR). These experiments reveal distinct binding modes and dynamic profiles associated with the 31P nuclei in the Michaelis complexes for the deamination and hydride transfer steps of the catalytic cycle. Importantly, the substrate is constrained and the cofactor is more dynamic in the deamination complex E·GMP·NADP+, whereas the substrate is more dynamic and the cofactor is constrained in the hydride transfer complex E·IMP·NADP+. The presence of D2O perturbed the relaxation of the 31P nuclei in E·IMP·NADP+ but not in E·GMP·NADP+, providing further evidence of distinct binding modes with different dynamic properties. dIMP and dGMP are poor substrates, and the dynamics of the cofactor complexes of dGMP/dIMP are disregulated relative to GMP/IMP. The substrate 2'-OH interacts with Asp219, and mutation of Asp219 to Ala decreases the value of Vmax by a factor of 30. Counterintuitively, loss of Asp219 makes both substrates and cofactors less dynamic. These observations suggest that the interactions between the substrate 2'-OH and Asp219 coordinate the dynamic properties of the Michaelis complexes, and these dynamics are important for progression through the catalytic cycle.

Hydration of MgO-Based Cement: Water Dynamics by 1H Fast Field-Cycling NMR Relaxometry

1H Fast Field-Cycling (FFC) NMR relaxometry was applied for the first time to monitor the state of water during the hydration reaction of MgO and silica that brings to the formation of magnesium silicate hydrate (M-S-H), the binder phase of innovative cements with promising applications in the containment of radioactive waste. To this aim water proton longitudinal relaxation rates (R1=1/T1) were measured in the Larmor frequency range between 10 kHz and 30 MHz at different hydration times ranging from 0.5 h to ~ 4 months. The obtained R1 vs frequency (NMRD) curves were analyzed considering fast exchange of water molecules between a hydration layer, where dynamics is affected by interactions with the surface of solids present in the reacting mixture, and a bulk phase. For the hydration layer, water molecules undergoing fast local molecular dynamics on the surface gave a constant contribution to R1 throughout the investigated frequency range; on the other hand, water molecules undergoing slow dynamics on the...

Correction of environmental magnetic fields for the acquisition of Nuclear magnetic relaxation dispersion profiles below Earth’s field

T1 relaxation times can be measured at a range of magnetic field strengths by Fast Field-Cycling (FFC) NMR relaxometry to provide T1-dispersion curves. These are valuable tools for the investigation of material properties as they provide information about molecular dynamics non-invasively. However, accessing information at fields below 230 μT (10kHz proton Larmor frequency) requires careful correction of unwanted environmental magnetic fields.In this work a novel method is proposed that compensates for the environmental fields on a FFC-NMR relaxometer and extends the acquisition of Nuclear Magnetic Relaxation Dispersion profiles to 2.3μT (extremely low field region), with direct application in the study of slow molecular motions. Our method is an improvement of an existing technique, reported by Anoardo and Ferrante in 2003, which exploits the non-adiabatic behaviour of the magnetisation in rapidly-varying magnetic fields and makes use of the oscillation of the signal amplitude to estimate the field strength. This increases the accuracy in measuring the environmental fields and allows predicting the optimal correction values by applying simple equations to fit the data acquired. Validation of the method is performed by comparisons with well-known dispersion curves obtained from polymers and benzene.

Dynamical regimes of lipids in additivated liposomes with enhanced elasticity: A field-cycling NMR relaxometry approach

We study the molecular dynamics of lipids in binary large unilamellar liposomes suspended in D2O composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or soy phosphatidylcholine (SPC) additivated with different percentiles of sodium deoxycholate (SDC). We use the fast field-cycling proton NMR relaxometry technique over a wide timescale and at diverse temperatures. A model previously validated in different formulations is here employed for the relaxometric analysis of elastic vesicles. A new dynamical regime is observed for the first time in additivated DMPC and additivated/non-additivated SPC liposomes. This surprising feature is discussed in terms of vesicle shape fluctuations, enhanced elasticity and lipid & additive diffusion within the membrane. The continuum elastic theory is revisited for a better understanding of recent experiments and those here presented. We address the point of deformability measurements across rigid permeable barriers versus measurements of the bending elastic modulus in free-standing vesicles.

Dynamics of poly(vinyl butyral) studied using dielectric spectroscopy and 1H NMR relaxometry.

Dielectric Spectroscopy (DS) and 1H Fast Field-Cycling (FFC) NMR relaxometry were applied for understanding the dynamic behavior of the amorphous ter-polymer poly(vinyl butyral) (PVB) across the glass transition temperature (Tg = 70 °C by Differential Scanning Calorimetry). Above Tg, main chain segmental motions (α relaxation) were detected and characterized using both DS and FFC NMR relaxometry. The correlation times extracted by the analysis of DS and FFC NMR relaxometry data agreed within a factor of three and showed a Vogel-Fulcher-Tammann temperature dependence, with an associated Tg of 69 °C and a fragility of 155 for PVB glass. Below Tg, a secondary process (β relaxation) was revealed by DS, and was ascribed to reorientations of the vinyl alcohol dipoles due to local twisting motions with an associated activation barrier of 11 kcal mol-1. The β process was also found to contribute to 1H NMR relaxation above Tg.

Dynamics of a Paradigmatic Linear Polymer: A Proton Field-Cycling NMR Relaxometry Study on Poly(ethylene–propylene)

The dynamics of melts of linear poly(ethylene-alt-propylene) (PEP) of different molar masses (M) is investigated by 1H field-cycling (FC) NMR relaxometry. Employing a commercial and a home-built relaxometer the spin-lattice relaxation rate R1(ω) is measured in the frequency range of 200 Hz to 30 MHz and the temperature range of 200–400 K. Transforming the FC NMR relaxation data to the susceptibility representation and applying frequency–temperature superposition, master curves for the dipolar correlation function CDD(t/τα) (containing intra- and intermolecular contributions) are constructed which extend up to six decades in amplitude and eight in time. Here, τα is the time scale of the structural (α-) relaxation, which is obtained over several decades. Comparison with previously reported FC data for polybutadiene (PB) discloses very similar CDD(t). Depending on M, all the five relaxation regimes of a polymer melt are covered: in addition to the α-process (0) and the terminal relaxation (IV), which are imm...

Measurement of the bending elastic modulus in unilamellar vesicles membranes by fast field cycling NMR relaxometry

The elastic properties of lipid membranes can be conveniently characterized through the bending elastic modulus κ. Elasticity directly affects the deformability of a membrane, morphological and shape transitions, fusion, lipid-protein interactions, etc. It is also a critical property for the formulation of ultradeformable liposomes, and of interest for the design of theranostic liposomes for efficient drug delivery systems and/or different imaging contrast agents. Measurements of κ in liposome membranes have been made using the fast field cycling nuclear magnetic relaxometry technique. We analyze the capability of the technique to provide a consistent value of the measured quantity under certain limiting conditions. Relaxation dispersions were measured acquiring a minimal quantity of points, within a reduced Larmor frequency range and, under inferior experimental conditions (in the presence of magnetic field in-homogeneity and lower power supply stability). A simplified model is discussed, showing practical advantages when fitting the data within the reduced frequency range. Experiments are contrasted with standard measurements performed in a state-of-the-art relaxometer. The methodology was tested in samples of 1,2-dimyristoyl-sn-glycero-3-phosphocholine with different percentiles of cholesterol. We observe a tendency to a decrease in κ with increasing temperature, and a tendency to increase with the cholesterol percentile.

Segmental Mean Square Displacement: Field-Cycling 1H Relaxometry vs Neutron Scattering

Proton (1H) field-cycling (FC) NMR relaxometry is applied to monitor the crossover in the segmental subdiffusion from the Rouse to the constrained Rouse regime in an entangled linear polymer melt. The method probes the dispersion of the spin–lattice relaxation rate R1(ω). Via Fourier transformation the segmental mean square displacement ⟨r2(t)⟩ is calculated from the intermolecular relaxation contribution R1inter(ω) to the total 1H spin–lattice relaxation dispersion R1(ω). As an example we chose poly(ethylene propylene) (M = 200k), and R1inter(ω) is singled out by performing an isotope dilution experiment. The ⟨r2(t)⟩ data obtained by FC NMR is directly compared to such of neutron scattering (NS) available from the literature. Because of different experimental time windows the NS data is converted to a reference temperature assuming frequency–temperature superposition. Absolute agreement is revealed between FC NMR and NS. The data on ⟨r2(t)⟩ confirm the predictions of the tube-reptation model; i.e., the c...

Substrate and Cofactor Dynamics on Guanosine Monophosphate Reductase Probed by High Resolution Field Cycling 31P NMR Relaxometry

Guanosine-5'-monophosphate reductase (GMPR) catalyzes the reduction of GMP to IMP and ammonia with concomitant oxidation of NADPH. Here we investigated the structure and dynamics of enzyme-bound substrates and cofactors by measuring 31P relaxation rates over a large magnetic field range using high resolution field cycling NMR relaxometry. Surprisingly, these experiments reveal differences in the low field relaxation profiles for the monophosphate of GMP compared with IMP in their respective NADP+ complexes. These complexes undergo partial reactions that mimic different steps in the overall catalytic cycle. The relaxation profiles indicate that the substrate monophosphates have distinct interactions in E·IMP·NADP+ and E·GMP·NADP+ complexes. These findings were not anticipated by x-ray crystal structures, which show identical interactions for the monophosphates of GMP and IMP in several inert complexes. In addition, the motion of the cofactor is enhanced in the E·GMP·NADP+ complex. Last, the motions of the substrate and cofactor are coordinately regulated; the cofactor has faster local motions than GMP in the deamination complex but is more constrained than IMP in that complex, leading to hydride transfer. These results show that field cycling can be used to investigate the dynamics of protein-bound ligands and provide new insights into how portions of the substrate remote from the site of chemical transformation promote catalysis.

Dry-cured ham tissue characterization by fast field cycling NMR relaxometry and quantitative magnetization transfer.

Fast field cycling (FFC) and quantitative magnetization transfer (qMT) NMR methods are two powerful tools in NMR analysis of biological tissues. The qMT method is well established in biomedical NMR applications, while the FFC method is often used in investigations of molecular dynamics on which longitudinal NMR relaxation times of the investigated material critically depend. Despite their proven analytical potential, these two methods were rarely used in NMR studies of food, especially when combined together. In our study, we demonstrate the feasibility of a combined FFC/qMT-NMR approach for the fast and nondestructive characterization of dry-curing ham tissues differing by protein content. The characterization is based on quantifying the pure quadrupolar peak area (area under the quadrupolar contribution of dispersion curve obtained by FFC-NMR) and the restricted magnetization pool size (obtained by qMT-NMR). Both quantities correlate well with concentration of partially immobilized, nitrogen-containing and proton magnetization exchanging muscle proteins. Therefore, these two quantities could serve as potential markers for dry-curing process monitoring. Copyright © 2016 John Wiley & Sons, Ltd.

Conformational Redistribution of Honey Components following Different Storage Conditions

The present study aims at the investigation of the changes in water distribution among the organic components of selected honey samples following honey storage at different temperatures. Results, achieved by application of fast field cycling NMR relaxometry, revealed that the organic constituents were homogeneously distributed within the whole samples stored at room temperature. Conversely, after four months of refrigeration at 4°C, the organic systems were included in persistent clusters, as a consequence of the water release due to the larger stability of the intramolecular interactions over the intermolecular ones. The new conformational arrangements of the honey constituents entailed enhancement of honey moisture content. For this reason, it can be suggested that honey refrigeration prior to storage at room temperature may be detrimental for its long-term storage. In fact, higher risk of fermentation may occur once the sample is warmed after the first refrigeration step.

Dynamics of PPI Dendrimers: A Study by Dielectric and2H NMR Spectroscopy and by Field-Cycling1H NMR Relaxometry

We investigate bulk poly(propyleneimine) dendrimers of generation (G) 2–5 by dielectric spectroscopy (DS), solid-state 2H NMR, and field-cycling 1H NMR relaxometry (FC 1H NMR) in a large temperature range (120–400 K). Three relaxation processes are identified by DS: a main (α-) relaxation (T > Tg) and two secondary processes (T < Tg). The α-process exhibits a super-Arrhenius temperature dependence typical of glass-forming liquids and changes only weakly with G, yielding Tg ∼ 200 K. The temperature dependence of the secondary relaxations is governed by an Arrhenius law. While one secondary process exhibits features characteristic for glasses, the other is atypical. Its time constant is virtually independent of G, and its spectral width does not increase with lowering temperature as is usually observed for sub-Tg relaxations. Regarding FC 1H NMR probing the dispersion of the spin–lattice rate R1 in the frequency range 200 Hz–30 MHz, transformation to the susceptibility representation, χ″(ω) ≡ ωR1(ω), and applying frequency–temperature superposition, an effective frequency range of 9 decades is covered by a master curve χ″(ωτα). In addition to the segmental time τα(T), which complements the results from DS up to high temperatures, a longer terminal relaxation τt(T) is identified. In between, an intermediate power-law regime is observed in χ″(ωτα) with an exponent of about 0.8. The broad relaxation spectrum is attributed to local dynamics, breathing modes, and overall tumbling and diffusion of the dendrimer molecule. In the low-frequency limit, R1(ω) is determined by intermolecular relaxation from which the molar mass dependence of the translational diffusion coefficient can be estimated. We find D(M) ∝ M–1.2±0.2.

Temperature Dependence of the Segmental Relaxation Time of Polymers Revisited

We analyze the temperature dependence of the segmental relaxation time τ of several low-Tg polymers with varying molar masses (M) as obtained from field-cycling 1H NMR relaxometry and dielectric spectroscopy. They are compared with those of molecular liquids (ML). Time constants in the range 3 × 10–12 s–1000 s, i.e., between Tg and 413 K, are covered. Describing τ(T) by the Vogel–Fulcher–Tammann (VFT) eq a systematic difference with respect to ML is found. While VFT fails for the latter it works well for polymers. The apparent activation energy at high temperatures shows a trend toward a temperature independent value E∞. For polymers, its M-dependence follows that of Tg(M), thus E∞(M) can be described by a Fox–Flory equation. Attempting to understand the difference among the two classes of liquids, we take recourse to our approach first applied to ML [J. Chem. Phys. 2013, 139, 084504]; i.e., we decompose the temperature-dependent activation energy E(T) controlling τ(T) in a constant high-temperature value...

New applications and perspectives of fast field cycling NMR relaxometry.

The field cycling NMR relaxometry method (also known as fast field cycling (FFC) when instruments employing fast electrical switching of the magnetic field are used) allows determination of the spin-lattice relaxation time (T1 ) continuously over five decades of Larmor frequency. The method can be exploited to observe the T1 frequency dependence of protons, as well as any other NMR-sensitive nuclei, such as (2) H, (13) C, (31) P, and (19) F in a wide range of substances and materials. The information obtained is directly correlated with the physical/chemical properties of the compound and can be represented as a 'nuclear magnetic resonance dispersion' curve. We present some recent academic and industrial applications showing the relevance of exploiting FFC NMR relaxometry in complex materials to study the molecular dynamics or, simply, for fingerprinting or quality control purposes. The basic nuclear magnetic resonance dispersion features are outlined in representative examples of magnetic resonance imaging (MRI) contrast agents, porous media, proteins, and food stuffs. We will focus on the new directions and perspectives for the FFC technique. For instance, the introduction of the latest Wide Bore FFC NMR relaxometers allows probing, for the first time, of the dynamics of confined surface water contained in the macro-pores of carbonate rock cores. We also evidence the use of the latest field cycling technology with a new cryogen-free variable-field electromagnet, which enhances the range of available frequencies in the 2D T1 -T2 correlation spectrum for separating oil and water in crude oil. Copyright © 2015 John Wiley & Sons, Ltd.