4,151 research outputs found
Lipidâprotein interactions in biological membranes: a structural perspective
AbstractLipid molecules bound to membrane proteins are resolved in some high-resolution structures of membrane proteins. An analysis of these structures provides a framework within which to analyse the nature of lipidâprotein interactions within membranes. Membrane proteins are surrounded by a shell or annulus of lipid molecules, equivalent to the solvent layer surrounding a water-soluble protein. The lipid bilayer extends right up to the membrane protein, with a uniform thickness around the protein. The surface of a membrane protein contains many shallow grooves and protrusions to which the fatty acyl chains of the surrounding lipids conform to provide tight packing into the membrane. An individual lipid molecule will remain in the annular shell around a protein for only a short period of time. Binding to the annular shell shows relatively little structural specificity. As well as the annular lipid, there is evidence for other lipid molecules bound between the transmembrane α-helices of the protein; these lipids are referred to as non-annular lipids. The average thickness of the hydrophobic domain of a membrane protein is about 29 Ă
, with a few proteins having significantly smaller or greater thicknesses than the average. Hydrophobic mismatch between a membrane protein and the surrounding lipid bilayer generally leads to only small changes in membrane thickness. Possible adaptations in the protein to minimise mismatch include tilting of the helices and rotation of side chains at the ends of the helices. Packing of transmembrane α-helices is dependent on the chain length of the surrounding phospholipids. The function of membrane proteins is dependent on the thickness of the surrounding lipid bilayer, sometimes on the presence of specific, usually anionic, phospholipids, and sometimes on the phase of the phospholipid
Three-loop HQET vertex diagrams for B^0 -\bar{B}^0 mixing
Three-loop vertex diagrams in HQET needed for sum rules for B^0 - \bar{B}^0
mixing are considered. They depend on two residual energies. An algorithm of
reduction of these diagrams to master integrals has been constructed. All
master integrals are calculated exactly in d dimensions; their epsilon
expansions are also obtained.Comment: JHEP documentclas
Baryon resonances from a novel fat-link fermion action
We present first results for masses of positive and negative parity excited
baryons in lattice QCD using an O(a^2) improved gluon action and a Fat Link
Irrelevant Clover (FLIC) fermion action in which only the irrelevant operators
are constructed with fat links. The results are in agreement with earlier
calculations of N^* resonances using improved actions and exhibit a clear mass
splitting between the nucleon and its chiral partner, even for the Wilson
fermion action. The results also indicate a splitting between the lowest J^P =
1/2^- states for the two standard nucleon interpolating fields.Comment: 5 pages, 3 figures, talk given by W.Melnitchouk at LHP 2001 workshop,
Cairns, Australi
D-wave-like nodal superconductivity in the organic conductor (TMTSF)2ClO4
We suggest theoretical explanation of the high upper critical magnetic field,
perpendicular to conducting chains, Hc2, experimentally observed in the
superconductor (TMTSF)2ClO4, in terms of singlet superconducting pairing. In
particular, we compare the results of d-wave-like nodal, d-wave-like node-less,
and s-wave scenarios of superconductivity. We show that, in d-wave-like nodal
scenario, superconductivity can naturally exceed both the orbital upper
critical magnetic field and Clogston-Shandrasekhar paramagnetic limit as well
as reach experimental value, Hc2 = 6T, in contrast to d-wave-like node-less and
s-wave scenarios. In our opinion, the obtained results are strongly in favor of
d-wave-like nodal superconductivity in (TMTSF)2ClO4, whereas, in a sister
compound, (TMTSF)2PF6, we expect either the existence of triplet order
parameter or the coexistence of triplet and singlet order parameters.Comment: Talk at the ECRYS-2011 international conferenc
Using Taguchi method to optimize welding pool of dissimilar laser welded components
In the present work CO2 continuous laser welding process was successfully applied and optimized for joining a dissimilar AISI 316 stainless steel and AISI 1009 low carbon steel plates. Laser power, welding speed, and defocusing distance combinations were carefully selected with the objective of producing welded joint with complete penetration, minimum fusion zone size and acceptable welding profile. Fusion zone area and shape of dissimilar austenitic stainless steel with ferritic low carbon steel were evaluated as a function of the selected laser welding parameters. Taguchi approach was used as statistical design of experiment (DOE) technique for optimizing the selected welding parameters in terms of minimizing the fusion zone. Mathematical models were developed to describe the influence of the selected parameters on the fusion zone area and shape, to predict its value within the limits of the variables being studied. The result indicates that the developed models can predict the responses satisfactorily
Conductance Fluctuations of Open Quantum Dots under Microwave Radiation
We develop a time dependent random matrix theory describing the influence of
a time-dependent perturbation on mesoscopic conductance fluctuations in open
quantum dots. The effect of external field is taken into account to all orders
of perturbation theory, and our results are applicable to both weak and strong
fields. We obtain temperature and magnetic field dependences of conductance
fluctuations. The amplitude of conductance fluctuations is determined by
electron temperature in the leads rather than by the width of electron
distribution function in the dot. The asymmetry of conductance with respect to
inversion of applied magnetic field is the main feature allowing to distinguish
the effect of direct suppression of quantum interference from the simple
heating if the frequency of external radiation is larger than the temperature
of the leads .Comment: 7 pages, 5 figure
Experimentally realizable characterizations of continuous variable Gaussian states
Measures of entanglement, fidelity and purity are basic yardsticks in quantum
information processing. We propose how to implement these measures using linear
devices and homodyne detectors for continuous variable Gaussian states. In
particular, the test of entanglement becomes simple with some prior knowledge
which is relevant to current experiments.Comment: 4 pages, This paper supersedes quant-ph/020315
Non-minimal neutral Higgs bosons at LEP2
We study the phenomenology of the neutral Higgs sector of a non-SUSY
non-minimal Standard Model. Models with more than one Higgs doublet are
possible, and may contain neutral Higgs scalars with branching ratios
significantly different to those of the Minimal Standard Model Higgs boson. We
show how these differences may be exploited at LEP2 in order to distinguish the
non-minimal Standard Model from the minimal version.Comment: 12 pages inc 4 figures, Latex, to appear in Physics Letters
Excited Baryons from the FLIC Fermion Action
Masses of positive and negative parity excited nucleons and hyperons are
calculated in quenched lattice QCD using an O(a^2) improved gluon action and a
fat-link clover fermion action in which only the irrelevant operators are
constructed with fat links. The results are in agreement with earlier N*
simulations with improved actions, and exhibit a clear mass splitting between
the nucleon and its parity partner, as well as a small mass splitting between
the two low-lying J^P={1/2}^- N* states. Study of different Lambda
interpolating fields suggests a similar splitting between the lowest two
{1/2}^- Lambda* states, although the empirical mass suppression of the
Lambda*(1405) is not seen.Comment: 3 pages, 3 figures, Lattice2002(QCD Spectrum and Quark Masses
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