1,991 research outputs found
Motor regulation results in distal forces that bend partially disintegrated Chlamydomonas axonemes into circular arcs
The bending of cilia and flagella is driven by forces generated by dynein
motor proteins. These forces slide adjacent microtubule doublets within the
axoneme, the motile cytoskeletal structure. To create regular, oscilla- tory
beating patterns, the activities of the axonemal dyneins must be coordinated
both spatially and temporally. It is thought that coordination is mediated by
stresses or strains, which build up within the moving axoneme, and somehow
regulate dynein activity. While experimenting with axonemes subjected to mild
proteolysis, we observed pairs of doublets associate with each other and form
bends with almost constant curvature. By model- ing the statics of a pair of
filaments, we show that the activity of the motors concentrates at the distal
tips of the doublets. Furthermore, we show that this distribution of motor
activity accords with models in which curvature, or curvature-induced normal
forces, regulates the activity of the motors. These observations, together with
our theoretical analysis, provide evidence that dynein activity can be
regulated by curvature or normal forces, which may, therefore, play a role in
coordinating the beating of cilia and flagella
Higher order relations in Fedosov supermanifolds
Higher order relations existing in normal coordinates between affine
extensions of the curvature tensor and basic objects for any Fedosov
supermanifolds are derived. Representation of these relations in general
coordinates is discussed.Comment: 11 LaTex pages, no figure
DVCS amplitude at tree level: Transversality, twist-3, and factorization
We study the virtual Compton amplitude in the generalized Bjorken region (q^2
-> Infinity, t small) in QCD by means of a light-cone expansion of the product
of e.m. currents in string operators in coordinate space. Electromagnetic gauge
invariance (transversality) is maintained by including in addition to the
twist-2 operators 'kinematical' twist-3 operators which appear as total
derivatives of twist-2 operators. The non-forward matrix elements of the
elementary twist-2 operators are parametrized in terms of two-variable spectral
functions (double distributions), from which twist-2 and 3 skewed distributions
are obtained through reduction formulas. Our approach is equivalent to a
Wandzura-Wilczek type approximation for the twist-3 skewed distributions. The
resulting Compton amplitude is manifestly transverse up to terms of order
t/q^2. We find that in this approximation the tensor amplitude for longitudinal
polarization of the virtual photon is finite, while the one for transverse
polarization contains a divergence already at tree level. However, this
divergence has zero projection on the polarization vector of the final photon,
so that the physical helicity amplitudes are finite.Comment: 34 pages, revtex, 1 eps figure included using epsf. Misprints
corrected, one reference adde
Moyal products -- a new perspective on quasi-hermitian quantum mechanics
The rationale for introducing non-hermitian Hamiltonians and other
observables is reviewed and open issues identified. We present a new approach
based on Moyal products to compute the metric for quasi-hermitian systems. This
approach is not only an efficient method of computation, but also suggests a
new perspective on quasi-hermitian quantum mechanics which invites further
exploration. In particular, we present some first results which link the Berry
connection and curvature to non-perturbative properties and the metric.Comment: 14 pages. Submitted to J Phys A special issue on The Physics of
Non-Hermitian Operator
The contribution of the Chirality-Induced Spin Selectivity (CISS) effect to the dispersion interaction between chiral molecules
Dispersion interactions are one of the components of van der Waals forces,
which play a key role in the understanding of intermolecular interactions in
many physical, chemical and biological processes. The theory of dispersion
forces was developed by London in the early years of quantum mechanics.
However, it was only in the 1960s that it was recognized that for molecules
lacking an inversion center such as chiral and helical molecules, there are
chirality-sensitive corrections to the dispersion forces proportional to the
rotatory power known from the theory of circular dichroism and with the same
distance scaling law R-6 as the London energy. The discovery of the
Chirality-Induced Spin Selectivity (CISS) effect in recent years has led to an
additional twist in the study of chiral molecular systems, showing a close
relation between spin and molecular geometry. Motivated by it, we propose in
this investigation that there may exist additional contributions to the
dispersion energy related to intermolecular, induced spin-orbit (ISOC)
interactions. Within a second-order perturbative approach, these forces
manifest as an effective intermolecular spin-spin exchange interaction.
Although they are weaker than the standard London forces, the ISOC interactions
turn out to be nevertheless not negligible and display the same R
distance scaling. Our results suggest that classical force field descriptions
of van-der Waals interactions may require additional modifications to include
the effects discussed here.Comment: 21 pages, 2 figure
Generalized plasma-like permittivity and thermal Casimir force between real metals
The physical reasons why the Drude dielectric function is not compatible with
the Lifshitz formula, as opposed to the generalized plasma-like permittivity,
are presented. Essentially, the problem is connected with the finite size of
metal plates. It is shown that the Lifshitz theory combined with the
generalized plasma-like permittivity is thermodynamically consistent.Comment: 17 pages, iopart.cls is used, to appear in J. Phys. A: Math. Theo
Histoplasmose pulmonar aguda e primeiro isolamento do Histoplasma capsulatum do solo do Rio Grande do Sul, Brasil
A case of acute pulmonary histoplasmosis, where the clinical histoiy and epidemiological data led to the identification of H. capsulatum natural source, is described. Specimens of spleen and liver, obtained after intraperitonial inoculation in mice, grew H. capsulatum in culture from the soil of rural area of General Câmara, by the first time in Rio Grande do Sul.Apresenta-se um caso de histoplasmose pulmonar aguda, onde a história clinica orientada levou a Identificação da fonte natural do Histoplasma capsulatum. O fungo foi obtido em cultivo a partir de fragmentos de baço e fígado de ratos inoculados intraperitonealmente com solo da zona rural de General Câmara, pela primeira vez no Rio Grande do Sul
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