Weak Dipolar Interactions Research Articles

Magnetic Differentiation of Pyrogallol[4]arene Tubular and Capsular Frameworks

The differences in magnetic properties of metal-based nanometric assemblies are due to distinct contributions from host-guest interactions, structural integrity, and magnetic interactions. To disentangle these contributions, it is necessary to control the self-assembly process that forms these entities. Herein we study the effect of host-to-guest ratios to identify remarkably different structural-magnetic contributions of C-methylpyrogallol[4]arene⊂ferrocene/(PgC1)2⊂Fc dimers vs (PgC1)3⊂Fc nanotubes. At low temperature, a weak anti-ferromagnetic alignment is observed, suggesting a weak dipolar interaction between Fc guest moieties within adjacent dimers or tubes. Also, differences are observed between magnetic atom occupancy as a function of guest (PgC1⊂Fc tube/dimer) versus magnetic atom occupancy within the framework wall (PgC3Ni hexamer/dimer). Identification of the role of the framework shape and metal-metal distances in the crystal lattice opens up unparalleled prospects for materials engineering.

Topology of the Trans-Membrane Peptide WALP23 in Model Membranes under Negative Mismatch Conditions

The organization and orientation of membrane-inserted helices is important for better understanding the mode of action of membrane-active peptides and of protein-membrane interactions. Here we report on the application of ESEEM (electron spin-echo envelope modulation) and DEER (double electron-electron resonance) techniques to probe the orientation and oligomeric state of an α-helical trans-membrane model peptide, WALP23, under conditions of negative mismatch between the hydrophobic cores of the model membrane and the peptide. Using ESEEM, we measured weak dipolar interactions between spin-labeled WALP23 and (2)H nuclei of either the solvent (D2O) or of lipids specifically deuterated at the choline group. The ESEEM data obtained from the deuterated lipids were fitted using a model that provided the spin label average distance from a layer of (2)H nuclei in the hydrophilic region of the membrane and the density of the (2)H nuclei in the layer. DEER was used to probe oligomerization through the dipolar interaction between two spin-labels on different peptides. We observed that the center of WALP23 does not coincide with the bilayer midplane and its N-terminus is more buried than the C-terminus. In addition, the ESEEM data fitting yielded a (2)H layer density that was much lower than expected. The DEER experiments revealed the presence of oligomers, the presence of which was attributable to the negative mismatch and the electrostatic dipole of the peptide. A discussion of a possible arrangement of the individual helices in the oligomers that is consistent with the ESEEM and DEER data is presented.

Refined Distances Between Paramagnetic Centers of a Multi‐Copper Nitrite Reductase Determined by Pulsed EPR (iDEER) Spectroscopy

Refined Distances Between Paramagnetic Centers of a Multi‐Copper Nitrite Reductase Determined by Pulsed EPR (<i>i</i>DEER) Spectroscopy

Recoupling of chemical shift anisotropy by R-symmetry sequences in magic angle spinning NMR spectroscopy

(13)C and (15)N chemical shift (CS) interaction is a sensitive probe of structure and dynamics in a wide variety of biological and inorganic systems, and in the recent years several magic angle spinning NMR approaches have emerged for residue-specific measurements of chemical shift anisotropy (CSA) tensors in uniformly and sparsely enriched proteins. All of the currently existing methods are applicable to slow and moderate magic angle spinning (MAS) regime, i.e., MAS frequencies below 20 kHz. With the advent of fast and ultrafast MAS probes capable of spinning frequencies of 40-100 kHz, and with the superior resolution and sensitivity attained at such high frequencies, development of CSA recoupling techniques working under such conditions is necessary. In this work, we present a family of R-symmetry based pulse sequences for recoupling of (13)C∕(15)N CSA interactions that work well in both natural abundance and isotopically enriched systems. We demonstrate that efficient recoupling of either first-rank (σ(1)) or second-rank (σ(2)) spatial components of CSA interaction is attained with appropriately chosen γ-encoded RN(n)(v) symmetry sequences. The advantage of these γ-encoded RN(n)(v)-symmetry based CSA (RNCSA) recoupling schemes is that they are suitable for CSA recoupling under a wide range of MAS frequencies, including fast MAS regime. Comprehensive analysis of the recoupling properties of these RN(n)(v) symmetry sequences reveals that the σ(1)-CSA recoupling symmetry sequences exhibit large scaling factors; however, the partial homonuclear dipolar Hamiltonian components are symmetry allowed, which makes this family of sequences suitable for CSA measurements in systems with weak homonuclear dipolar interactions. On the other hand, the γ-encoded symmetry sequences for σ(2)-CSA recoupling have smaller scaling factors but they efficiently suppress the homonuclear dipole-dipole interactions. Therefore, the latter family of sequences is applicable for measurements of CSA parameters in systems with strong homonuclear dipolar couplings, such as uniformly-(13)C labeled biological solids. We demonstrate RNCSA NMR experiments and numerical simulations establishing the utility of this approach to the measurements of (13)C and (15)N CSA parameters in model compounds, [(15)N]-N-acetyl-valine (NAV), [U-(13)C, (15)N]-alanine, [U-(13)C,(15)N]-histidine, and present the application of this approach to [U-(13)C∕(15)N]-Tyr labeled C-terminal domain of HIV-1 CA protein.

Pressure effects on the magnetic behaviour of copper (II) compounds: magnetic ordering of layered organic/inorganic magnets

The high hydrostatic pressure effect on the magnetic properties of the layered hybrid compounds Cu2(OH)3(CnH2n+1CO2)⋅mH2O with distance between magnetic layers of up to 40 Å is studied. It is shown that the temperature of the ferromagnetic ordering decreases linearly with pressure increase. From measurements of susceptibility in the paramagnetic region, using both quantum Heisenberg and Ising exchange coupling models in layers and dipole interaction between layers, the in- and interlayer interactions are deduced. The dipole interactions are calculated and are shown to coincide with the model of Ising interactions in the layers. The value and decrease of Tc under pressure are mainly driven by the value and decrease of the in-plane interactions. The formation of the long range ordering in the layered sample with dipolar interaction between layers is analysed. As a conclusion it is suggested that for designing high temperature ferromagnetism in layer compounds it is enough to have large in-plane interactions of ions with specific symmetry in layers and weak dipole interactions between layers.

Surface effects on the magnetic behavior of nanoparticle assemblies

We study the interplay between intrinsic properties and collective effects in assemblies of magnetic nanoparticles. Analytical expressions for the magnetization are obtained for weak dipolar interactions in dilute assemblies. Our study is based on thermodynamic perturbation theory for the effective macrospin model where a nanoparticle is represented by its macroscopic magnetic moment, taking into account surface effects of each nanoparticle. The approximate analytical expressions for the magnetization are compared to Monte Carlo simulations and their range of validity is established. Our calculations show that the magnetization is influenced by the nanoparticle anisotropy and the shape of the assembly and that in all cases the effect of the second-order dipolar interaction term of the perturbation theory is the reduction of the magnetization.

Magnetic reversal of an artificial square ice: dipolar correlation and charge ordering

Magnetic reversal of an artificial square ice pattern subject to a sequence of magnetic fields applied slightly off the diagonal axis is investigated via magnetic force microscopy of the remanent states that result. Sublattice independent reversal is observed via correlated incrementally pinned flip cascades along parallel dipolar chains, as evident from analysis of vertex populations and dipolar correlation functions. Weak dipolar interactions between adjacent chains favour antialignment and give rise to weak charge ordering of ‘monopole’ vertices during the reversal process.

Synchronization of magnetic dipole rotation in an ac magnetic field

The synchronization of the rotation of magnetic dipoles due to weak dipolar interactions is studied. The set of equations is analyzed by the time averaging technique. It is found that dipoles synchronously oscillate at low applied fields and rotate synchronously at large applied fields. The mean angular velocity of synchronous rotation increases with the field strength and reaches a constant value equal to the angular frequency of the field above the critical value of the field strength. The critical value of the field strength above which the synchronous rotation takes place can be calculated from dimensionless parameters using a model derived from first principles by others. The values thus obtained are in good agreement with the values we obtain from a numerical simulation. Thus, we may conclude that the liquid flow observed in these systems may be caused by synchronized rotations of the dipoles.

Tunable dipolar resonances and Einstein-de Haas effect in aRb87-atom condensate

We theoretically study a spinor condensate of $^{87}\mathrm{Rb}$ atoms in a $F=1$ hyperfine state confined in an optical dipole trap. Putting initially all atoms in an ${m}_{F}=1$, component we observe a significant transfer of atoms to other, initially empty Zeeman states exclusively due to dipolar forces. Because of conservation of a total angular momentum the atoms going to other Zeeman components acquire an orbital angular momentum and circulate around the center of the trap. This is a realization of the Einstein-de Haas effect in a system of cold gases. We show that the transfer of atoms via dipolar interactions is possible only when the energies of the initial and the final sates are equal. This condition can be fulfilled utilizing a resonant external magnetic field, which tunes energies of involved states via the linear Zeeman effect. We found that there are many final states of different spatial density, which can be tuned selectively to the initial state. We show a simple model explaining high selectivity and controllability of weak dipolar interactions in the condensate of $^{87}\mathrm{Rb}$ atoms.

Effect of weak dipolar interaction on the magnetic properties of Ni nanoparticles assembly analyzed with different protocols

The effect of weak dipolar interactions (DIs) between Ni nanoparticles (NPs) in samples with different Ni concentrations was investigated by performing a detailed characterization of their structural and magnetic properties. From the determination of several physical parameters of Ni NP assemblies, it was found that the ac and dc magnetic susceptibility measurements are valuable for identifying the DIs between NPs while hysteresis loops measurements showed to be very insensitive, provided that the strength of the DI field is much smaller than the maximum coercive field. Therefore, the sensitivity of the observed static and dynamical magnetic properties to the effect of weak DI depends on the measurement protocols used.

A simulation study on the magnetic ordering in an artificial geometrically frustrated lattice

A Monte Carlo simulation on the magnetic ordering of the artificial geometrically frustrated square lattice shows that the system exhibits the spin ice state and the disorder state for strong and weak dipolar interactions. We demonstrate that the long-range dipolar interactions are significant for the short-range spin ice state.

Magnetic properties of Fe2O3 nanoparticles embedded in hollows of periodic nanoporous silica

The magnetic properties of the nanocomposite consisting of hematite (α-Fe2O3) nanoparticles embedded in the hollows of the periodic nanoporous silica matrix were investigated. The magnetic measurements showed on the superparamagnetic behavior of the nanoparticles. This behavior was evidenced by the irreversibility of magnetization curves measured in zero-field-cooled and field cooled regimes, by the presence of a maximum in zero-field-cooled magnetization curve related to blocking temperature TB∼37 K and also by revealing of the coercivity HC below TB. The value of the magnetic moment mP∼300 μB of each iron-oxide particle was estimated. The interparticle interactions were investigated from dynamic properties using ac susceptibility. The susceptibility study shows, that the in-phase ac susceptibility χ′ is frequency dependent and the peak position increases with the increase in the frequency. The quantitative analysis of χ′ using theoretical models shows on the existence of weak dipolar interactions between particles. It was shown from correlation of experimental data with Arrhenius and Vogel–Fulcher laws that the incorporation of the nanoparticles into porous SiO2 matrix provides the effective method for the reducing of dipolar interactions between magnetic nanoparticles with the size below 10 nm.

Self-association based on orthogonal CO⋯CO interactions in the solid and liquid state

A network of orthogonal CO⋯CO interactions was identified in the X-ray crystal structure of an α,α-difluorocyclopentanone derivative. This finding inspired investigations of self-association driven by these weak dipolar interactions in apolar solvents, which was proven by 1H NMR spectroscopy.

Role of dipolar interactions in a system of Ni nanoparticles studied by magnetic susceptibility measurements

The role of dipolar interactions among Ni nanoparticles (NPs) embedded in an amorphous ${\text{SiO}}_{2}/\text{C}$ matrix with different concentrations has been studied performing ac magnetic susceptibility ${\ensuremath{\chi}}_{\text{ac}}$ measurements. For very diluted samples, with Ni concentrations $&lt;4\text{ }\text{wt}\text{ }%$ Ni or very weak dipolar interactions, the data are well described by the N\'eel-Arrhenius law. Increasing Ni concentration to values up to $12.8\text{ }\text{wt}\text{ }%$ Ni results in changes in the N\'eel-Arrhenius behavior, the dipolar interactions become important, and need to be considered to describe the magnetic response of the NPs system. We have found no evidence of a spin-glasslike behavior in our Ni NP systems even when dipolar interactions are clearly present.

Fermi surface of a trapped dipolar Fermi gas

Under the framework of the semiclassical theory, we investigate the equilibrium-state properties of a spin polarized dipolar Fermi gas through full numerical calculation. We show that the Fermi surfaces in both real and momentum spaces are stretched along the attractive direction of dipolar interaction. We further verify that the deformed Fermi surfaces can be well approximated by ellipsoids. In addition, the deformation parameters slightly depend on the local real- and momentum-space densities. We also study the interaction strength dependence of the energy and real- and momentum-space densities. By comparing them with variational results, we find that the ellipsoidal ansatz usually generates accurate results for weak dipolar interaction, while under strong dipolar interaction limit, notable discrepancy can be observed. Finally, we map out the stability boundary of the system.

Dipolar Driven Spontaneous Self Assembly of Superparamagnetic Co Nanoparticles into Micrometric Rice-Grain like Structures

Superparamagnetic single crystal single domain Co nanoparticles of 6 nm in diameter evaporated onto highly pyrolytic oriented graphite spontaneously self-assemble into super structures with an elongated shape. These structures have been studied by optical and scanning electron microscopies, atomic and magnetic force microscopy, electron dispersive X-ray analysis, and SQUID magnetometry. We propose that the weak dipolar interactions between superparamagnetic dipoles of the cobalt nanoparticles are responsible for the formation of these structures when the dipolar magnetic interactions are strong enough to influence the general process of self-assembly dominated by van der Waals forces between neighboring nanoparticles and between nanoparticles and the substrate during evaporation of the solvent.

Interaction forces and adhesion of supported myelin lipid bilayers modulated by myelin basic protein

Force-distance measurements between supported lipid bilayers mimicking the cytoplasmic surface of myelin at various surface coverages of myelin basic protein (MBP) indicate that maximum adhesion and minimum cytoplasmic spacing occur when each negative lipid in the membrane can bind to a positive arginine or lysine group on MBP. At the optimal lipid/protein ratio, additional attractive forces are provided by hydrophobic, van der Waals, and weak dipolar interactions between zwitterionic groups on the lipids and MBP. When MBP is depleted, the adhesion decreases and the cytoplasmic space swells; when MBP is in excess, the bilayers swell even more. Excess MBP forms a weak gel between the surfaces, which collapses on compression. The organization and proper functioning of myelin can be understood in terms of physical noncovalent forces that are optimized at a particular combination of both the amounts of and ratio between the charged lipids and MBP. Thus loss of adhesion, possibly contributing to demyelination, can be brought about by either an excess or deficit of MBP or anionic lipids.

Towards Allosteric Receptors: Adjustment of the Rotation Barrier of 2,2′‐Bipyridine Derivatives

What a difference! The energy differences between anti and syn conformers as well as the energy barrier for the rotation around the aryl-aryl bond of a number of 2,2'-bipyridine molecules were examined by quantum-chemical methods. The energy differences were found to be governed by the substituents directly attached to the bipyridine and their ability to form intramolecular hydrogen bonds.Quantum-chemical calculations at the BP86/TZVP level of theory were performed to determine the energy differences between the syn and the anti conformers, as well as the energy barrier for the rotation of the aryl-aryl bond of 2,2'-bipyridine molecules and a number of disubstituted derivatives. Substitutents with hydrogen-bond donor (or electron acceptor) functions or hydrogen-bond acceptors (or electron donors) are generally found to have large effects on the difference and the barrier. Substitution with a hydrogen-bond donor (or an electron acceptor) at position 6 and 6' leads to a decrease owing to a charge transfer from the pyridine nitrogen lone pair to the donor, which is caused by the formation of weak intramolecular hydrogen bonds and/or dipolar interactions, respectively. Conversely, substitution at position 4 and 4' causes an increase in the energy barrier. Substitution with a hydrogen-bond acceptor (or an electron donor) shows the opposite behavior, which can be explained by the weak intramolecular interactions. Interestingly, even very weak CH hydrogen-bond donors (electron acceptors) such as methyl groups have a significant influence. This indicates the importance of such weak interactions for the structure and energetics of supramolecular systems. The energy differences are mainly governed by the substituents directly attached to the bipyridine core as the introduction of sterically demanding groups in the periphery hardly influences the barriers or energy differences of the conformers. These findings are important for the design of heterotropic positive cooperative allosteric receptors with 2,2'-bipyridines as the allosteric centre.

Biomimetic Modelling of Copper Enzymes: Synthesis, Characterization, EPR Analysis and Enantioselective Catalytic Oxidations by a New Chiral Trinuclear Copper(II) Complex

AbstractThe new octadentate ligand (R)‐(+)‐N,N′‐dimethyl‐N,N′‐bis{‐2‐[bis(1‐methyl‐2‐benzimidazolylmethyl)]‐2‐methylaminoethyl}‐1,1′‐binaphthyl‐2,2′diamine [(R)‐(+)‐DABN‐L‐Ala‐Bz4; L] was employed for the synthesis of dinuclear and trinuclear copper(II) complexes. The ligand design is based on the insertion of chiral residues derived from L‐alanine between the diaminobinaphthyl (R)‐(+)‐DABN spacer and the aminobis(benzimidazole) metal binding units. The chiroptical properties of the ligand and the complexes are described. EPR experiments were performed on [Cu2L]4+ and [Cu3L]6+ at low temperatures. In the case of [Cu2L]4+, a weak dipolar interaction between the two spin centres was found. A similar weak spin interaction occurred in the trinuclear copper cluster, which could be treated likewise as a weakly coupled three‐spin system. The analysis was substantiated by studying the complex EPR temperature behaviour, where population of the quartet state occurred only at high temperature (77 K), whereas at cryogenic temperatures (4 K) the system adopted a doublet state as a ground‐state spin configuration. Titration with sodium azide of [Cu2L]4+ was consistent with terminal binding of one N3– molecule to each copper ion. Furthermore, dipolar interactions between spin centres were strongly suppressed in this case. For [Cu3L]6+, the adduct formed by the interaction with two azido molecules induced formation of a μ‐azido bridges among the three Cu2+ ions and led to population of the quartet state even at cryogenic temperatures. This bridged configuration was, however, lost upon further addition of azido molecules. The copper(II) complexes were tested as catalysts in the oxidation of biogenic catechols and flavonoids by dioxygen to give the corresponding quinones, which were trapped as adducts with MBTH. The dinuclear complex [Cu2L]4+ displays poor substrate enantiodifferentiating ability, even though it exhibits catalytic activity comparable to that of [Cu3L]6+. The trinuclear complex [Cu3L]6+ exhibits significant enantioselectivity in the oxidations of the catecholamines L‐/D‐dopa methyl ester and L‐/D‐norepinephrine. The origin of this enantioselectivity must be associated with the mode of substrate binding, as it depends almost entirely on KM.(© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2009)

Ultracentrifugation of Single-Domain Magnetite Particles and the De Gennes−Pincus Approach to Ferromagnetic Colloids in the Dilute Regime

We report an analytical ultracentrifugation study on sedimentation in dilute stable dispersions of uniform, magnetic iron oxide (Fe3O4) colloids. On increase of the dipolar coupling constant, tuned by the average particle size, the linear concentration dependence of the sedimentation velocity shows an abrupt transition from the hindered sedimentation expected for hard spheres to a marked acceleration already for weak dipolar interactions. This transition is not reproduced by sedimentation theory derived from an effective, isotropic pair correlation function of the type proposed by De Gennes and Pincus, for reasons which are made clear. Accelerated settling, instead, follows a scaling based on the mass action law for dimer formation with dipoles in head-to-tail configuration. Our work illustrates that orientational averaging of dipole interactions of ferromagnetic colloids in the dilute regime is inapplicable, with an obvious implication for the possible existence of isotropic gas-liquid criticality for such colloids.