Convection compensated electrophoretic NMR

A novel method of convection compensated ENMR (CC-ENMR) has been developed to detect electrophoretic motion of ionic species in the presence of bulk solution convection. This was accomplished using a gradient moment nulling technique to remove spectral artifacts from heat-induced convection and using the polarity switch of the applied electric field to retain spin phase modulations due to electrophoretic flow. Experiments were carried out with a mixture of 100 mM L-aspartic acid and 100 mM 4,9-dioxa-1,12-dodecanediamine to demonstrate this new method of ENMR. CC-ENMR enhances our previously developed capillary array ENMR (CA-ENMR) in solving the convection problem. The combined CA- and CC-ENMR approach strengthens the potential of multidimensional ENMR in simultaneous structural determination of coexisting proteins and protein conformations in biological buffer solutions of high ionic strength. Structural mapping of interacting proteins during biochemical reactions becomes possible in the future using ENMR techniques, which may have a profound impact on the understanding of biological events, including protein folding, genetic control, and signal transduction in general.     

埋管换热器热阻分析

地源热泵系统中的竖直埋管换热器是地源热泵系统的一个关键部分,从U管内至岩土间的传热热阻影响热泵系统的性能.文中介绍在静态负荷下,利用垂直U型埋管换热器换热的一维模型和二维模型,从传热热阻的角度进行了埋管管径、钻孔直径与管内对流热阻、钻孔热阻间的分析,结果显示,对钻孔热阻与管径的优化选择可降低热阻,同时对提高埋管换热器的...     

High flow-resolution for mobility estimation in 2D-ENMR of proteins using maximum entropy method (MEM-ENMR)

Multidimensional electrophoretic NMR (nD-ENMR) is a potentially powerful tool for structural characterization of co-existing proteins and protein conformations. By applying a DC electric field pulse, the electrophoretic migration rates of different proteins were detected experimentally in a new dimension of electrophoretic flow. The electrophoretic mobilities were employed to differentiate protein signals. In U-shaped ENMR sample chambers, individual protein components in a solution mixture followed a cosinusoidal electrophoretic interferogram as a function of its unique electrophoretic migration rate. After Fourier transformation in the electrophoretic flow dimension, the protein signals were resolved at different resonant frequencies proportional to their electrophoretic mobilities. Currently, the mobility resolution of the proteins in the electrophoretic flow dimension is limited by severe truncations of the electrophoretic interferograms due to the finite electric field strength available before the onset of heat-induced convection. In this article, we present a successful signal processing method, the Burg's maximum entropy method (MEM), to analyze the truncated ENMR signals (MEM-ENMR). Significant enhancement in flow resolution was demonstrated using two-dimensional ENMR of two protein samples: a lysozyme solution and a solution mixture of bovine serum albumin (BSA) and ubiquitin. The electrophoretic mobilities of lysozyme, BSA and ubiquitin were measured from the MEM analysis as 7.5x10(-5), 1.9x10(-4) and 8.7x10(-5) cm2 V-1 s-1, respectively. Results from computer simulations confirmed a complete removal of truncation artifacts in the MEM-ENMR spectra with 3- to 6-fold resolution enhancement.     

Deterministic and stochastic effects near the convective onset

The pure conduction state of a horizontal layer of fluid heated from below becomes unstable with respect to a convecting state when the temperature difference exceeds a critical value. We examine the question of how real, physical systems evolve from conduction to convection. Most experimental cells contain geometric or thermal inhomogeneities which render the bifurcation to convection imperfect. In that case the pure conduction state never exists and the convecting state evolves continuously and smoothly as the temperature difference is raised. When a sufficiently perfect experimental cell is constructed to eliminate this route to convection, then dynamic imperfections will usually prevail. When the temperature difference across the cell is raised, the vertical gradients in the sidewalls evolve at a rate which differs from that in the fluid. The resultingtransient horizontal thermal gradients initiate the convective flow. This phenomenon can be eliminated by providing sidewalls which have the same thermal diffusivity as that of the fluid. When that is done, the convective flow is started by random noise which exists in any experimental system. Analysis of experiments shows that the noise source is considerably stronger than thermal noise, but its origin is unclear at this time.     

Diffusion-broadened velocity spectra of convection in variable-temperature BP-LED experiments

When NMR diffusion experiments are performed at temperatures different from ambient temperature, temperature gradients due to probe design can cause thermal convection and therefore significantly affect the signal amplitude. Fourier transformation of the signal amplitude gives rise to a diffusion-broadened velocity spectrum, which contains information about the convection velocity. It is shown that when the diffusion broadening factor is smaller than the maximum velocity, the broadening has little effect on the determination of the maximum velocity. Thus, convection velocity can be determined in the presence of diffusion.     

Investigation of the dielectric properties of BaTiO3 single crystals of different qualities by the thermal noise method

Comparative dielectric investigations of bulk barium titanate crystals grown using different techniques have been performed by the thermal noise and traditional methods. The temperature dependences of the permittivity have been used to determine the phase transition temperatures and the Curie-Weiss constants. The investigation performed using the thermal noise method has revealed a number of phenomena that complicate the separation of the noise associated with the change in the impedance of the sample. Possible approaches to the elimination or minimization of contributions from different effects to the total noise voltage have been described. The piezoelectric resonances that also contribute to the total noise signal have been found for the first time and investigated by the thermal noise method. The possible nature of the obtained temperature dependences of the piezoelectric resonance frequencies has been discussed.     

Measurement of Convection and Temperature Profiles in Liquid Samples

This paper describes a method for measuring the rate of convective flow in a liquid sample used for high-resolution NMR. The measurement is straightforward and achieves a clean separation of convection from other effects such as diffusion and relaxation. Convection results from temperature gradients within the sample, and it is shown how these can be measured with the aid of a simple chemical shift imaging experiment of a sample whose spectrum shows a strong and well characterized temperature dependence. The use of these two methods is illustrated by showing how the rate of convection and the temperature profile depend on the solvent, temperature, and gas flow rate of the temperature regulating system. It is also shown that broadband (13)C decoupling results in significant temperature gradients and associated convection.     

Constant-time multidimensional electrophoretic NMR

Multidimensional electrophoretic NMR (ENMR) has been introduced to determine structures of coexisting proteins and protein conformations in solution. Signals of different proteins are separated in a new dimension of electrophoretic flow according to their characteristic electrophoretic mobilities. The electrophoretic interferograms have been generated in the flow dimension in two approaches by incrementing either the amplitude or the duration of the electric field. The ENMR method of incrementing the duration of the electric field, however, introduces severe signal decays due to molecular diffusion and spin relaxation, limiting the effectiveness of the method. In this study, an improved method of constant-time multidimensional ENMR (CT-ENMR) has been proposed and successfully tested. The time delays between the magnetic field gradients and the RF pulses are kept constant in this new method so that the molecular diffusion and spin relaxation processes contribute to only a constant factor of signal amplitude. As an alternative approach of incrementing the amplitude of the electric field, this novel method significantly enhances our capability and potential in characterizing structural changes of interacting proteins during biological signaling processes. The CT-ENMR method is particularly useful in studies where the amplitude-incrementing of the electric field is not optimal. For example, the CT-ENMR method is superior when the electric field is applied in the direction not parallel to the static magnetic field B(0) to the xy-magnetization. The new method was successfully demonstrated with a sample solution containing 100 mM 4,9-dioxa-1,12-dodecanediamine and 100 mM L-aspartic acid in D(2)O.     

Measured thermal dissipation field in turbulent Rayleigh-Bénard convection

The time-averaged local thermal dissipation rate epsilonN(r) in turbulent convection is obtained from direct measurements of the temperature gradient vector in a cylindrical cell filled with water. It is found that epsilonN(r) contains two contributions. One is generated by thermal plumes, present mainly in the plume-dominated bulk region, and decreases with increasing Rayleigh number Ra. The other contribution comes from the mean temperature gradient, being concentrated in the thermal boundary layers, and increases with Ra. The experiment thus provides a new physical picture about the thermal dissipation field in turbulent convection.     

非能动传热球床堆有效导热系数的壁面效应分析

本文针对非能动传热机制下简单立方球床堆有效导热系数进行了数值研究,根据有效导热系数的空间分布特性,对球床堆的近壁面区域和主体区域作了划分;分析了不同非能动传热机制下的有效导热系数的壁面效应;最后分析了导热、辐射和自然对流对近壁面和主体区域有效导热系数的贡献。结果发现,近壁面区域是在壁面附近一个球径范围内的区域;由于辐射和自然对流的影响,相同温度下近壁面有效导热系数比主体区域的有效导热系数小了近15%。当温度分别超过950 K和1080 K时,辐射成为球床堆主体区域和近壁面区域的主导传热机制;自然对流对有效导热系数的贡献并不大,当温度超过600 K时,自然对流可以忽略。研究结果可以为高温球床堆的设计与优化提供理论基础。     

Three-dimensional NMR microscopy: improving SNR with temperature and microcoils.

It is widely held that the spatial resolution achievable by NMR microscopic imaging is limited in biological systems by diffusion to approximately 1-5 microns. However, these estimates were developed for specific imaging techniques and represent practical rather than fundamental limits. NMR imaging is limited by the signal-to-noise ratio (SNR). Diffusion effects on spatial resolution can be made arbitrarily small in principle by increasing the gradient strength. The exponential signal attenuation from random spin motion in a gradient, however, will reduce the signal far below the noise level when the voxel size is reduced much below 5 microns. Two factors can be optimized to improve the SNR: (1) the inductive linkage between microscopic samples and the detection apparatus and (2) the temperature of the rf probe. In this work, the filling factor was optimized using inductors with diameters less than 1 mm. It is furthermore shown that probe circuit cooling results in significant improvements in SNR, whereas cooling of the preamplifier is of little value when proper noise matching between the resonant circuit and preamplifier is accomplished. Using three-dimensional Fourier imaging techniques, we have obtained images of single-cell organisms with spatial resolution of approximately 6 microns. Practical limitations include mechanical stability of the apparatus, thermal shielding between the sample and probe, and the magnetic susceptibility of the sample.     

Three-dimensional electrophoretic NMR correlation spectroscopy

A novel method of three-dimensional electrophoretic NMR correlation spectroscopy (3D EP-COSY) has been proposed, developed, and implemented. It has a demonstrated potential of facilitating simultaneous structural assignments of multiple proteins in mixtures. The principle is to add a pulsed DC electric field that introduces a new dimension of electrophoretic flow, in which resonances of different molecules can be separated by their electrophoretic migration rates without physical separation. As a result, two COSY spectra were simultaneously obtained in a single 3D EP-COSY experiment from a mixture of 150 mM l-aspartic acid and 148 mM 4, 9-dioxa-1,12-dodecanediamine with concurrent resolution of their chemical shifts and J-coupling constants. This approach creates a new horizon of multidimensional electrophoretic NMR. The technical advance opens doors for structure characterization of complex protein systems and protein interactions, which are at the basis of biochemical mechanisms and the phenomena of living systems.     

Capillary array electrophoretic NMR of proteins in biological buffer solutions

The capillary array electrophoretic NMR (CA-ENMR) was developed to study protein mixtures in biological buffer solutions of high ionic strength. By enhancing the strength of the effective electric field across the sample, the technique permits the detection of the electrophoretic motion of 1 mM lysozyme in 50 mM NaH(2)PO(4) aqueous solution, which was previously not achievable using conventional ENMR. Heat-induced convection was dramatically reduced by blocking convective current loops and by improving the efficiency of heat exchange. Thus, the capability of ENMR study of electrolyte solutions was extended from low to high ionic strength. In addition, capillary walls reduced rf-induced electrical eddy current, thereby maintaining good probe Q factors. Because of its parallel configuration to the static magnetic field, the capillary array chamber produced no susceptibility distortions of the ENMR signal. The technique offers great potential in characterizing multiple protein conformations and protein interactions in solution.     

Electrokinetic transport of nanoparticles to opening of nanopores on cell membrane during electroporation

Nanoparticle transport to the opening of the single nanopore created on the cell membrane during the electroporation is studied. First, the permeabilization of a single cell located in a microchannel is investigated. When the nanopores are created, the transport of the nanoparticles from the surrounding liquid to the opening of one of the created nanopores is examined. It was found that the negatively charged nanoparticles preferably move into the nanopores from the side of the cell membrane that faces the negative electrode. Opposite to the electro-osmotic flow effect, the electrophoretic force tends to draw the negatively charged nanoparticles into the opening of the nanopores. The effect of the Brownian force is negligible in comparison with the electro-osmosis and the electrophoresis. Smaller nanoparticles with stronger surface charge transport more easily to the opening of the nanopores. Positively charged nanoparticles preferably enter the nanopores from the side of the cell membrane that faces the positive electrode. On this side, both the electrophoretic and the electro-osmotic forces are in the same directions and contribute to bring the positively charged particles into the nanopores.     

The lever-coil: a simple, inexpensive sensor for respiratory and cardiac motion in MRI experiments

Thoracic and abdominal magnetic resonance imaging studies generally require some type of compensation for respiratory and cardiac motions in order to yield artifact-free images with good signal-to-noise ratio. Most techniques for respiratory compensation require the use of a non-NMR sensing device to monitor the subject's chest motion, while cardiac motion compensation generally requires the use of ECG leads within the magnet. An inductive pickup coil placed on the subject's chest is perhaps the simplest and least expensive means of monitoring respiration in a MR scanner. However, due to inductive coupling between the pickup coil, radio frequency resonator and gradient set, this arrangement often results in both NMR and respiratory signal artifacts and can also present a burn hazard to the subject depending on the placement and orientation of the pickup coil. Moreover, the presence of a pickup coil on the chest can degrade local magnetic field homogeneity and thus degrade image quality. Similar problems arise when ECG leads must be connected to the subject for cardiac monitoring and gating. To preserve the benefits of the simple pickup coil while circumventing these limitations, a "lever-coil" sensor is presented in which a pickup coil is mechanically coupled to the subject but is not located within the resonator or gradient coil. This results in much lower mutual inductance between the pickup coil and the resonator or gradients. The optimization of the geometry of the apparatus is discussed and lever-coil signal traces are shown which demonstrate the sensor's ability to simultaneously detect both respiratory and cardiac motion in mice. Finally, respiratory-gated and cardiac-triggered spin echo images of the rat abdomen and mouse heart are presented to demonstrate the utility of the lever-coil sensor.     

Cardinal series to sort out defective samples in magnetic resonance data sets

NMR signals are unavoidably impaired with noise stemming from the electronic circuits of the spectrometer. This noise is most often white and Gaussian and can be greatly reduced by applying low pass analogue and digital filters. Nevertheless, extra noise with other statistics than Gaussian may interfere with the signal, e.g. when auxiliary electrical devices are placed near the magnet of the NMR spectrometer. This paper reports on how one can make use of this difference in statistics to remove the noise caused by electrical devices before any further data processing. The algorithm is based on the use of a new linear low pass filter, which consists in fitting NMR data in the time domain with a cardinal series and whose spectral width can be controlled. Over other filtering methods such filter has the advantage of not distorting the signal neither at the beginning nor the end of the acquisition period. The performance of the method is demonstrated by applying it to a data set collected in a flow PGSE experiment and impaired with noise emanated from a brushed DC electric motor.     

High-resolution NMR with resistive and hybrid magnets: deconvolution using a field-fluctuation signal

A method for compensating effect of field fluctuation is examined to attain high-resolution NMR spectra with resistive and hybrid magnets. In this method, time dependence of electromotive force induced for a pickup coil attached near a sample is measured synchronously with acquisition of NMR. Observed voltage across the pickup coil is converted to field fluctuation data, which is used to deconvolute NMR signals. The feasibility of the method is studied by (79)Br MAS NMR of KBr under a 30T magnetic field of a hybrid magnet. Twenty single-scan NMR signals were accumulated after the manipulation, resulting in a high-resolution NMR spectrum.     

Effects of Thermal Convection on NMR and Their Elimination by Sample Rotation

It is shown that the presence of thermal convection in a sample tube may lead to a variety of anomalous phenomena in prolonged multiple-pulse NMR experiments. They are investigated by applying inversion-recovery pulse sequences to¹²⁹Xe of xenon gas dissolved in deuterated cyclohexane and acetonitrile, and to¹⁹F in xenon difluoride (XeF₂) dissolved in deuterated acetonitrile. If convection is present, the recovery of the magnetization after the π pulse may be very different from the recovery due to the spin–lattice relaxation alone. It may be much faster, very sensitive to temperature, and nonexponential, exhibiting even oscillatory behavior. In addition, the shape of the spectral lines may be seriously distorted. The results show that convection and the resulting anomalies can be effectively eliminated by rotating the sample tube at a spinning speed on the order of 10 Hz. These phenomena may provide novel methods for investigating thermal convection.     

Characteristics of sign and rank algorithms of detection with fixed sample sizes in noise and chaotic pulse interference flow with arbitrary distribution laws

It is shown that the sign, rank, and binary rank detectors with fixed sample sizes retain the property of nonparametricity under the joint action of noise and chaotic pulse interference flow with arbitrary distribution laws. An expression has been derived for the rank distribution in noise and regular pulse interference flow with arbitrary distribution laws. It is shown that the sign detector with a fixed sample size retains the nonparametricity while the rank and binary rank detectors with fixed sample sizes acquire the property of quasinonparametricity under these conditions. State Technical University of Vladimir (Murom Branch), Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 11, pp. 1433–1447, November, 1997.     

Multiplicative or t 1 Noise in NMR Spectroscopy

The signal in a nuclear magnetic resonance (NMR) experiment is highly sensitive to fluctuations of the environment of the sample. If, for example, the static magnetic field B ₀, the amplitude and phase of radio frequency (rf) pulses, or the resonant frequency of the detection circuit are not perfectly stable and reproducible, the magnetic moment of the spins is altered and becomes a noisy quantity itself. This kind of noise depends on the presence of a signal, to which it is usually proportional. Since all the spins at a particular location in a sample experience the same environment at any given time, such multiplicative noise primarily affects the reproducibility of an experiment. It is mainly of importance in the indirect dimensions of a multidimensional experiment, when intense lines are suppressed with a phase cycle, or for difference spectroscopy techniques. Equivalently, experiments which are known to be problematic with regard to their reproducibility, like flow experiments or experiments with a mobile target, tend to be affected more strongly by multiplicative noise. In this article it is demonstrated how multiplicative noise can be identified and characterized using very simple, repetitive experiments. An error estimation approach is developed to give an intuitive, yet quantitative understanding of its properties. The consequences for multidimensional NMR experiments are outlined, implications for data analysis are shown, and strategies for the optimization of experiments are summarized. Author's address: Josef Granwehr, Sir Peter Mansfield Magnetic Resonance Center, School of Physics and Astronomy, Univesity of Nottingham, Nottingham NG7 2RD, UK