75 research outputs found
Double-spiral magnetic structure of the Fe/Cr multilayer revealed by nuclear resonance scattering
We have studied the magnetization depth profiles in a [57Fe(dFe)/Cr(dCr)]x30
multilayer with ultrathin Fe layers and nominal thickness of the chromium
spacers dCr 2.0 nm using nuclear resonance scattering of synchrotron radiation.
The presence of a broad pure-magnetic half-order (1/2) Bragg reflection has
been detected at zero external field. The joint fit of the reflectivity curves
and Mossbauer spectra of reflectivity measured near the critical angle and at
the "magnetic" peak reveals that the magnetic structure of the multilayer is
formed by two spirals, one in the odd and another one in the even iron layers,
with the opposite signs of rotation. The double-spiral structure starts from
the surface with the almost antiferromagnetic alignment of the adjacent Fe
layers. The rotation of the two spirals leads to nearly ferromagnetic alignment
of the two magnetic subsystems at some depth, where the sudden turn of the
magnetic vectors by ~180 deg (spin-flop) appears, and both spirals start to
rotate in opposite directions. The observation of this unusual double-spiral
magnetic structure suggests that the unique properties of giant
magneto-resistance devices can be further tailored using ultrathin magnetic
layers.Comment: 9 pages, 3 figure
Degradation versus self-assembly of block copolymer micelles
The stability of micelles self-assembled from block copolymers can be altered
by the degradation of the blocks. Slow degradation shifts the equilibrium size
distribution of block copolymer micelles and change their properties.
Quasi-equilibrium scaling theory shows that the degradation of hydrophobic
blocks in the core of micelles destabilize the micelles reducing their size,
while the degradation of hydrophilic blocks forming coronas of micelles favors
larger micelles and may, at certain conditions, induce the formation of
micelles from individual chains.Comment: Published in Langmuir http://pubs.acs.org/doi/pdf/10.1021/la204625
Biomolecule surface patterning may enhance membrane association
Under dehydration conditions, amphipathic Late Embryogenesis Abundant (LEA)
proteins fold spontaneously from a random conformation into alpha-helical
structures and this transition is promoted by the presence of membranes. To
gain insight into the thermodynamics of membrane association we model the
resulting alpha-helical structures as infinite rigid cylinders patterned with
hydrophobic and hydrophilic stripes oriented parallel to their axis.
Statistical thermodynamic calculations using Single Chain Mean Field (SCMF)
theory show that the relative thickness of the stripes controls the free energy
of interaction of the alpha-helices with a phospholipid bilayer, as does the
bilayer structure and the depth of the equilibrium penetration of the cylinders
into the bilayer. The results may suggest the optimal thickness of the stripes
to mimic the association of such protein with membranes.Comment: Published in ACS Nano http://pubs.acs.org/doi/pdf/10.1021/nn204736
Nematic Ordering of Rigid Rods in a Gravitational Field
The isotropic-to-nematic transition in an athermal solution of long rigid
rods subject to a gravitational (or centrifugal) field is theoretically
considered in the Onsager approximation. The new feature emerging in the
presence of gravity is a concentration gradient which coupled with the nematic
ordering. For rodlike molecules this effect becomes noticeable at centrifugal
acceleration g ~ 10^3--10^4 m/s^2, while for biological rodlike objects, such
as tobacco mosaic virus, TMV, the effect is important even for normal
gravitational acceleration conditions. Rods are concentrated near the bottom of
the vessel which sometimes leads to gravity induced nematic ordering. The
concentration range corresponding to phase separation increases with increasing
g. In the region of phase separation the local rod concentration, as well as
the order parameter, follow a step function with height.Comment: Full article http://prola.aps.org/abstract/PRE/v60/i3/p2973_
Micellization of Sliding Polymer Surfactants
Following up a recent paper on grafted sliding polymer layers (Macromolecules
2005, 38, 1434-1441), we investigated the influence of the sliding degree of
freedom on the self-assembly of sliding polymeric surfactants that can be
obtained by complexation of polymers with cyclodextrins. In contrast to the
micelles of quenched block copolymer surfactants, the free energy of micelles
of sliding surfactants can have two minima: the first corresponding to small
micelles with symmetric arm lengths, and the second corresponding to large
micelles with asymmetric arm lengths. The relative sizes and concentrations of
small and large micelles in the solution depend on the molecular parameters of
the system. The appearance of small micelles drastically reduces the kinetic
barrier signifying the fast formation of equilibrium micelles.Comment: Submitted to Macromolecule
Surface patterning of carbon nanotubes can enhance their penetration through a phospholipid bilayer
Nanotube patterning may occur naturally upon the spontaneous self-assembly of
biomolecules onto the surface of single-walled carbon nanotubes (SWNTs). It
results in periodically alternating bands of surface properties, ranging from
relatively hydrophilic to hydrophobic, along the axis of the nanotube. Single
Chain Mean Field (SCMF) theory has been used to estimate the free energy of
systems in which a surface patterned nanotube penetrates a phospholipid
bilayer. In contrast to un-patterned nanotubes with uniform surface properties,
certain patterned nanotubes have been identified that display a relatively low
and approximately constant system free energy (10 kT) as the nanotube traverses
through the bilayer. These observations support the hypothesis that the
spontaneous self-assembly of bio-molecules on the surface of SWNTs may
facilitate nanotube transduction through cell membranes.Comment: Published in ACS Nano http://pubs.acs.org/doi/abs/10.1021/nn102763
Possible origins of macroscopic left-right asymmetry in organisms
I consider the microscopic mechanisms by which a particular left-right (L/R)
asymmetry is generated at the organism level from the microscopic handedness of
cytoskeletal molecules. In light of a fundamental symmetry principle, the
typical pattern-formation mechanisms of diffusion plus regulation cannot
implement the "right-hand rule"; at the microscopic level, the cell's
cytoskeleton of chiral filaments seems always to be involved, usually in
collective states driven by polymerization forces or molecular motors. It seems
particularly easy for handedness to emerge in a shear or rotation in the
background of an effectively two-dimensional system, such as the cell membrane
or a layer of cells, as this requires no pre-existing axis apart from the layer
normal. I detail a scenario involving actin/myosin layers in snails and in C.
elegans, and also one about the microtubule layer in plant cells. I also survey
the other examples that I am aware of, such as the emergence of handedness such
as the emergence of handedness in neurons, in eukaryote cell motility, and in
non-flagellated bacteria.Comment: 42 pages, 6 figures, resubmitted to J. Stat. Phys. special issue.
Major rewrite, rearranged sections/subsections, new Fig 3 + 6, new physics in
Sec 2.4 and 3.4.1, added Sec 5 and subsections of Sec
A CLASP-modulated cell edge barrier mechanism drives cell-wide cortical microtubule organization in Arabidopsis
It is well known that the parallel order of microtubules in the plant cell cortex defines the direction of cell expansion, yet it remains unclear how microtubule orientation is controlled, especially on a cell-wide basis. Here we show through 4D imaging and computational modelling that plant cell polyhedral geometry provides spatial input that determines array orientation and heterogeneity. Microtubules depolymerize when encountering sharp cell edges head-on, whereas those oriented parallel to those sharp edges remain. Edge-induced microtubule depolymerization, however, is overcome by the microtubule-associated protein CLASP, which accumulates at specific cell edges, enables microtubule growth around sharp edges and promotes formation of microtubule bundles that span adjacent cell faces. By computationally modelling dynamic 'microtubules on a cube' with edges differentially permissive to microtubule passage, we show that the CLASP-edge complex is a 'tuneable' microtubule organizer, with the inherent flexibility to generate the numerous cortical array patterns observed in nature
RETROGRADE СOLANGIOPANCREATOGRAPHY (RCG) AND ENDOSCOPIC PAPILLOSPHINCTEROGRAPHY (EPG) IN NEW SCHEME OF ENDOSURGICAL STRATEGY OF TREATMENT OF CHOLECYSTITIS ACCOMPANIED BY BILIRUBINEMIA
The article is devoted to the substantiation of strategy changes in acute cholecystitis and the bilirubinemia of obscure etiology. At the first stage the treatment was started from RCG in 120 patients and RCG with EPG in 232 patients. The results obtained were not very good, there being many organizational problems. At the second stage the laparoscopic cholecystectomy with drainage of the common bile duct (228 patients) were performed in 248 patients with acute cholecystitis and increased bilirubin from 29,54 till 167,16 millimole/l. Futhermore, the postoperative transdrainage cholangiography was made in 184 (74,2%) patients, the stones or the obstruction of bile secretion weren’t found. The classical intervention on common bile duct was required only to 4 patients, for the rest EPG was performed according to the indications. EPG was the most effective and the results were the best
Transformation of the System of Training Specialists during the Transition to FSES 3++
Currently, the universities in the Russian Federation are engaged in the updating of the educational programs being implemented, in accordance with the requirements of the Federal State Educational Standard 3 ++. The article provides information on the conceptual changes in the State Educational Standard, Federal State Educational Standard, and Federal State Educational Standard 3 ++ , as well as on the new opportunities universities have in connection with the introduction of the “two plus” generation standard. By an example of the project “NetworkElectronicUniversityof theRepublicofBashkortostan” the article shows the network interaction on issues of the formation of universal competences. New opportunities for universities during the transition to FSES 3 ++ are considered in the context of globalization conditions. The universities are suggested to be considered not so much as a “talent foundry”, but rather as a presence territory development driver, a space for constant positive changes (4P) due to the comprehensive development of scientific, innovative, socio-cultural activities with concurrent integration into the educational environment. The implementation of such a model within the transition to FSES 3 ++ is proposed to be done by individualizing mass education, building flexible educational trajectories that most meet the needs of the parties involved. At the same time, the so-called soft skills should become an indispensable superstructure for the formation of professional competencies, as without them it is extremely difficult to train specialists able to adapt as quickly as possible to changes in the external environment, and, in conditions of uncertainty of occupational functions in the future, they become more important than hard skills
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