119 research outputs found
Long-Range Excitons in Optical Absorption Spectra of Electroluminescent Polymer Poly(para-phenylenevinylene)
The component of photoexcited states with large spatial extent is
investigated for poly(para-phenylenevinylene) using the intermediate exciton
theory. We find a peak due to long-range excitons at the higher-energy side of
the lowest main feature of optical spectra. The fact that the onset of
long-range excitons is located near the energy gap is related to the mechanisms
of large photocurrents measured in such energy regions. We show that a large
value of the hopping integral is realistic for characterizing optical
excitations.Comment: To be published in J. Phys. Soc. Jpn. (Letters
Correlated theory of triplet photoinduced absorption in phenylene-vinylene chains
In this paper we present results of large-scale correlated calculations of
triplet photoinduced absorption (PA) spectrum of oligomers of
poly-(para)phenylenevinylene (PPV) containing up to five phenyl rings. In
particular, the high-energy features in the triplet PA spectrum of oligo-PPVs
are the focus of this study, which, so far, have not been investigated
theoretically, or experimentally. The calculations were performed using the
Pariser-Parr-Pople (PPP) model Hamiltonian, and many-body effects were taken
into account by means of multi-reference singles-doubles configuration
interaction procedure (MRSDCI), without neglecting any molecular orbitals. The
computed triplet PA spectrum of oligo-PPVs exhibits rich structure consisting
of alternating peaks of high and low intensities. The predicted higher energy
features of the triplet spectrum can be tested in future experiments.
Additionally, theoretical estimates of exciton binding energy are also
presented.Comment: To appear in Phys. Rev.
A theoretical investigation of the low lying electronic structure of poly(p-phenylene vinylene)
The two-state molecular orbital model of the one-dimensional phenyl-based
semiconductors is applied to poly(p-phenylene vinylene). The energies of the
low-lying excited states are calculated using the density matrix
renormalization group method. Calculations of both the exciton size and the
charge gap show that there are both Bu and Ag excitonic levels below the band
threshold. The energy of the 1Bu exciton extrapolates to 2.60 eV in the limit
of infinite polymers, while the energy of the 2Ag exciton extrapolates to 2.94
eV. The calculated binding energy of the 1Bu exciton is 0.9 eV for a 13
phenylene unit chain and 0.6 eV for an infinite polymer. This is expected to
decrease due to solvation effects. The lowest triplet state is calculated to be
at ca. 1.6 eV, with the triplet-triplet gap being ca. 1.6 eV. A comparison
between theory, and two-photon absorption and electroabsorption is made,
leading to a consistent picture of the essential states responsible for most of
the third-order nonlinear optical properties. An interpretation of the
experimental nonlinear optical spectroscopies suggests an energy difference of
ca. 0.4 eV between the vertical energy and ca. 0.8 eV between the relaxed
energy, of the 1Bu exciton and the band gap, respectively.Comment: LaTeX, 19 pages, 7 eps figures included using epsf. To appear in
Physical Review B, 199
CK2 Phosphorylates Sec31 and Regulates ER-To-Golgi Trafficking
Protein export from the endoplasmic reticulum (ER) is an initial and rate-limiting step of molecular trafficking and secretion. This is mediated by coat protein II (COPII)-coated vesicles, whose formation requires small GTPase Sar1 and 6 Sec proteins including Sec23 and Sec31. Sec31 is a component of the outer layer of COPII coat and has been identified as a phosphoprotein. The initiation and promotion of COPII vesicle formation is regulated by Sar1; however, the mechanism regulating the completion of COPII vesicle formation followed by vesicle release is largely unknown. Hypothesizing that the Sec31 phosphorylation may be such a mechanism, we identified phosphorylation sites in the middle linker region of Sec31. Sec31 phosphorylation appeared to decrease its association with ER membranes and Sec23. Non-phosphorylatable mutant of Sec31 stayed longer at ER exit sites and bound more strongly to Sec23. We also found that CK2 is one of the kinases responsible for Sec31 phosphorylation because CK2 knockdown decreased Sec31 phosphorylation, whereas CK2 overexpression increased Sec31 phosphorylation. Furthermore, CK2 knockdown increased affinity of Sec31 for Sec23 and inhibited ER-to-Golgi trafficking. These results suggest that Sec31 phosphorylation by CK2 controls the duration of COPII vesicle formation, which regulates ER-to-Golgi trafficking
A Genomic Approach for the Identification and Classification of Genes Involved in Cell Wall Formation and its Regulation in Saccharomyces Cerevisiae
Using a hierarchical approach, 620 non-essential single-gene yeast deletants generated by
EUROFAN I were systematically screened for cell-wall-related phenotypes. By analyzing
for altered sensitivity to the presence of Calcofluor white or SDS in the growth medium,
altered sensitivity to sonication, or abnormal morphology, 145 (23%) mutants showing at
least one cell wall-related phenotype were selected. These were screened further to identify
genes potentially involved in either the biosynthesis, remodeling or coupling of cell wall
macromolecules or genes involved in the overall regulation of cell wall construction and to
eliminate those genes with a more general, pleiotropic effect. Ninety percent of the mutants
selected from the primary tests showed additional cell wall-related phenotypes. When
extrapolated to the entire yeast genome, these data indicate that over 1200 genes may
directly or indirectly affect cell wall formation and its regulation. Twenty-one mutants with
altered levels of β1,3-glucan synthase activity and five Calcofluor white-resistant mutants
with altered levels of chitin synthase activities were found, indicating that the
corresponding genes affect β1,3-glucan or chitin synthesis. By selecting for increased
levels of specific cell wall components in the growth medium, we identified 13 genes that
are possibly implicated in different steps of cell wall assembly. Furthermore, 14 mutants
showed a constitutive activation of the cell wall integrity pathway, suggesting that they
participate in the modulation of the pathway either directly acting as signaling components
or by triggering the Slt2-dependent compensatory mechanism. In conclusion, our screening
approach represents a comprehensive functional analysis on a genomic scale of gene
products involved in various aspects of fungal cell wall formation
NS2 Protein of Hepatitis C Virus Interacts with Structural and Non-Structural Proteins towards Virus Assembly
Growing experimental evidence indicates that, in addition to the physical virion components, the non-structural proteins of hepatitis C virus (HCV) are intimately involved in orchestrating morphogenesis. Since it is dispensable for HCV RNA replication, the non-structural viral protein NS2 is suggested to play a central role in HCV particle assembly. However, despite genetic evidences, we have almost no understanding about NS2 protein-protein interactions and their role in the production of infectious particles. Here, we used co-immunoprecipitation and/or fluorescence resonance energy transfer with fluorescence lifetime imaging microscopy analyses to study the interactions between NS2 and the viroporin p7 and the HCV glycoprotein E2. In addition, we used alanine scanning insertion mutagenesis as well as other mutations in the context of an infectious virus to investigate the functional role of NS2 in HCV assembly. Finally, the subcellular localization of NS2 and several mutants was analyzed by confocal microscopy. Our data demonstrate molecular interactions between NS2 and p7 and E2. Furthermore, we show that, in the context of an infectious virus, NS2 accumulates over time in endoplasmic reticulum-derived dotted structures and colocalizes with both the envelope glycoproteins and components of the replication complex in close proximity to the HCV core protein and lipid droplets, a location that has been shown to be essential for virus assembly. We show that NS2 transmembrane region is crucial for both E2 interaction and subcellular localization. Moreover, specific mutations in core, envelope proteins, p7 and NS5A reported to abolish viral assembly changed the subcellular localization of NS2 protein. Together, these observations indicate that NS2 protein attracts the envelope proteins at the assembly site and it crosstalks with non-structural proteins for virus assembly
Mechanisms of the Thermal and Catalytic Redistributions, Oligomerizations, and Polymerizations of Linear Diborazanes
Investigating the plasma chemistry for the synthesis of carbon nanotubes/nanofibres in an inductively coupled plasma enhanced CVD system: the effect of different gas mixtures
Assembly, organization, and function of the COPII coat
A full mechanistic understanding of how secretory cargo proteins are exported from the endoplasmic reticulum for passage through the early secretory pathway is essential for us to comprehend how cells are organized, maintain compartment identity, as well as how they selectively secrete proteins and other macromolecules to the extracellular space. This process depends on the function of a multi-subunit complex, the COPII coat. Here we describe progress towards a full mechanistic understanding of COPII coat function, including the latest findings in this area. Much of our understanding of how COPII functions and is regulated comes from studies of yeast genetics, biochemical reconstitution and single cell microscopy. New developments arising from clinical cases and model organism biology and genetics enable us to gain far greater insight in to the role of membrane traffic in the context of a whole organism as well as during embryogenesis and development. A significant outcome of such a full understanding is to reveal how the machinery and processes of membrane trafficking through the early secretory pathway fail in disease states
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