Displacement Of Loop Research Articles

Comment on “Thickness dependence of exchange bias and coercivity in a ferromagnetic layer coupled with an antiferromagnetic layer” [J. Appl. Phys. 94, 2529 (2003)

Hu, Jin, and Ma have proposed a theoretical investigation on the influence of the antiferromagnetic layer thickness on the magnetic properties of ferromagnetic∕antiferromagnetic bilayers [J. Appl. Phys. 94, 2529 (2003)], considering both the bilinear and biquadratic exchange couplings, and have claimed that from their formulas for the hysteresis loop displacement and coercivity many interesting conclusions can be extracted. Unfortunately, the mathematical procedure used to find the equilibrium of the system is based on inadequate stability conditions and has led to nonphysical results. More importantly, the simple phenomenological model, employed by the authors, is intrinsically not capable to give the antiferromagnetic layer thickness dependence of the magnetic properties of such exchange-coupled bilayers.

Transconformations of the SERCA1 Ca-ATPase: A Normal Mode Study

The transport of Ca2+ by Ca-ATPase across the sarcoplasmic reticulum membrane is accompanied by several transconformations of the protein. Relying on the already established functional importance of low-frequency modes in dynamics of proteins, we report here a normal mode analysis of the Ca2+-ATPase based on the crystallographic structures of the E1Ca2 and E2TG forms. The lowest-frequency modes reveal that the N and A(+Nter) domains undergo the largest amplitude movements. The dynamical domain analysis performed with the DomainFinder program suggests that they behave as rigid bodies, unlike the highly flexible P domain. We highlight two types of movements of the transmembrane helices: i), a concerted movement around an axis perpendicular to the membrane which “twists open” the lumenal side of the protein and ii), an individual translational and rotational mobility which is of lower amplitude for the helices hosting the calcium binding sites. Among all modes calculated for E1Ca, only three are enough to describe the transition to E2TG; the associated movements involve almost exclusively the A and N domains, reflecting the closure of the cytoplasmic headpiece and high displacement of the L7-8 lumenal loop. Subsequently, we discuss the potential contribution of the remaining low-frequency normal modes to the transconformations occurring within the overall calcium transport cycle.

Ferromagnetic–antiferromagnetic state in manganites

Magnetic and electrical properties of La 1− x Sr x MnO 3 ( x=0.1, 0.15 and 0.3) single-crystals, Eu 0.7A 0.3MnO 3 (A=Ca, Sr) ceramics, and La 0.84Sr 0.16MnO 3 and La 0.1Pr 0.6Ca 0.3MnO 3 epitaxial thin films are investigated. Curves of T-dependence of the electrical resistivity and the magnetoresistance show the maxima near Curie point and magnetoresistance is colossal in maximum. Distinction between the magnetizations and the remanences of zero-field-cooled and field-cooled sample and displacement of hysteresis loop of field-cooled sample are found in La 0.9Sr 0.1MnO 3 single-crystals, Eu 0.7A 0.3MnO 3 (A=Ca, Sr) ceramics, and La 0.84Sr 0.16MnO 3 and La 0.1Pr 0.6Ca 0.3MnO 3 epitaxial thin films. This displacement denotes that magnetic two-phase ferro–antiferromagnetic state is inherent in them. From this displacement the exchange integral |J|=10 −7−10 −6 eV, described the Mn–O–Mn connection across droplet/host interface, is calculated. The presence of ferromagnetic (F) and antiferromagnetic (AF) states in Eu 0.7A 0.3MnO 3 (A=Ca, Sr) is confirmed by next facts: magnetization isotherms at T< T N ( T N is Neel temperature) are superposition of linear part, as for AF, and small spontaneous magnetization due to the F droplets; paramagnetic Curie point is approximately three times as much than in EuMnO 3, whereas T N is slightly varied. Behavior of electrical and magnetic properties are explained by presence of magnetic two-phase F–AF state due to strong s– d exchange.

Exchange coupling in ultrathin epitaxial yttrium iron garnet films

Magnetic exchange coupling has been observed for ultrathin films of yttrium iron garnet (Y3Fe5O12 or YIG). Single-crystalline YIG films were prepared on yttrium aluminium garnet (Y3Al5O12 or YAG) substrates by pulsed laser deposition. (111) and (110) oriented substrates were used. Film thicknesses were varied from 180 A to 4600 A. Epitaxial growth of YIG on YAG was obtained in spite of the lattice mismatch of 3%. Magnetic hysteresis loops recorded for ultrathin YIG films have a “bee-waist” shape and show a coupling between two different magnetic phases. The first phase is magnetically soft YIG. A composition study by secondary ion mass spectroscopy shows the second phase to be Y3Fe5-xAlxO12 due to the interdiffusion of Fe and Al at the film/substrate interface. This compound is known to be magnetically harder and to have weaker magnetization than YIG. The coupling of the two phases leads to a hysteresis loop displacement at low temperatures. This displacement varies differently with film thickness for two substrate orientations. Assuming an interfacial coupling, the maximal interaction energy is estimated to be about 0.17 erg/cm2 at 5 K for (111) oriented sample.

Multicopy Crystallographic Refinement of a Relaxed Glutamine Synthetase from Mycobacterium tuberculosis Highlights Flexible Loops in the Enzymatic Mechanism and Its Regulation

The crystal structure of glutamine synthetase (GS) from Mycobacterium tuberculosis determined at 2.4 A resolution reveals citrate and AMP bound in the active site. The structure was refined with strict 24-fold noncrystallographic symmetry (NCS) constraints and has an R-factor of 22.7% and an R-free of 25.5%. Multicopy refinement using 10 atomic models and strict 24-fold NCS constraints further reduced the R-factor to 20.4% and the R-free to 23.2%. The multicopy model demonstrates the range of atomic displacements of catalytic and regulatory loops in glutamine synthesis, simulating loop motions. A comparison with loop positions in substrate complexes of GS from Salmonella typhimurium shows that the Asp50 and Glu327 loops close over the active site during catalysis. These loop closures are preceded by a conformational change of the Glu209 beta-strand upon metal ion or ATP binding that converts the enzyme from a relaxed to a taut state. We propose a model of the GS regulatory mechanism based on the loop motions in which adenylylation of the Tyr397 loop reverses the effect of metal ion binding, and regulates intermediate formation by preventing closure of the Glu327 loop.

The displacement field of a rectangular Volterra dislocation loop

The displacement field of a rectangular Volterra dislocation loop having three non-zero Burgers vector components is obtained in an analytical closed form. The solution is obtained via integrating the Burgers equation for the displacement field of any closed dislocation curve and is expressed in a relatively compact form. The current solution has utility in a number of problems including dislocation-particle interaction problems, where the boundary condition involved imposes certain restrictions on the displacements in the medium, and modelling of cracks of arbitrary shapes. The solution is also useful in benchmarking newly emerging dislocation dynamics codes which discretize a curved dislocation line in some form or another. Several verification steps of the solution correctness are made including a comparison with the displacement and stress fields of a circular Volterra dislocation loop of equal Burgers vector and comparable size.

Magnetic two-phase ferromagnetic–antiferromagnetic state in manganites

Distinction between the magnetizations and the remanences of zero-field-cooled and field-cooled sample and displacement of hysteresis loop of field-cooled sample are found in La 0.9Sr 0.1MnO 3 single crystals, Eu 0.7A 0.3MnO 3 (A=Ca, Sr) ceramics, and La 0.84Sr 0.16MnO 3 and La 0.1Pr 0.6Ca 0.3MnO 3 thin films. This displacement denotes that ferro–antiferromagnetic state is inherent in them. From this displacement we calculated the exchange integral | J|=10 −7–10 −6 eV, described the MnOMn connection across droplet/host interface. The behavior of CMR and volume magnetostriction near T C are similar in La 1− x Sr x MnO 3 and explained magnetic two-phase state due to strong s– d exchange.

Penetration mechanisms of non-deforming projectiles into reinforced concrete barriers

Static and dynamic penetration tests of reinforced concrete (RC) slab specimens are described and discussed. The experimental study was aimed at a better understanding of mechanisms that are involved in dynamic penetration, through their identification in static tests, and by establishing their relative influence in similar dynamic cases. The RC specimens were square plates, and they were made of 30 MPa concrete. The non-deforming steel penetrator was a 50-mm diameter steel rod with a conical nose of 1.5 aspect ratio. Impact penetration tests were carried out with an air gun, which launched the projectiles at velocities of up to 300 m/sec. The static tests were conducted using a closed loop displacement control actuator, where the penetrator was pushed at a constant rate of displacement into the specimen. The static tests reveal important mechanisms that govern the penetration process and therefore contribute to a better understanding of RC barriers resistance to non-deforming projectiles impact.

Analytical solutions for exchange bias and coercivity in ferromagnetic/antiferromagnetic bilayers

Analytical expressions have been derived for the exchange bias field, coercivity, and effective anisotropy field in ferromagnetic/antiferromagnetic bilayers in the framework of a model assuming the formation of a planar domain wall at the antiferromagnetic side of the interface with the reversal of the ferromagnetic orientation. It is shown that there are five different sets of analytical expressions for the hysteresis loop displacement and coercivity, which depend on the interfacial exchange coupling strength and ferromagnetic anisotropy, and only one expression for the effective anisotropy field. These expressions are compared with the previously reported theoretical results, and the validity of the latter is discussed. It is shown that in the framework of the present model, the hysteresis loop, ac susceptibility, and ferromagnetic resonance measurements of exchange anisotropy should give the same values for the exchange bias field. The difference between the exchange bias field values, estimated experimentally by ac susceptibility and through hysteresis loop measurements for Co/CoO bilayers, is explained as well.

Hysteresis and relaxation of hard–soft nanocomposite samples

We present experimental results on the hysteresis and relaxation of Fe/Ba hexaferrite composite samples prepared by ball milling starting from precursor Fe and Ba hexaferrite particles. Our results show that, for maximum applied fields of up to 8 kOe, the measured loops show a shift of up to 85 Oe. The compositional dependence of that loop displacement presents two maxima (at approximately x=0.3 and x=0.7) and a minimum at x=0.5. These results are discussed in terms of the dipolar coupling between both phases.

Dipolar interactions in hard-soft nanocomposites

We report on the hysteretic behavior of nanocomposite samples formed by Fe and Ba hexaferrite particles. Our most interesting result corresponds to the observation of a loop displacement (in the sense of the negative fields) which depended in a complex way on the composition of the samples. That result was supported by the observation of measurable differences between the high field values of the isothermal and demagnetization remanences. We discuss our experimental data in terms of the characteristics of the dipolar field created into the Fe regions by the hard Ba hexaferrite particles.

Exchange interaction between ferromagnetic and antiferromagnetic phases in La 1/3Nd 1/3Ca 1/3MnO 3

Magnetization measurements of a La 1/3Nd 1/3Ca 1/3MnO 3 perovskite at magnetic field up to 6 T have revealed an anomalous behaviour – above 130 K the material exhibits a loop displacement about a field of 2 T. We assume that this is the result of an exchange interaction between ferromagnetic and antiferromagnetic phases in a magnetically inhomogeneous compound. At about 115 K a transition from a semiconducting to a metallic-like state has been observed.

Displacement of OmpF loop 3 is not required for the membrane translocation of colicins N and A in vivo

The pore-forming colicins N and A require the porin, OmpF, in order to translocate across the outer membrane of Escherichia coli. We investigated the hypothesis that in vivo, colicins N and A may traverse the outer membrane through the OmpF channel. In order to accommodate a polypeptide in the pore, the mid-channel constriction loop of OmpF, L3, would need to undergo a conformational change. We used five OmpF cystine mutants, which fix L3 in the conformation determined by X-ray crystallography, to investigate L3 movement during colicin activity in vivo. Sensitivity to colicins N and A of E. coli cells expressing these OmpF cystine mutants was determined using cell survival and in vivo potassium efflux and fluorescence assays. Results indicate that gross movement of L3 is not required for colicin N or A activity and that neither of these colicins crosses the outer membrane of E. coli through the lumen of the OmpF pore.

Two-step mechanism of inhibition of cathepsin B by cystatin C due to displacement of the proteinase occluding loop

Stopped-flow kinetics showed that the inhibition of the lysosomal cysteine proteinase, cathepsin B, by its endogenous inhibitor, cystatin C, occurs by a two-step mechanism, in which an initial, weak interaction is followed by a conformational change. The initial interaction most likely involves binding of the N-terminal region of the inhibitor to the proteinase. Considerable evidence indicates that the subsequent conformational change is due to the inhibitor displacing the occluding loop of the proteinase that partially obscures the active site. The presence of this loop, which allows the enzyme to function as an exopeptidase, thus complicates the inhibition mechanism, rendering cathepsin B much less susceptible than other cysteine proteinases to inhibition by cystatins.

Optimizing arthroscopic knots using braided or monofilament suture

Surgeons need to know how the material properties of a suture affect the security of a surgical knot. The purpose of this study was to compare the security of some clinically important arthroscopic knots when tied using a braided multifilament suture and to draw comparisons with results of similar knots tied with monofilament suture. Permanent braided polyester suture was used to test 10 knot configurations. Eight of the knots included (1) two types of initial cinching knots followed by (2) one of four combinations of half-hitches. We also tested the taut-line hitch locked with half-hitches and the original Revo knot. Each knot was subjected to cyclic loading followed by an ultimate load to failure. Clinical failure was defined as the maximum force that resulted in 3 mm of loop displacement. Force versus displacement data were obtained, and the maximal loop holding capacities were compared statistically. The Duncan loop with switched-post half-hitches and the Revo knot (Linvatec, Largo, FL) showed the highest knot-holding capacities (mean, 87N and 92N, respectively) when compared with all other configurations ( P < .0001) for braided suture. A similar knot- holding capacity was described for monofilament suture using the Duncan loop locked with switched-post, reversed-direction half-hitches (mean, 81 N). All knots without post switching slipped completely at significantly lower loads than knots with post switching (monofilament, P < .001; braided, P < .0001). When compared with results of knots tied with monofilament suture, the braided switched-post configurations had smaller cyclic displacements (braided, 0.7 mm; monofilament, 1.7 mm). Although the Revo knot showed good strength for braided suture, it was significantly weaker than other configurations when tied with monofilament suture. Therefore, it is important to test the knot strength for a given suture material before applying it clinically. Arthroscopy 1998 Jan-Feb;14(1):57-65

Ligand-induced conformational changes in ras p21: a normal mode and energy minimization analysis

A normal mode and energy minimization of ras p21 is used to determine the flexibility of the protein and the origin of the conformational differences between GTP and GDP-bound forms. To preserve the integrity of the structures, a hydration shell of water molecules was included as part of the system. Certain low-frequency modes were found to have high involvement coefficients with the conformational transition between the GTP and GDP-bound structures; the involvement coefficients of some of the modes increase when the γ-phosphate group is removed. Two unstable modes that appear in the GTP-bound structure upon deletion of the γ-phosphate group were determined and shown to have dominant contributions in the regions of switch I and switch II; there was also a significant displacement of loop 1. The initial motion in these regions is predicted by the modes to be approximately perpendicular to the direction of the transition from the GTP-bound state to the GDP-bound state. The overall conformational change in the switch I and II regions involves rearrangements of the protein backbone within these regions, rather than rigid body motion. Differences in the low-frequency modes of the GTP and GDP-bound forms appear to play a role in ligand binding. A coupling between the helix α3 position and the deletion of the γ-phosphate group may be involved in the interaction with GAP. The oncogenic mutation G12D leads to a global increase in the rigidity of the protein. Thus, the mutant is likely to have a higher barrier for the conformational change to the inactive form; this would slow the transition and could be related to its oncogenic properties.

Photoinduced exchange anisotropy in a ferromagnet/semiconductor hybrid

It is shown theoretically that the exchange coupling of a ferromagnet film with optically oriented electrons in deep centers near the ferromagnet/semiconductor interface induces an effective magnetic field acting upon the ferromagnet. The effective field with its sign depending on the helicity of circular polarized light leads to the displacement of the magnetic hysteresis loop of the film, i.e. to the phenomenon of photoinduced exchange (unidirectional) anisotropy.

Essential dynamics of lipase binding sites: the effect of inhibitors of different chain length.

The biochemical activity of enzymes, such as lipases, is often associated with structural changes in the enzyme resulting in selective and stereospecific reactions with the substrate. To investigate the effect of a substrate and its chain length on the dynamics of the enzyme, we have performed molecular dynamics simulations of the native Rhizomucor miehei lipase (Rml) and lipase-dialkylphosophate complexes, where the length of the alkyl chain ranges from two to 10 carbon atoms. Simulations were performed in water and trajectories of 400 ps were used to analyse the essential motions in these systems. Our results indicate that the internal motions of the Rml and Rml complexes occur in a subspace of only a few degrees of freedom. A high flexibility is observed in solvent-exposed segments, which connect beta-sheets and helices. In particular, loop regions Gly35-Lys50 and Thr57-Asn63 fluctuate extensively in the native enzyme. Upon activation and binding of the inhibitor, involving the displacement of the active site loop, these motions are considerably suppressed. With increasing chain length of the inhibitor, the fluctuations in the essential subspace increase, levelling off at a chain length of 10, which corresponds to the size of the active-site groove.

Effect of annealing on the magnetic properties and microstructure of amorphous Co 75Si 10B 15

The influence of annealing on low and high frequency magnetic behaviour in amorphous ribbons of nominal composition, Co 75Si 10B 15 was investigated at temperatures both above and below that required for the crystallisation of ε-Co from the amorphous matrix. Low-frequency (0–400 Hz) soft magnetic properties improved on annealing below the crystallisation temperature while high frequency (1–50 kHz) properties were optimised in ribbons annealed at temperatures resulting in approximately 1% crystallisation. Crystallisation resulted in asymmetrically displaced hysteresis loops, the loop displacement increasing with increasing number of discrete ε-Co crystals or crystal clusters. Based on magnetic measurements, differential scanning calorimetry and microstructural observations, it was concluded that the shift in the hysteresis loops originates from the presence of ε-Co crystals within the bulk, rather than at the ribbon surfaces. This type of shift is manifest as a displacement of the coercivities towards the direction of the initial magnetisation and occurs in a range of amorphous alloys which crystallise the ε-Co phase in the bulk. Shifts associated with surface magnetic phases in both Fe- and Co-based alloys may be characterised by a dip in the second quadrant of the B-H loop and/or a skewing of the loop towards lower inductions for a given magnetisation.

Optimizing arthroscopic knots

Arthroscopic repairs, such as those for shoulder instability, are commonly performed. However, the failure rate after arthroscopic repair appears to be higher than with open surgery. These failures may relate to the challenge of tying secure knots arthroscopically. Many knots tied arthroscopically commonly consist of an initial slip knot to remove slack, and a series of half-hitches. Half-hitches, instead of square throws, are difficult to avoid and result when asymmetrical tension is applied to the strands. For this reason, the security of knots tied arthroscopically may not be equivalent to square knots and a greater rate of failure may occur. The purpose of this study was to determine (1) the security of various arthroscopic knots under cyclic and peak loading conditions, (2) how the surgeon can modify the method or sequence of half-hitch throws to minimize knot slippage or breakage, and (3) whether using an arthroscopic knot pusher affects the security of the same knot tied by hand. The most secure knot configurations were achieved by reversing the half-hitch throws and alternating the posts. These knots performed significantly better than all other knots tested ( P < .002). Thus the surgeon can control the holding capacity and minimize suture loop displacement by proper alternation of the tying strands and reversal of the loop when placing the hitches. Under our testing conditions, the loop holding capacity of hand-tied knots was superior to identical knots tied using a pusher ( P < .002). Because arthroscopic surgeons must tie their knots with a pusher, the best way to optimize knot security is by careful attention to the specific technique with which the knot throws are made.