265 research outputs found
The structure of haemoglobin bound to the haemoglobin receptor IsdH from Staphylococcus aureus shows disruption of the native α-globin haem pocket
© 2015 International Union of Crystallography. Staphylococcus aureus is a common and serious cause of infection in humans. The bacterium expresses a cell-surface receptor that binds to, and strips haem from, human haemoglobin (Hb). The binding interface has previously been identified; however, the structural changes that promote haem release from haemoglobin were unknown. Here, the structure of the receptor-Hb complex is reported at 2.6 Å resolution, which reveals a conformational change in the α-globin F helix that disrupts the haem-pocket structure and alters the Hb quaternary interactions. These features suggest potential mechanisms by which the S. aureus Hb receptor induces haem release from Hb
Combining type I and type II seesaw mechanisms in the minimal 3-3-1 model
The minimal 3-3-1 model is perturbative until energies around 4-5TeV, posing
a challenge to generate neutrino masses at eV scale, mainly if one aims to take
advantage of the seesaw mechanism. As a means to circumvent this problem we
propose a modification of the model such that it accommodates the type I and
type II seesaw mechanisms altogether. We show that the conjunction of both
mechanisms yield a neutrino mass expression suppressed by a high power of the
cutoff scale, , in its denominator. With such a suppression term we
naturally obtain neutrino masses at eV scale when is around few TeV. We
also investigate the size of lepton flavor violation through the process .Comment: about 15 pages, no figure
A Simple Realization of the Inverse Seesaw Mechanism
Differently from the canonical seesaw mechanism, which is grounded in grand
unified theories, the inverse seesaw mechanism lacks a special framework that
realizes it naturally. In this work we advocate that the 3-3-1 model with
right-handed neutrinos has such an appropriate framework to accommodate the
inverse seesaw mechanism. We also provide an explanation for the smallness of
the parameter and estimate the branching ratio for the rare lepton flavor
violation process .Comment: About 14pages, no figures, basis corrected, to appear at the PR
Structural basis for hemoglobin capture by Staphylococcus aureus cell-surface protein, IsdH
Pathogens must steal iron from their hosts to establish infection. In mammals, hemoglobin (Hb) represents the largest reservoir of iron, and pathogens express Hb-binding proteins to access this source. Here, we show how one of the commonest and most significant human pathogens, Staphylococcus aureus, captures Hb as the first step of an iron-scavenging pathway. The x-ray crystal structure of Hb bound to a domain from the Isd (iron-regulated surface determinant) protein, IsdH, is the first structure of a Hb capture complex to be determined. Surface mutations in Hb that reduce binding to the Hb-receptor limit the capacity of S. aureus to utilize Hb as an iron source, suggesting that Hb sequence is a factor in host susceptibility to infection. The demonstration that pathogens make highly specific recognition complexes with Hb raises the possibility of developing inhibitors of Hb binding as antibacterial agents. © 2011 by The American Society for Biochemistry and Molecular Biology, Inc
Obtaining the equation of motion for a fermionic particle in a generalized Lorentz-violating system framework
Using a generalized procedure for obtaining the dispersion relation and the
equation of motion for a propagating fermionic particle, we examine previous
claims for a preferred axis at (), embedded
in the framework of very special relativity (VSR). We show that, in a
relatively high energy scale, the corresponding equation of motion is reduced
to a conserving lepton number chiral equation previously predicted in the
literature. Otherwise, in a relatively low energy scale, the equation is
reduced to the usual Dirac equation for a free propagating fermionic particle.
It is accomplished by the suggestive analysis of some special cases where a
nonlinear modification of the action of the Lorentz group is generated by the
addition of a modified conformal transformation which, meanwhile, preserves the
structure of the ordinary Lorentz algebra in a very peculiar way. Some feasible
experiments, for which Lorentz violating effects here pointed out may be
detectable, are suggested.Comment: 10 page
The experience of enchantment in human-computer interaction
Improving user experience is becoming something of a rallying call in human–computer interaction but experience is not a unitary thing. There are varieties of experiences, good and bad, and we need to characterise these varieties if we are to improve user experience. In this paper we argue that enchantment is a useful concept to facilitate closer relationships between people and technology. But enchantment is a complex concept in need of some clarification. So we explore how enchantment has been used in the discussions of technology and examine experiences of film and cell phones to see how enchantment with technology is possible. Based on these cases, we identify the sensibilities that help designers design for enchantment, including the specific sensuousness of a thing, senses of play, paradox and openness, and the potential for transformation. We use these to analyse digital jewellery in order to suggest how it can be made more enchanting. We conclude by relating enchantment to varieties of experience.</p
Physical limits of inference
I show that physical devices that perform observation, prediction, or
recollection share an underlying mathematical structure. I call devices with
that structure "inference devices". I present a set of existence and
impossibility results concerning inference devices. These results hold
independent of the precise physical laws governing our universe. In a limited
sense, the impossibility results establish that Laplace was wrong to claim that
even in a classical, non-chaotic universe the future can be unerringly
predicted, given sufficient knowledge of the present. Alternatively, these
impossibility results can be viewed as a non-quantum mechanical "uncertainty
principle". Next I explore the close connections between the mathematics of
inference devices and of Turing Machines. In particular, the impossibility
results for inference devices are similar to the Halting theorem for TM's.
Furthermore, one can define an analog of Universal TM's (UTM's) for inference
devices. I call those analogs "strong inference devices". I use strong
inference devices to define the "inference complexity" of an inference task,
which is the analog of the Kolmogorov complexity of computing a string. However
no universe can contain more than one strong inference device. So whereas the
Kolmogorov complexity of a string is arbitrary up to specification of the UTM,
there is no such arbitrariness in the inference complexity of an inference
task. I end by discussing the philosophical implications of these results,
e.g., for whether the universe "is" a computer.Comment: 43 pages, updated version of Physica D version, which originally
appeared in 2007 CNLS conference on unconventional computatio
Analyticity, Unitarity and One-loop Graviton Corrections to Compton Scattering
We compute spin-flip cross section for graviton photoproduction on a spin-1/2
target of finite mass. Using this tree-level result, we find one-loop graviton
correction to the spin-flip low-energy forward Compton scattering amplitude by
using Gerasimov-Drell-Hearn sum rule. We show that this result agrees with the
corresponding perturbative computations, implying the validity of the sum rule
at one-loop level, contrary to the previous claims. We discuss possible effects
from the black hole production and string Regge trajectory exchange at very
high energies. These effects seem to soften the UV divergence present at
one-loop graviton level. Finally, we discuss the relation of these observations
with the models that involve extra dimensions.Comment: 15 pages, 3 figure
How the Inverse See-Saw Mechanism Can Reveal Itself Natural, Canonical and Independent of the Right-Handed Neutrino Mass
The common lore in the literature of neutrino mass generation is that the
canonical see-saw mechanism beautifully offers an explanation for the tiny
neutrino mass but at the cost of introducing right-handed neutrinos at a scale
that is out of range for the current experiments. The inverse see-saw mechanism
is an interesting alternative to the canonical one once it leads to tiny
neutrino masses with the advantage of being testable at TeV scale. However,
this last mechanism suffers from an issue of naturalness concerning the scale
responsible for such small masses, namely, the parameter that is related
to lepton number violation and is supposed to be at the keV scale, much lower
than the electroweak one. However, no theoretical framework was built that
offers an explanation for obtaining this specific scale. In this work we
propose a variation of the inverse see-saw mechanism by assuming a minimal
scalar and fermionic set of singlet fields, along with a
symmetry, that allows a dynamical explanation for the smallness of ,
recovering the neat canonical see-saw formula and with right-handed (RH)
neutrinos free to be at the electroweak scale, thus testable at LHC and current
neutrino experiments.Comment: 10 pages, stability of the potential verified, accepted for
publication in Physical review
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