6,877 research outputs found
Import of honeybee prepromelittin into the endoplasmic reticulum
Honeybee prepromelittin is correctly processed and imported by dog pancreas microsomes. Insertion of prepromelittin into microsomal membranes, as assayed by signal sequence removal, does not depend on signal recognition particle (SRP) and docking protein. We addressed the question as to how prepromelittin bypasses the SRP/docking protein system. Hybrid proteins between prepromelittin, or carboxy-terminally truncated derivatives, and the cytoplasmic protein dihydrofolate reductase from mouse were constructed. These hybrid proteins were analysed for membrane insertion and sequestration into microsomes. The results suggest the following: (i) The signal sequence of prepromelittin is capable of interacting with the SRP/docking protein system, but this interaction is not mandatory for membrane insertion; this is related to the small size of prepromelittin. (ii) In prepromelittin a cluster of negatively charged amino acids must be balanced by a cluster of positively charged amino acids in order to allow membrane insertion. (iii) In general, a signal sequence can be sufficient to mediate membrane insertion independently of SRP and docking protein in the case of short precursor proteins; however, the presence and distribution of charged amino acids within the mature part of these precursors can play distinct roles
't Hooft anomalies of discrete gauge theories and non-abelian group cohomology
We study discrete symmetries of Dijkgraaf-Witten theories and their gauging
in the framework of (extended) functorial quantum field theory. Non-abelian
group cohomology is used to describe discrete symmetries and we derive concrete
conditions for such a symmetry to admit 't Hooft anomalies in terms of the
Lyndon-Hochschild-Serre spectral sequence. We give an explicit realization of a
discrete gauge theory with 't Hooft anomaly as a state on the boundary of a
higher-dimensional Dijkgraaf-Witten theory. This allows us to calculate the
2-cocycle twisting the projective representation of physical symmetries via
transgression. We present a general discussion of the bulk-boundary
correspondence at the level of partition functions and state spaces, which we
make explicit for discrete gauge theories.Comment: 46 pages, 1 figure; v2: minor corrections and clarifying comments
added, references updated; Final version to appear in Communications in
Mathematical Physic
Extended quantum field theory, index theory and the parity anomaly
We use techniques from functorial quantum field theory to provide a geometric
description of the parity anomaly in fermionic systems coupled to background
gauge and gravitational fields on odd-dimensional spacetimes. We give an
explicit construction of a geometric cobordism bicategory which incorporates
general background fields in a stack, and together with the theory of symmetric
monoidal bicategories we use it to provide the concrete forms of invertible
extended quantum field theories which capture anomalies in both the path
integral and Hamiltonian frameworks. Specialising this situation by using the
extension of the Atiyah-Patodi-Singer index theorem to manifolds with corners
due to Loya and Melrose, we obtain a new Hamiltonian perspective on the parity
anomaly. We compute explicitly the 2-cocycle of the projective representation
of the gauge symmetry on the quantum state space, which is defined in a
parity-symmetric way by suitably augmenting the standard chiral fermionic Fock
spaces with Lagrangian subspaces of zero modes of the Dirac Hamiltonian that
naturally appear in the index theorem. We describe the significance of our
constructions for the bulk-boundary correspondence in a large class of
time-reversal invariant gauge-gravity symmetry-protected topological phases of
quantum matter with gapless charged boundary fermions, including the standard
topological insulator in 3+1 dimensions.Comment: 63 pages, 3 figures; v2: clarifying comments and references added;
Final version to be published in Communications in Mathematical Physic
Copyright and Open Access for Academic Works
In a recent paper, Prof. Steven Shavell (see Shavell, 2009) has argued strongly in favor of eliminating copyright from academic works. Based upon solid economic arguments, Shavell analyses the pros and cons of removal of copyright and in its place to have a pure open access system, in which authors (or more likely their employers) would provide the funds that keep journals in business. In this paper we explore some of the arguments in Shavell’s paper, above all the way in which the distribution of the sources of journal revenue would be altered, and the feasible effects upon the quality of journal content. We propose a slight modification to a pure open access system which may provide for the best of both the copyright and open access worlds.Open Access, Academic Works, Effects of Removal of Copyrights
Spin correlation functions in random-exchange s=1/2 XXZ chains
The decay of (disorder-averaged) static spin correlation functions at T=0 for
the one-dimensional spin-1/2 XXZ antiferromagnet with uniform longitudinal
coupling and random transverse coupling is investigated
by numerical calculations for ensembles of finite chains. At (XX
model) the calculation is based on the Jordan-Wigner mapping to free lattice
fermions for chains with up to N=100 sites. At Lanczos
diagonalizations are carried out for chains with up to N=22 sites. The
longitudinal correlation function is found to exhibit a
power-law decay with an exponent that varies with $\Delta$ and, for nonzero
$\Delta$, also with the width of the $\lambda_i$-distribution. The results for
the transverse correlation function show a crossover from
power-law decay to exponential decay as the exchange disorder is turned on.Comment: RevTex manuscript (7 pages), 4 postscript figure
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