222 research outputs found
Rigid unit modes in tetrahedral crystals
The 'rigid unit mode' (RUM) model requires unit blocks, in our case
tetrahedra of SiO_4 groups, to be rigid within first order of the displacements
of the O-ions. The wave-vectors of the lattice vibrations, which obey this
rigidity, are determined analytically. Lattices with inversion symmetry yield
generically surfaces of RUMs in reciprocal space, whereas lattices without this
symmetry yield generically lines of RUMs. Only in exceptional cases as in
beta-quartz a surface of RUMs appears, if inversion symmetry is lacking. The
occurence of planes and bending surfaces, straight and bent lines is discussed.
Explicit calculations are performed for five modifications of SiO_2 crystals.Comment: 18 pages, 6 figures, improved notatio
Amorphization induced by pressure: results for zeolites and general implications
We report an {\sl ab initio} study of pressure-induced amorphization (PIA) in
zeolites, which are model systems for this phenomenon. We confirm the
occurrence of low-density amorphous phases like the one reported by Greaves
{\sl et al.} [Science {\bf 308}, 1299 (2005)], which preserves the crystalline
topology and might constitute a new type of glass. The role of the zeolite
composition regarding PIA is explained. Our results support the correctness of
existing models for the basic PIA mechanim, but suggest that energetic, rather
than kinetic, factors determine the irreversibility of the transition.Comment: 4 pages with 3 figures embedded. More information at
http://www.icmab.es/dmmis/leem/jorg
Simulation study of pressure and temperature dependence of the negative thermal expansion in Zn(CN)(2)
12 pages, 16 figures12 pages, 16 figures12 pages, 16 figures12 pages, 16 figure
Are Vicinal Metal Surfaces Stable?
Quantum Matter and Optic
Is bisexuality invisible? A review of sexualities scholarship 1970–2015
This article provides a review of sexualities scholarship within the social sciences between 1970 and 2015. It takes an innovative approach by focusing on the way in which bisexuality is addressed in this body of literature. The article reveals the marginalisation, under-representation and invisibility of bisexuality within and across the social sciences in relation to both bisexual experience and identity. Reasons for this varied across the different eras, including the heterosexist nature of the literature, the impact of gay and lesbian-focused identity politics, and queer deconstructionism. In addition, patterns of bisexual erasure and invisibility were uneven, with some scholarship taking inclusive approaches or criticising prejudice against bisexuality. The initial findings of the review were enriched by critical commentary from key relevant sociologists and political scientists. The article concludes that future sexualities scholarship could be enhanced by greater consideration of bisexuality
Maturation-Induced Cloaking of Neutralization Epitopes on HIV-1 Particles
To become infectious, HIV-1 particles undergo a maturation process involving proteolytic cleavage of the Gag and Gag-Pol polyproteins. Immature particles contain a highly stable spherical Gag lattice and are impaired for fusion with target cells. The fusion impairment is relieved by truncation of the gp41 cytoplasmic tail (CT), indicating that an interaction between the immature viral core and gp41 within the particle represses HIV-1 fusion by an unknown mechanism. We hypothesized that the conformation of Env on the viral surface is regulated allosterically by interactions with the HIV-1 core during particle maturation. To test this, we quantified the binding of a panel of monoclonal antibodies to mature and immature HIV-1 particles by immunofluorescence imaging. Surprisingly, immature particles exhibited markedly enhanced binding of several gp41-specific antibodies, including two that recognize the membrane proximal external region (MPER) and neutralize diverse HIV-1 strains. Several of the differences in epitope exposure on mature and immature particles were abolished by truncation of the gp41 CT, thus linking the immature HIV-1 fusion defect with altered Env conformation. Our results suggest that perturbation of fusion-dependent Env conformational changes contributes to the impaired fusion of immature particles. Masking of neutralization-sensitive epitopes during particle maturation may contribute to HIV-1 immune evasion and has practical implications for vaccine strategies targeting the gp41 MPER
Counteraction of Tetherin Antiviral Activity by Two Closely Related SIVs Differing by the Presence of a Vpu Gene
In different primate lentiviruses, three proteins (Vpu, Env and Nef) have been shown to have anti-tetherin activities. SIVden is a primate lentivirus harbored by a Cercopithecus denti (C. denti) whose genome code for a Vpu gene. We have compared the activity of HIV-1 Vpu and of SIVden Vpu on tetherin proteins from humans, from C. denti and from Cercopithecus neglectus (C. neglectus), a monkey species that is naturally infected by SIVdeb, a virus closely related to SIVden but which does not encode a Vpu protein. Here, we demonstrate that SIVden Vpu, is active against C. denti tetherin, but not against human tetherin. Interestingly, C. neglectus tetherin was more sensitive to SIVden Vpu than to HIV-1 Vpu. We also identify residues in the tetherin transmembrane domains that are responsible for the species-specific Vpu effect. Simultaneous mutation (P40L and T45I) of human tetherin conferred sensitivity to SIVden Vpu, while abolishing its sensitivity to HIV-1 Vpu. We next analyzed the anti-tetherin activity of the Nef proteins from HIV-1, SIVden and SIVdeb. All three Nef proteins were unable to rescue virus release in the presence of human or C. denti tetherin. Conversely, SIVdeb Nef enhanced virus release in the presence of C. neglectus tetherin, suggesting that SIVdeb relies on Nef in its natural host. Finally, while HIV-1 Vpu not only removed human tetherin from the cell surface but also directed it for degradation, SIVden Vpu only induced the redistribution of both C. denti and C. neglectus tetherins, resulting in a predominantly perinuclear localization
- …