Magnetic Dipole Coupling Research Articles

An encodable lanthanide binding tag with reduced size and flexibility for measuring residual dipolar couplings and pseudocontact shifts in large proteins

Metal ions serve important roles in structural biology applications from long-range perturbations seen in magnetic resonance experiments to electron-dense signatures in X-ray crystallography data; however, the metal ion must be secured in a molecular framework to achieve the maximum benefit. Polypeptide-based lanthanide-binding tags (LBTs) represent one option that can be directly encoded within a recombinant protein expression construct. However, LBTs often exhibit significant mobility relative to the target molecule. Here we report the characterization of improved LBTs sequences for insertion into a protein loop. These LBTs were inserted to connect two parallel alpha helices of an immunoglobulin G (IgG)-binding Z domain platform. Variants A and B bound Tb(3+) with high affinity (0.70 and 0.13μM, respectively) and displayed restricted LBT motion. Compared to the parent construct, the metal-bound A experienced a 2.5-fold reduction in tag motion as measured by magnetic field-induced residual dipolar couplings and was further studied in a 72.2kDa complex with the human IgG1 fragment crystallizable (IgG1 Fc) glycoprotein. The appearance of both pseudo-contact shifts (-0.221 to 0.081ppm) and residual dipolar couplings (-7.6 to 14.3Hz) of IgG1 Fc resonances in the IgG1 Fc:(variant A:Tb(3+))2 complex indicated structural restriction of the LBT with respect to the Fc. These studies highlight the applicability of improved LBT sequences with reduced mobility to probe the structure of macromolecular systems.

Isotope shifts and hyperfine structure of the laser-cooling Fe I 358-nm line

We report on the measurement of the isotope shifts of the $3{d}^{7}4s\phantom{\rule{0.28em}{0ex}}a\phantom{\rule{0.28em}{0ex}}{}^{5}{F}_{5}\text{--}3{d}^{7}4p\phantom{\rule{0.28em}{0ex}}z\phantom{\rule{0.28em}{0ex}}{}^{5}{G}_{6}^{o}$ Fe i line at 358 nm between all four stable isotopes $^{54}\text{Fe},\phantom{\rule{0.16em}{0ex}}^{56}\text{Fe},\phantom{\rule{0.16em}{0ex}}^{57}\text{Fe}$, and $^{58}\text{Fe}$, as well as the hyperfine structure of that line for $^{57}\text{Fe}$, the only stable isotope having a nonzero nuclear spin. This line is of primary importance for laser-cooling applications. In addition, an experimental value of the field and specific mass shift coefficients of the transition is reported as well as the hyperfine structure magnetic dipole coupling constant $A$ of the transition excited state in $^{57}\text{Fe}$, namely $A(3{d}^{7}4p\phantom{\rule{0.28em}{0ex}}z\phantom{\rule{0.28em}{0ex}}{}^{5}{G}_{6}^{o})=31.241(48)$ MHz. The measurements were carried out by means of laser-induced fluorescence spectroscopy performed on an isotope-enriched iron atomic beam. All measured frequency shifts are reported with relative uncertainties below one third percent.

Simple expressions of the nuclear relaxation rate enhancement due to quadrupole nuclei in slowly tumbling molecules.

For slowly tumbling entities or quasi-rigid lattices, we derive very simple analytical expressions of the quadrupole relaxation enhancement (QRE) of the longitudinal relaxation rate R1 of nuclear spins I due to their intramolecular magnetic dipolar coupling with quadrupole nuclei of arbitrary spins S ≥ 1. These expressions are obtained by using the adiabatic approximation for evaluating the time evolution operator of the quantum states of the quadrupole nuclei S. They are valid when the gyromagnetic ratio of the spin S is much smaller than that of the spin I. The theory predicts quadrupole resonant peaks in the dispersion curve of R1 vs magnetic field. The number, positions, relative intensities, Lorentzian shapes, and widths of these peaks are explained in terms of the following properties: the magnitude of the quadrupole Hamiltonian and the asymmetry parameter of the electric field gradient (EFG) acting on the spin S, the S-I inter-spin orientation with respect to the EFG principal axes, the rotational correlation time of the entity carrying the S-I pair, and/or the proper relaxation time of the spin S. The theory is first applied to protein amide protons undergoing dipolar coupling with fast-relaxing quadrupole (14)N nuclei and mediating the QRE to the observed bulk water protons. The theoretical QRE agrees well with its experimental counterpart for various systems such as bovine pancreatic trypsin inhibitor and cartilages. The anomalous behaviour of the relaxation rate of protons in synthetic aluminium silicate imogolite nano-tubes due to the QRE of (27)Al (S = 5/2) nuclei is also explained.

Separating read and write units in multiferroic devices

Strain-mediated multiferroic composites, i.e., piezoelectric-magnetostrictive heterostructures, hold profound promise for energy-efficient computing in beyond Moore’s law era. While reading a bit of information stored in the magnetostrictive nanomagnets using a magnetic tunnel junction (MTJ), a material selection issue crops up since magnetostrictive materials in general cannot be utilized as the free layer of the MTJ. This is an important issue since we need to achieve a high magnetoresistance for technological applications. We show here that magnetically coupling the magnetostrictive nanomagnet and the free layer e.g., utilizing the magnetic dipole coupling between them can circumvent this issue. By solving stochastic Landau-Lifshitz-Gilbert equation of magnetization dynamics in the presence of room-temperature thermal fluctuations, we show that such design can eventually lead to a superior energy-delay product.

Low-Energy Bead-Mill Dispersion of Agglomerated Core-Shell α-Fe/Al₂O₃ and α″-Fe₁₆N₂/Al₂O₃ Ferromagnetic Nanoparticles in Toluene.

Magnetic materials such as α″-Fe16N2 and α-Fe, which have the largest magnetic moment as hard and soft magnetic materials, are difficult to produce as single domain magnetic nanoparticles (MNPs) because of quasistable state and high reactivity, respectively. The present work reports dispersion of agglomerated plasma-synthesized core-shell α″-Fe16N2/Al2O3 and α-Fe/Al2O3 in toluene by a new bead-mill with very fine beads to prepare single domain MNPs. As a result, optimization of the experimental conditions (bead size, rotation speed, and dispersion time) enables the break-up of agglomerated particles into primary particles without destroying the particle structure. Slight deviation from the optimum conditions, i.e., lower or higher dispersion energy, gives undispersed or broken particles due to fragile core-shell structure against stress or impact force of beads. The dispersibility of α″-Fe16N2/Al2O3 is more restricted than that of α-Fe/Al2O3, because of the preparation conditions. Especially for α″-Fe16N2/Al2O3, no change on crystallinity (98% α″-Fe16N2) or magnetization saturation after dispersion was observed, showing that this method is appropriate to disperse α″-Fe16N2/Al2O3 MNPs. A different magnetic hysteresis behavior is observed for well-dispersed α″-Fe16N2/Al2O3 MNPs, and the magnetic coercivity of these NPs is constricted when the magnetic field close to zero due to magnetic dipole coupling among dispersed α″-Fe16N2 MNPs.

Selective mode excitation in three-chained magnetic vortices

We report on collective dynamics of three-chained magnetic vortices interacting with each other through magnetic dipolar coupling. Three different eigenstates characterized by the phase difference of the rotating cores were clearly observed by means of the spin torque diode effect. We also selectively excited such eigenstates with the same system by tuning the phase difference. These experimental results are well reproduced by analytical calculations based on the linearized Thiele’s equation. Moreover, we demonstrate that the spectra obtained for the selective excitations correspond to a part of the dispersion relation for one-dimensional chained vortices.

MESMER: minimal ensemble solutions to multiple experimental restraints

Macromolecular structures and interactions are intrinsically heterogeneous, temporally adopting a range of configurations that can confound the analysis of data from bulk experiments. To obtain quantitative insights into heterogeneous systems, an ensemble-based approach can be employed, in which predicted data computed from a collection of models is compared to the observed experimental results. By simultaneously fitting orthogonal structural data (e.g. small-angle X-ray scattering, nuclear magnetic resonance residual dipolar couplings, dipolar electron-electron resonance spectra), the range and population of accessible macromolecule structures can be probed. We have developed MESMER, software that enables the user to identify ensembles that can recapitulate experimental data by refining thousands of component collections selected from an input pool of potential structures. The MESMER suite includes a powerful graphical user interface (GUI) to streamline usage of the command-line tools, calculate data from structure libraries and perform analyses of conformational and structural heterogeneity. To allow for incorporation of other data types, modular Python plugins enable users to compute and fit data from nearly any type of quantitative experimental data. Conformational heterogeneity in three macromolecular systems was analyzed with MESMER, demonstrating the utility of the streamlined, user-friendly software. https://code.google.com/p/mesmer/

Experimental implementation of quantum gates through actuator qubits

Universal quantum computation requires the implementation of arbitrary control operations on the quantum register. In most cases, this is achieved by external control fields acting selectively on each qubit to drive single-qubit operations. In combination with a drift Hamiltonian containing interactions between the qubits, this allows the implementation of arbitrary gate operations. Here, we demonstrate an alternative scheme that does not require local control for all qubits: we implement one- and two-qubit gate operations on a set of target qubits indirectly, through a combination of gates on directly controlled actuator qubits with a drift Hamiltonian that couples actuator and target qubits. Experiments are performed on nuclear spins, using radio-frequency pulses as gate operations and magnetic-dipole couplings for the drift Hamiltonian.

Quantum beat spectroscopy: Stimulated emission probe of hyperfine quantum beats in the atomic Cs8p2P3/2level

Measurements of hyperfine polarization quantum beats are used determine the magnetic dipole (A) and electric quadrupole (B) coupling constants in the excited atomic Cs 8p level. The experimental approach is a novel combination of pulsed optical pumping and time-delayed stimulated emission probing of the excited level. From the measured evolution of the atomic linear polarization degree as a function of probe delay time, we determine the hyperfine coupling constants A = 7.42(6) MHz and B = 0.14(29) MHz.

Charge order, superconducting correlations, and positive muons

The recent discoveries of short-range charge-density wave fluctuations in the normal state of several hole-doped cuprate superconductors constitute a significant addition to the known intrinsic properties of these materials. Besides likely being associated with the normal-state pseudogap, the charge-density wave order presumably influences the build-up of known superconducting correlations as the temperature is lowered toward the superconducting state. As a pure magnetic probe, muon spin rotation ($\mu$SR) is not directly sensitive to charge order, but may sense its presence via the effect it has on the magnetic dipolar coupling of the muon with the host nuclei at zero or low magnetic field. At higher field where $\mu$SR is completely blind to the effects of charge order, experiments have revealed a universal inhomogeneous normal-state response extending to temperatures well above $T_c$. The measured inhomogeneous line broadening has been attributed to regions of superconducting correlations that exhibit varying degrees of fluctuation diamagnetism. Here, the compatibility of these results with other measurements showing charge order correlations or superconducting fluctuations above $T_c$ is discussed.

Vibrational branching ratios and hyperfine structure of 11BH and its suitability for laser cooling

The simple structure of the BH molecule makes it an excellent candidate for direct laser cooling. We measure the branching ratios for the decay of the A^{1}Pi (v'=0) state to vibrational levels of the ground state, X^{1}Sigma^{+}, and find that they are exceedingly favourable for laser cooling. We verify that the branching ratio for the spin-forbidden transition to the intermediate a^{3}Pi state is inconsequentially small. We measure the frequency of the lowest rotational transition of the X state, and the hyperfine structure in the relevant levels of both the X and A states, and determine the nuclear electric quadrupole and magnetic dipole coupling constants. Our results show that, with a relatively simple laser cooling scheme, a Zeeman slower and magneto-optical trap can be used to cool, slow and trap BH molecules.

Precision measurement of the hyperfine structure of 8d2D3/2 state of 133Cs by the radio-frequency phase modulation technique

We report the measurement of the hyperfine structure in the 8d2D3/2 state of 133Cs isotope by Doppler free two-photon fluorescence spectroscopy in a gas cell. The hyperfine level separations were measured by the radio-frequency phase modulation technique using a frequency stabilized Ti:sapphire laser. The frequency separation between various hyperfine levels of the excited state has been measured with a precision of ~60kHz. From the measured hyperfine separations of the excited state, we have determined the magnetic dipole coupling constant (A) and the electric quadrupole coupling constant (B) for the 8d2D3/2 state. The determined hyperfine structure constants are A=3.98 (0.01) MHz and B=−0.03 (0.09) MHz. Our deduced magnetic dipole coupling constant (A) value is thus by far the most precise and was found to be in good agreement with the 3σ level with the earlier reported values. This work reports the electric quadrupole coupling constant (B) value for the 8d2D3/2 state for the first time.

Hyperfine structure analysis in the intense spectral lines of the neutral Cu atom falling in the 353–809 nm wavelength region using a Fourier transform spectrometer

Hyperfine structure analyses have been performed in the high-resolution spectrum of the neutral copper atom covering the wavelength region of 353–809nm using Fourier transform spectroscopy. A DC discharge of natural copper produced in a liquid nitrogen cooled hollow cathode lamp used as a light source and a photomultiplier tube as well as Si photodiodes were employed as the light detectors. The hfs studies in 17 transitions of the neutral copper atom originating from 17 energy levels for 63Cu have been reported here. The present investigation has provided the magnetic dipole coupling constant A and electric quadrupole coupling constant B for the first time for the following 6 even-parity levels lying at 49,935, 49,942cm−1, of 3d104d configuration, 52,848cm−1 of 3d106s configuration, 55,387, 55,391cm−1 3d105d configuration and 71,978cm−1 of 3d104s4d configuration. The sign convention of the previously-reported hfs A value amounting to 1920MHz for the level at 44,963cm−1 of 3d94s4p configuration has been revised to −1920MHz. Measurements reported earlier of A and B hfs constants for the 11 odd-parity energy levels also have been confirmed.

Ultra-broadband electromagnetically induced transparency using tunable self-asymmetric planar metamaterials

We numerically and experimentally demonstrate a tunable electromagnetically induced transparency (EIT)-like spectral response in a self-asymmetric planar metamaterial in microwave region. This EIT-like physical mechanism is characterized by magnetic dipole coupling induced transparency, which is different from classical properties of EIT-like phenomenon. Our proposed system can be better explained by making an analogy to three-level Λ-type system. Further study also revealed that an ultra-broadband transparency window with the maximum bandwidth up to 7.8 GHz, extending from 6.1 GHz to 13.9 GHz, could be realized by simply adjusting the depth of the structure.

Nuclear magnetic relaxation induced by exchange-mediated orientational randomization: Longitudinal relaxation dispersion for a dipole-coupled spin-1/2 pair

In complex biological or colloidal samples, magnetic relaxation dispersion (MRD) experiments using the field-cycling technique can characterize molecular motions on time scales ranging from nanoseconds to microseconds, provided that a rigorous theory of nuclear spin relaxation is available. In gels, cross-linked proteins, and biological tissues, where an immobilized macromolecular component coexists with a mobile solvent phase, nuclear spins residing in solvent (or cosolvent) species relax predominantly via exchange-mediated orientational randomization (EMOR) of anisotropic nuclear (electric quadrupole or magnetic dipole) couplings. The physical or chemical exchange processes that dominate the MRD typically occur on a time scale of microseconds or longer, where the conventional perturbation theory of spin relaxation breaks down. There is thus a need for a more general relaxation theory. Such a theory, based on the stochastic Liouville equation (SLE) for the EMOR mechanism, is available for a single quadrupolar spin I = 1. Here, we present the corresponding theory for a dipole-coupled spin-1/2 pair. To our knowledge, this is the first treatment of dipolar MRD outside the motional-narrowing regime. Based on an analytical solution of the spatial part of the SLE, we show how the integral longitudinal relaxation rate can be computed efficiently. Both like and unlike spins, with selective or non-selective excitation, are treated. For the experimentally important dilute regime, where only a small fraction of the spin pairs are immobilized, we obtain simple analytical expressions for the auto-relaxation and cross-relaxation rates which generalize the well-known Solomon equations. These generalized results will be useful in biophysical studies, e.g., of intermittent protein dynamics. In addition, they represent a first step towards a rigorous theory of water (1)H relaxation in biological tissues, which is a prerequisite for unravelling the molecular basis of soft-tissue contrast in clinical magnetic resonance imaging.

Effects of Counterion and Solvent on Proton Location and Proton Transfer Dynamics of N–H···N Hydrogen Bond of Monoprotonated 1,8-Bis(dimethylamino)naphthalene

The proton location and proton transfer (PT) dynamics of a hydrogen bond are under the influence of the static and dynamical properties of the solvent and counterions. In the present study, the N-H distances were determined for salts of 1,8-bis(dimethylamino)naphthalene, DMANH(+)X(-) (X(-) = BPh4(-), ClO4(-), and Cl(-)), in acetonitrile (AN) solution, and DMANH(+)Br(-) in water by observing the (15)N spin-lattice relaxation caused by the (15)N-(1)H magnetic dipolar coupling under assumption that the PT time was shorter than the NH reorientation time (∼10(-11) s). The obtained N-H distances decreased in the following order: DMANH(+)BPh4(-) > DMANH(+)ClO4(-) > DMANH(+)Br(-)/H2O > DMANH(+)Cl(-), indicating that interactions with the environment affect the PT potentials. To understand the results at the molecular level, Car-Parrinello molecular dynamics simulations were performed for DMANH(+), DMANH(+) in water, and DMANH(+)-Cl(-) ion-pair in AN. The results of simulation suggest that (1) the N-H distance decreases in the presence of a solvent and counterion; (2) the PT time is probably ∼10(-12) s, which confirms the above assumption used for the NMR relaxation data analyses; and (3) fluctuation of the interactions with the solvent or counterion has a significant role in PT. Quantum nuclear effects on the hydrogen bond were also examined.

Novel one-pot preparation of CoFe2O4–Ag nanocrystalline powders

We report the synthesis of the nanocomposite Ag100−x(CoFe2O4)x with x=20 and 93wt%. The nanocomposite was synthesized in the pores of the biopolymer chitosan. The samples were analyzed by x-ray diffraction, transmission electron microscopy, scanning electron microscopy and magnetometry. The average sizes of the Ag nanoparticles ranged from 8.6 to 13.6nm and the CoFe2O4 nanoparticles from 8.5 to 9.5nm. The sizes of the CoFe2O4 particles were very close to the critical diameter for superparamagnetic behavior at room temperature. The zero field cooled magnetization measurements for both samples showed peak temperatures above 290K, the coercive fields measured at 300K were smaller than 100Oe and increased with the size and the percentage of cobalt ferrite phase. Magnetization versus field measurement at 5K for x=20wt% showed coercive fields of 9.62kOe, typical of hard cobalt ferrite, and for x=93wt% complex magnetic behavior was observed. The results were discussed taking into account the magnetic dipolar coupling between the CoFe2O4 nanoparticles.

Structures of the Excited States of Phospholamban and Shifts in Their Populations upon Phosphorylation

Phospholamban is an integral membrane protein that controls the calcium balance in cardiac muscle cells. As the function and regulation of this protein require the active involvement of low populated states in equilibrium with the native state, it is of great interest to acquire structural information about them. In this work, we calculate the conformations and populations of the ground state and the three main excited states of phospholamban by incorporating nuclear magnetic resonance residual dipolar couplings as replica-averaged structural restraints in molecular dynamics simulations. We then provide a description of the manner in which phosphorylation at Ser16 modulates the activity of the protein by increasing the sizes of the populations of its excited states. These results demonstrate that the approach that we describe provides a detailed characterization of the different states of phospholamban that determine the function and regulation of this membrane protein. We anticipate that the knowledge of conformational ensembles enable the design of new dominant negative mutants of phospholamban by modulating the relative populations of its conformational substates.

Constraints on anomalous color dipole operators from Higgs boson production at the LHC

Physics beyond the standard model (SM) can be parameterized with an effective Lagrangian that respects the symmetries of the standard model and contains many operators of dimension six. We consider the subset of these operators that is responsible for flavor diagonal anomalous color magnetic (CMDM) and electric (CEDM) dipole couplings between quarks and gluons. Invariance of these operators under the SM implies that they contribute to Higgs boson production at the LHC and we study the corresponding constraints that can be placed on them. For the case of the top-quark we first review constraints from top-quark pair production and decay, and then compare them to what can be achieved by studying $t\bar{t}h$ production. We also constrain the corresponding couplings for $b$-quarks and light quarks by studying $pp \to b\bar{b}h$ and $pp \to hX$ respectively.

Hyperfine structure of the7d2D3/2level in cesium measured by Doppler-free two-photon spectroscopy

We report the measurement of the hyperfine structure in the 7$d$ ${}^{2}{D}_{3/2}$ state of ${}^{133}$Cs isotope by Doppler-free two-photon fluorescence spectroscopy in a gas cell. The hyperfine level separations were measured using a frequency-stabilized Ti:sapphire laser locked to one of the hyperfine transitions and an acousto-optic modulator locked to another hyperfine transition. The frequency separation between various hyperfine levels has been measured with a precision of \ensuremath{\sim}100 kHz. From the measured hyperfine separations of the excited state, we have determined the magnetic dipole coupling constant ($A$) and electric quadrupole coupling constant ($B$) for the 7$d$ ${}^{2}{D}_{3/2}$ state. The determined hyperfine structure constants are $A=7.38$ (0.01) MHz and $B=\ensuremath{-}0.18$ (0.1) MHz. The values measured are found to be in good agreement with the earlier reported results using stepwise excitation process.