171 research outputs found
A Budding-Defective M2 Mutant Exhibits Reduced Membrane Interaction, Insensitivity To Cholesterol, And Perturbed Interdomain Coupling
Influenza A M2 is a membrane-associated protein with a C-terminal amphipathic helix that plays a cholesterol-dependent role in viral budding. An M2 mutant with alanine substitutions in the C-terminal amphipathic helix is deficient in viral scission. With the goal of providing atomic-level understanding of how the wild-type protein functions, we used a multipronged site-directed spin labeling electron paramagnetic resonance spectroscopy (SDSL-EPR) approach to characterize the conformational properties of the alanine mutant. We spin-labeled sites in the transmembrane (TM) domain and the C-terminal amphipathic helix (AH) of wild-type (WT) and mutant M2, and collected information on line shapes, relaxation rates, membrane topology, and distances within the homotetramer in membranes with and without cholesterol. Our results identify marked differences in the conformation and dynamics between the WT and the alanine mutant. Compared to WT, the dominant population of the mutant AH is more dynamic, shallower in the membrane, and has altered quaternary arrangement of the C-terminal domain. While the AH becomes more dynamic, the dominant population of the TM domain of the mutant is immobilized. The presence of cholesterol changes the conformation and dynamics of the WT protein, while the alanine mutant is insensitive to cholesterol. These findings provide new insight into how M2 may facilitate budding. We propose the AH–membrane interaction modulates the arrangement of the TM helices, effectively stabilizing a conformational state that enables M2 to facilitate viral budding. Antagonizing the properties of the AH that enable interdomain coupling within M2 may therefore present a novel strategy for anti-influenza drug design
Influence of Collision Cascade Statistics on Pattern Formation of Ion-Sputtered Surfaces
Theoretical continuum models that describe the formation of patterns on
surfaces of targets undergoing ion-beam sputtering, are based on Sigmund's
formula, which describes the spatial distribution of the energy deposited by
the ion. For small angles of incidence and amorphous or polycrystalline
materials, this description seems to be suitable, and leads to the classic BH
morphological theory [R.M. Bradley and J.M.E. Harper, J. Vac. Sci. Technol. A
6, 2390 (1988)]. Here we study the sputtering of Cu crystals by means of
numerical simulations under the binary-collision approximation. We observe
significant deviations from Sigmund's energy distribution. In particular, the
distribution that best fits our simulations has a minimum near the position
where the ion penetrates the surface, and the decay of energy deposition with
distance to ion trajectory is exponential rather than Gaussian. We provide a
modified continuum theory which takes these effects into account and explores
the implications of the modified energy distribution for the surface
morphology. In marked contrast with BH's theory, the dependence of the
sputtering yield with the angle of incidence is non-monotonous, with a maximum
for non-grazing incidence angles.Comment: 12 pages, 13 figures, RevTe
Global Structure of Moduli Space for BPS Walls
We study the global structure of the moduli space of BPS walls in the Higgs
branch of supersymmetric theories with eight supercharges. We examine the
structure in the neighborhood of a special Lagrangian submanifold M, and find
that the dimension of the moduli space can be larger than that naively
suggested by the index theorem, contrary to previous examples of BPS solitons.
We investigate BPS wall solutions in an explicit example of M using Abelian
gauge theory. Its Higgs branch turns out to contain several special Lagrangian
submanifolds including M. We show that the total moduli space of BPS walls is
the union of these submanifolds. We also find interesting dynamics between BPS
walls as a byproduct of the analysis. Namely, mutual repulsion and attraction
between BPS walls sometimes forbid a movement of a wall and lock it in a
certain position; we also find that a pair of walls can transmute to another
pair of walls with different tension after they pass through.Comment: 42 pages, 11 figures; a few comments adde
A nonlocal connection between certain linear and nonlinear ordinary differential equations/oscillators
We explore a nonlocal connection between certain linear and nonlinear
ordinary differential equations (ODEs), representing physically important
oscillator systems, and identify a class of integrable nonlinear ODEs of any
order. We also devise a method to derive explicit general solutions of the
nonlinear ODEs. Interestingly, many well known integrable models can be
accommodated into our scheme and our procedure thereby provides further
understanding of these models.Comment: 12 pages. J. Phys. A: Math. Gen. 39 (2006) in pres
Nonlinear Dirac operator and quaternionic analysis
Properties of the Cauchy-Riemann-Fueter equation for maps between
quaternionic manifolds are studied. Spaces of solutions in case of maps from a
K3-surface to the cotangent bundle of a complex projective space are computed.
A relationship between harmonic spinors of a generalized nonlinear Dirac
operator and solutions of the Cauchy-Riemann-Fueter equation are established.Comment: Cosmetic changes onl
A direct probe of cosmological power spectra of the peculiar velocity field and the gravitational lensing magnification from photometric redshift surveys
The cosmological peculiar velocity field (deviations from the pure Hubble
flow) of matter carries significant information on dark energy, dark matter and
the underlying theory of gravity on large scales. Peculiar motions of galaxies
introduce systematic deviations between the observed galaxy redshifts z and the
corresponding cosmological redshifts z_cos. A novel method for estimating the
angular power spectrum of the peculiar velocity field based on observations of
galaxy redshifts and apparent magnitudes m (or equivalently fluxes) is
presented. This method exploits the fact that a mean relation between z_cos and
m of galaxies can be derived from all galaxies in a redshift-magnitude survey.
Given a galaxy magnitude, it is shown that the z_cos(m) relation yields its
cosmological redshift with a 1-sigma error of sigma_z~0.3 for a survey like
Euclid (~10^9 galaxies at z<~2), and can be used to constrain the angular power
spectrum of z-z_cos(m) with a high signal-to-noise ratio. At large angular
separations corresponding to l<~15, we obtain significant constraints on the
power spectrum of the peculiar velocity field. At 15<~l<~60, magnitude shifts
in the z_cos(m) relation caused by gravitational lensing magnification
dominate, allowing us to probe the line-of-sight integral of the gravitational
potential. Effects related to the environmental dependence in the luminosity
function can easily be computed and their contamination removed from the
estimated power spectra. The amplitude of the combined velocity and lensing
power spectra at z~1 can be measured with <~5% accuracy.Comment: 22 pages, 3 figures; added a discussion of systematic errors,
accepted for publication in JCA
Entropy and Wigner Functions
The properties of an alternative definition of quantum entropy, based on
Wigner functions, are discussed. Such definition emerges naturally from the
Wigner representation of quantum mechanics, and can easily quantify the amount
of entanglement of a quantum state. It is shown that smoothing of the Wigner
function induces an increase in entropy. This fact is used to derive some
simple rules to construct positive definite probability distributions which are
also admissible Wigner functionsComment: 18 page
A unification in the theory of linearization of second order nonlinear ordinary differential equations
In this letter, we introduce a new generalized linearizing transformation
(GLT) for second order nonlinear ordinary differential equations (SNODEs). The
well known invertible point (IPT) and non-point transformations (NPT) can be
derived as sub-cases of the GLT. A wider class of nonlinear ODEs that cannot be
linearized through NPT and IPT can be linearized by this GLT. We also
illustrate how to construct GLTs and to identify the form of the linearizable
equations and propose a procedure to derive the general solution from this GLT
for the SNODEs. We demonstrate the theory with two examples which are of
contemporary interest.Comment: 8 page
Optimization of the Kinematic Chain of the Thumb for a Hand Prosthesis Based on the Kapandji Opposition Test
Ponènica presentada a International Symposium on Computer Methods in Biomechanics and Biomedical Engineering - CMBBE 2019The thumb plays a key role in the performance of the hand for grasp-ing and manipulating objects. In artificial hands the complex thumb’s kinematic chain (TKC) is simplified and its five degrees of freedom are reduced to only one or two with the consequent loss of dexterity of the hand. The Kapandji op-position test (KOT) has been clinically used in pathological human hands for evaluating the thumb opposition and it has also been employed in some previ-ous studies as reference for the design of the TKC in artificial hands, but with-out a clearly stated methodology. Based on this approaches, in this study we present a computational method to optimize the whole TKC (base placement, link lengths and joint orientation angles) of an artificial hand based on its per-formance in the KOT. The cost function defined for the optimization (MPE) is a weighted mean position error when trying to reproduce the KOT postures and can be used also as a metric to quantify thumb opposition in the hand. As a case study, the method was applied to the improvement of the TKC of an artificial hand developed by the authors and the MPE was reduced to near one third of that of the original design, increasing significantly the number of reachable po-sitions in the KOT. The metric proposed based on the KOT can be used directly or in combination with other to improve the kinematic chain of artificial hands
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