In recent years, low-field magnetic resonance has increasingly demonstrated its application of relaxation and diffusion measurements for the study of materials, catalysts, cement hydration, fluid transport in rocks and soil, geological prospecting, and characterization of tissue properties for medical diagnosis. In particular, the application on porous materials has benefited tremendously from the development in the multi-dimensional methods. Porous media are ubiquitous in our environment and their microstructure (μm to mm scale) is essential in determining their properties and applications. This article will summarize a few key advances in basic physics and NMR methodology, and their applications.

Abstract：G-rich repetitive DNA and RNA sequences can form four-stranded structures named G-quadruplexes. Solution NMR spectroscopy has played a central role in examining G-quadruplex structure, dynamics, and interaction. Here we review NMR methods to study structure and interaction of G-quadruplexes.

Post-translational modification by Small Ubiquitin-like MOdifier (SUMO) proteins regulates a diverse array of cellular events. The signaling process is initiated by attaching SUMO to the E1 activating protein. In the second step SUMO is transferred to E2 conjugating protein (Ubc9). Lastly, Ubc9 couples SUMO to a target substrate covalently. The process is terminated by protease removal of SUMOs from the substrates. Sumoylation is regulated primarily through specific recognition of the sumoylation motif (SM) by Ubc9 and, in some cases, by E3-substrate recognition. The functional consequences of SUMO modification are mostly mediated by recruitment of effector proteins that contain a SUMO Interaction Motif (SIM). Furthermore, SUMO can form poly-SUMO conjugate, which can be recognized by proteins containing poly-SIMs, such as the RING-finger 4 (RNF4) ubiquitin E3 ligase. RNF4 contains four SIMs that
facilitate poly-SUMO-specific ubiquitination and targets poly-sumoylated proteins for degradation. Here we review NMR structure-functional studies, conducted in our laboratory and aimed at dissecting the molecular basis of SUMO-mediated pathway.

Structural biology has been paying more attention on biomolecular complexes over the past decades, since they are crucial for many biological processes. Among these techniques for structural determination, nuclear magnetic resonance (NMR) has its advantage when dealing with biomolecules with high flexibility in solution. Small-angle X-ray scattering (SAXS) is a very important complementary technique that provides information on global shape of biomolecules. For biomolecular complexes, it can be much easier to determine atomic structures of individual subunits through NMR. In addition, NMR can also provide other structural information, such as the interface and orientations between subunits, and long range distance and angular restraints. Therefore, to construct structural models of biomolecular complexes, it would be very appropriate to combine experimental restraints obtained through NMR and low-resolution shape information from SAXS by utilizing computational tools, which is the main topic of this review.

Nitric oxide (NO) is a simple gas with free radical properties. It appears as a major signaling molecule in cardiovascular, immune and nervous systems. NO is generated by three isoforms of NO synthase, endothelial NO synthase (eNOS), neuronal NO synthase (nNOS) and inducible NO synthase (iNOS) in different cells. It is known that NO is an important physiological molecule in the regulation of blood pressure, vascular tone, however, excessive NO produced by iNOS results in inhibition of cardiac contractility, impairment of mitochondrial respiration and apoptosis. NO has been shown to influence neurotransmitter release and synaptogenetic processes, modulate the synaptic plasticity, indicating that NO plays an important role in the development, maintenance and regulation of brain circuits, as signaling molecule in mammary and learning. Increasing evidences indicate that excessive NO may damage cardiovascular and neurons and even cause cardiovascular and neurodegenerative diseases. We review the results about the “double edge” effects of NO in cardio-brain-vascular health and diseases and the protective effects of antioxidants in recent years studied by ESR.

Solid-state NMR offers a unique way to monitor the local and collective chain motions in semi-crystalline polymers. In this review paper, we will briefly survey recent results on the solid-state NMR study of chain motions in semi-crystalline polyethylenes (PEs). We will demonstrate that the state-of-the-art solid-state NMR now can provide very detailed knowledge about the local and collective chain motions in semi-crystalline PEs, which is of great relevance to our understanding of the mechanical behaviors of polyethylene in the microscopic level.

Layered double hydroxides (LDHs) are very important inorganic supramolecular materials that have widespread applications including catalysis, ion exchange and biological sciences. Solid-state NMR spectroscopy has been a powerful tool for the investigations of local structure and dynamics of LDHs, and has provided rich information. In particular, key advances (e.g., cation ordering information) have been made with solid-state NMR only very recently. This review introduces significant progress in the solid-state NMR studies of LDHs in the last 40 years.

Magnetic resonance hyperthermia is a new nano-medical therapy that emerges in recent years. In the presence of external alternating magnetic fields produced by MR instruments, magnetic nanoparticles accumulated at the tumor site can generate heat through Neel relaxation and/or Brownian relaxation. Through magnetic resonance hyperthermia, magnetic nanoparticles can serve as “molecular bullets” to kill cancer cells, leaving surrounding healthy tissues unaffected. Such hyperthermic effects can also be used for thermal activation and control releasing of cancer drugs. One major challenge of magnetic resonance hyperthermia is to optimize the heating efficiency of magnetic nanoparticle suspension. Heating efficiency depends on the size, physical properties, and aggregation state of magnetic nanoparticles. In this study, the thermodynamic behavior of magnetic nanoparticles and the aggregation/disruption of monomers/clusters under different temperatures were studied by 3D Metropolis Monte Carlo method. The relationship between the critical temperature for aggregation/disruption and the frequency of external magnetic field has been established through revised Langevin function.
Simulation results show that the relative content of aggregates in colloidal magnetic nanoparticle suspension decreased with the increase in temperature, and the aggregates disrupted completely into monomers at or above the critical temperature. In addition, increasing the frequency of external alternating magnetic field significantly lowered down the critical temperature, and there existed a critical frequency where the critical temperature stabilized and became unaffected by the frequency. Preheating the suspension under critical frequency will disrupt the aggregates into monomers and thus optimize the heating efficiency of magnetic nanoparticles.

Hyperpolarized ³He or ¹²⁹Xe diffusion MRI has been demonstrated as a promising technique for the detection of microanatomical changes in chronic obstructive pulmonary disease (COPD). Compared with ³He, ¹²⁹Xe is more available for the potential clinical applications. However, the measurement of ¹²⁹Xe apparent diffusion coefficient (ADC) possesses more challenges due to the relevant low gyromagnetic ratio and spin polarization. In this present study, a single b value (b = 14 s/cm2) diffusion-weighted hyperpolarized ¹²⁹Xe MRI sequence was used to image a balloon phantom, healthy rats, and the COPD rats, respectively. All COPD rats were induced by second-hand smoke and lipopolysaccharide (LPS). The lung ¹²⁹Xe ADC maps were obtained on a 7 T MRI scanner. The mean lung parenchymal ¹²⁹Xe ADCs were 0.044 22±0.002 9 and 0.042 34±0.002 3 cm2/s (Δ = 0.8/1.2 ms) for the COPD rats, which showed significant increasements in comparison with healthy ones (0.037 7±0.002 3 and 0.036 7±0.001 3 cm2/s). Furthermore, the corresponding ADC histogram of the COPD rats exhibited a broader distribution as compared with the healthy ones. Our experiments demonstrated that the alveolar airspace
enlargement in the COPD rats are able to be quantitatively evaluated by hyperpolarized xenon diffusion-weighted MRI.

A new biocompatible gadolinium (III)-macromolecule (AP-EDA-DOTA-Gd) was developed as a magnetic resonance imaging (MRI) contrast agent. Poly (aspartic acid-cophenylalanine) was synthesized, modified via ethylenediamine, conjugated with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and finally chelated gadolinium (III), yielding gadolinium (III)-based macromolecule (AP-EDA-DOTA-Gd). The hemolytic tests showed the hemocompatibility of this gadolinium (III)-based macromolecular conjugate. In vitro, AP-EDA-DOTA-Gd could be degraded, when it was incubated with cathepsin B in phosphate buffered solution (pH = 5.5). The T1-relaxivity (15.95 mmol–1·L·s–1) of AP-EDA-DOTA-Gd was 2.9 times of that (5.59 mmol–1·L·s–1) of the clinical MRI contrast agent (Gd-DOTA) at 1.5 T and 25 ℃. The liver enhancement of AP-EDA-DOTA-Gd was 63.5±6.1% during the maximum enhancement time (50-80 min), which was much better than that of Gd-DOTA (24.2±2.9%, 10-30 min). AP-EDA-DOTA-Gd was expected to be a potential liver MRI contrast agent.

Human salivary statherin is a 43-residue acidic phosphoprotein present in human saliva, possessing a high affinity for calcium phosphate minerals such as hydroxyapatite. The N-terminal 15-residue fragment of statherin (SN-15) is known to bind strongly to the crystallites of hydroxyapatite. In this work, we investigate the conformation of SN-15 in aqueous solution by NMR. Analysis of the CD spectra shows that SN-15 adopts an α-helical structure in phosphate buffer. High-resolution proton NMR spectra (COSY, TOCSY, and NOESY) have been acquired, from which the NOE patterns and J-couplings of amide hydrogens have been obtained. Together with the constraints obtained from amide-hydrogen exchange experiments, the molecular structure of SN-15 has shown to be a continuous α-helical structure.

The twin-arginine transport (Tat) systems in bacteria and plant chloroplasts translocate cargo proteins across cellular membranes in their folded states. The single-pass transmembrane protein TatA forms the protein translocation channel via self-oligomerization. Herein, we present the structure of Bacillus subtilis TatAy protein in dodecylphosphocholine micelles determined by solution NMR method. TatAy adopts an L-shaped conformation formed by a transmembrane helix (TMH) and an amphipathic helix (APH). Structural comparison of TatAy protein with the previously reported B. subtilis TatAd protein highlights essential residues at the hinge region for maintaining the L-shaped conformation, and suggests a few conserved structural features for the
TatA protein family. Possible roles for the conserved residues in the TatA channel formation are discussed.

Tellurite (TeO₃^2–), an oxyanion of tellurium, is highly toxic to most microorganisms. Several tellurite resistance genes (terZABCDEF) have been identified in many pathogenic bacteria. Previously, we determined the NMR solution structure of the tellurite resistance protein TerD and suggested that TerD may function as a calcium sensor in bacteria. TerZ, which shares 40% sequence identity with TerD, contains an extra 9-residue segment of L36FGSIFGGN44 and exhibits much weaker Ca²⁺-binding affinity. Interestingly, TerZdel in which the extra segment is deleted has comparable binding affinity to TerD. Based on chemical shift index and homology modeling results, it was revealed that the extra segment is unstructured and forms an extended loop, which may disturb the conformation of Ca2+-binding sites and also prevent Ca²⁺ from contacting its binding site, hence significantly reduce Ca²⁺-binding affinity.

The let-7 miRNA (microRNA) family control many cell-fate determination genes to influence pluripotency, differentiation, and transformation. Lin28 is a specific, posttranscriptional inhibitor of let-7 biogenesis. The C-terminal Zn-knuckle domain (ZKD) of Lin28 specially interacts with a conserved GGAG or GGAG-like motif in let-7 miRNA. We here report the NMR structure of human Lin28 binding to let-7 RNA with a sequence of 5′-A–2A–1G1G2A3G4-3′,
demonstrating that the two folded domains of Lin28 ZKD recognize the region G1G2A3G4 of the RNA. All bases in bound RNA adopt anti conformation, and the backbone of RNA is bent due to Lin28 binding, consistent with the observations in the previous crystal structure, but different from those in the reported NMR structure, further confirming the structural basis for how Lin28 specially recognizes this RNA.

Human ubiquitin C-terminal hydrolase, UCH-L1, is a highly abundant neuronal protein that is implicated in Parkinson´s disease (PD). Familial mutations and post-translational modifications of UCH-L1 have been reported to cause increased aggregation propensity and loss of de-ubiquitination activity, both of which may be pathogenic. We have recently demonstrated that a PD-associated mutation of UCH-L1, namely I93M, significantly destabilizes the folding
stability and accelerates the unfolding kinetics (Andersson et al. J Mol Biol, 2011, 407: 261－272). Here we report the use of solution state NMR spectroscopy, including side-chain methyl chemical shift, backbone relaxation dynamics and residual dipolar coupling (RDC) analyses, to further elucidate how the I93M mutation affects the structure and dynamics of UCH-L1. The results revealed altered side-chain packing within the hydrophobic core around the mutation site. However, such structural perturbation does not affect the fast backbone dynamics on the ns timescale. Furthermore, comparative RDC analysis suggests that the solution structure of UCH-L1 deviates considerably from the reported crystal structure and that the I93M mutation results in long-range structural perturbations far beyond the mutation site. These solution state-based structural findings complement previously reported crystallographic data to provide detailed insights into the impacts of the PD-associated mutation on UCH-L1.

A procedure based on the ¹H NMR fingerprints of biologics was established for the assessment of higher order structure of biosimilar samples. The global correlation coefficients r and R² of binned NMR spectra were used to quantify structural similarity between biosimilars and originators, and a local correlation analysis method was used to find out the specific regions that contribute to the structure differences. This method was applied to the quantitative
assessment of daptomycin, a lipopeptide antibiotic with a molecular weight of 1 620, by comparing the higher order structure of the biosimilar active pharmaceutical ingredient (API), its injection and originator injection Cubicin. The results showed that there were significant spectral differences among daptomycin samples, indicating structural differences between daptomycin originator and its biosimilars. Local correlation analysis further revealed the specific spectral regions that contributed to these differences. The method was also applied to a monoclonal antibody drug Trastuzumab, which is a large protein with a molecular weight of 145 423. ¹H NMR spectra of the originator drug Herceptin and four lots of its biosimilar were recorded and binned for statistical analysis. The correlation coefficients r and R2 showed that Trastuzumab biosimilars seem to be highly similar to Herceptin. These results demonstrated that the combination of NMR fingerprints and statistical analysis is a powerful and robust technique in biosimilar quality control.

By using advanced solid-state NMR techniques, the heterogeneous structure and inter-polymer miscibility of chitosan-poly (N-3-acrylamidophenyl-boronic acid) nanoparticles (CS-PAPBA NPs) were investigated. The ¹³C CPMAS experiments indicate the conformational change of chitosan when incorporated with phenyl boronic acid. Two-dimensional (2D) ¹³C-¹H HETCOR experiments further demonstrate that a portion of chitosan is closely bound with
PAPBA in the interphase by means of boronic coordination interaction. 2D ¹H-¹H spin-exchange with spin-diffusion experiments demonstrate that the interfacial mixing of chitosan and PAPBA, and micro-phase separation is of the order of 15–20 nanometers.

A method to significantly enhance the sensitivity of the widely used multiple quantum magic angle spinning (MQMAS), satellite transition magic angle spinning (STMAS) and their variants is proposed. By introducing a preparatory period prior to the normal multiple quantum excitation or satellite transition pulse, the initial state can be optimized so that the recycle delay time can be substantially reduced. The performance of this method, for both MQMAS and
STMAS in two different magnetic fields, has been demonstrated with a number of representative nuclear species (²³Na, 11B and 87Rb). This method can be implemented, without any additional hardware or software accessories, on all conventional solid-state NMR spectrometers with operating magnetic field as low as 4.7 T and sample spinning speed as slow as 6 kHz. Moreover, it is a very flexible method so that it can be customized, i.e., for individual compounds, the experimental parameters can be optimized for each compound following a step-by-step procedure specified in this work although a full theoretical exposition will be provided separately.

Quadrupolar echo NMR spectroscopy of solids often requires RF pulse excitation that covers spectral widths exceeding 100 kHz. In a recent work we found out that a composite pulse COM-II (9018090135 45) , provided robust broadband excitation for deuterium quadrupolar echo spectroscopy. Moreover, when combined with an 8-step phase cycle, spectral distortions arising from finite pulse widths were greatly suppressed. In this paper we report on a theoretical
analysis of COM-II with 8-step phase cycle by average Hamiltonian theory. This treatment is combined with the fictitious spin-1 operator formalism, and the mechanism of the 8-step phase cycling that minimizes the spectral distortions is discussed.

The method for NMR spectra post processing, especially for a set of parallel biological NMR spectra, is crucial in metabolomics studies. Here, an efficient spectra post processing method for peak alignment and peak extraction is proposed, which not only did well in accurate results with high resolution but also had advantages in batch processing and time consumption. The spectrum alignment was completed with the shift of the spectra without changing the profiles of the NMR spectra, and the results were evaluated by the regression coefficient R. The peak extraction step was achieved by repeated searching the maximum value in the NMR spectrum, and was a prerequisite for metabolomics analysis. The extraction of all relevant peaks contained in the complex mixture spectra, rid of any non-significant signal could be easily applied to the statistical analysis. This new approach was applied to a set of 1H NMR spectra of rat plasma and urine to demonstrate the efficiency of the method. The whole theory is compiled in Matlab, and the implementation code is available upon request.

In this report, we present a pulsed dynamic nuclear polarization (DNP) system, being reconfigurable for DNP enhanced magnetic resonance imaging (DNP-MRI) and magnetic resonance spectroscopy (DNP-MRS). Several novel console designs are proposed in the presented DNP system. A distributed digital architecture based on Peripheral Component Interconnect Express (PCIe) has been developed, so as to significantly improve the efficiency of data transmission and communication reliability as well as the precise control of pulse sequence. An external high speed Double Data Rate (DDR) memory chip is used for storing FID data and pulse sequence elements, greatly speeding up the execution of the pulse sequence and reducing the interval of TR and improving the accuracy of TR in image sequence. Using clock phase-shift technology, we can produce digital pulse accurately with high timing resolution of nanosecond timescales. Finally, two experimental examples for DNP-MRS and DNP-MRI are shown.