Electronic spectra of linear HC5_5H and cumulene carbene H2_2C5_5

The $1 ^3\Sigma_u^- \leftarrow X^3\Sigma_g^-$ transition of linear HC$_5$H (A) has been observed in a neon matrix and gas phase. The assignment is based on mass-selective experiments, extrapolation of previous results of the longer HC$_{2n+1}$H homologues, and density functional and multi-state CASPT2 theoretical methods. Another band system starting at 303 nm in neon is assigned as the $1 ^1 A_1 \leftarrow X ^1 A_1$ transition of the cumulene carbene pentatetraenylidene H$_2$C$_5$ (B).Comment: 7 pages, 4 figures, 5 table

Structure and oligomerization of the periplasmic domain of GspL from the type II secretion system of Pseudomonas aeruginosa

The ability of bacteria to infect a host relies in part on the secretion of molecular virulence factors across the cell envelope.Pseudomonas aeruginosa, a ubiquitous environmental bacterium causing opportunistic infections in humans, employs the type II secretion system (T2SS) to transport effector proteins across its cellular envelope as part of a diverse array of virulence strategies.General secretory pathway protein L (GspL) is an essential inner-membrane component of the T2SS apparatus, and is thought to facilitate transduction of the energy from ATP hydrolysis in the cytoplasm to the periplasmic components of the system.However,our incomplete understanding of the assembly principles of the T2SS machinery prevents the mechanistic deconvolution of T2SS-mediated protein secretion.Here we show via two crystal structures that the periplasmic ferredoxin-like domain of GspL (GspL(fld)) is a dimer stabilized by hydrophobic interactions, and that this interface may allow significant interdomain plasticity.The general dimerization mode of GspL(fld) is shared with GspL from Vibrio parahaemolyticus suggesting a conserved oligomerization mode across the GspL family. Furthermore, we identified a tetrameric form of the complete periplasmic segment of GspL (GspL(Peri)) which indicates that GspL may be able to adopt multiple oligomeric states as part of its dynamic role in the T2SS apparatus

Identification of H2_2CCC as a diffuse interstellar band carrier

We present strong evidence that the broad, diffuse interstellar bands (DIBs) at 4881 and 5450\,\AA are caused by the B\,^1B$_1$\,$\leftarrow$\,X\,^1A$_1$ transition of H$_2$CCC (l-C$_3$H$_2$). The large widths of the bands are due to the short lifetime of the B\,^1B$_1$ electronic state. The bands are predicted from absorption measurements in a neon matrix and observed by cavity ring-down in the gas phase and show exact matches to the profiles and wavelengths of the two broad DIBs. The strength of the 5450\,\AA DIB leads to a l-C$_3$H$_2$ column density of $\sim5\times10^{14}$ cm$^{-2}$ towards HD\,183143 and $\sim2\times10^{14}$\,cm$^{-2}$ to HD\,206267. Despite similar values of $E$($B-V$), the 4881 and 5450\,\AA DIBs in HD\,204827 are less than one third their strength in HD\,183143, while the column density of interstellar C$_3$ is unusually high for HD\,204827 but undetectable for HD\,183143. This can be understood if C$_3$ has been depleted by hydrogenation to species such as l-C$_3$H$_2$ towards HD\,183143. There are also three rotationally resolved sets of triplets of l-C$_3$H$_2$ in the 6150$-$6330\,\AA region. Simulations, based on the derived spectroscopic constants and convolved with the expected instrumental and interstellar line broadening, show credible coincidences with sharp, weak DIBs for the two observable sets of triplets. The region of the third set is too obscured by the $\alpha$-band of telluric O$_2$.Comment: 22 pages, 9 figure

Polyhedral Analysis using Parametric Objectives

The abstract domain of polyhedra lies at the heart of many program analysis techniques. However, its operations can be expensive, precluding their application to polyhedra that involve many variables. This paper describes a new approach to computing polyhedral domain operations. The core of this approach is an algorithm to calculate variable elimination (projection) based on parametric linear programming. The algorithm enumerates only non-redundant inequalities of the projection space, hence permits anytime approximation of the output

Structure and oligomerization of the periplasmic domain of GspL from the type II secretion system of Pseudomonas aeruginosa

The ability of bacteria to infect a host relies in part on the secretion of molecular virulence factors across the cell envelope.Pseudomonas aeruginosa, a ubiquitous environmental bacterium causing opportunistic infections in humans, employs the type II secretion system (T2SS) to transport effector proteins across its cellular envelope as part of a diverse array of virulence strategies.General secretory pathway protein L (GspL) is an essential inner-membrane component of the T2SS apparatus, and is thought to facilitate transduction of the energy from ATP hydrolysis in the cytoplasm to the periplasmic components of the system.However,our incomplete understanding of the assembly principles of the T2SS machinery prevents the mechanistic deconvolution of T2SS-mediated protein secretion.Here we show via two crystal structures that the periplasmic ferredoxin-like domain of GspL (GspL(fld)) is a dimer stabilized by hydrophobic interactions, and that this interface may allow significant interdomain plasticity.The general dimerization mode of GspL(fld) is shared with GspL from Vibrio parahaemolyticus suggesting a conserved oligomerization mode across the GspL family. Furthermore, we identified a tetrameric form of the complete periplasmic segment of GspL (GspL(Peri)) which indicates that GspL may be able to adopt multiple oligomeric states as part of its dynamic role in the T2SS apparatus

A southern hemisphere survey of the 5780 and 6284 {\AA} diffuse interstellar bands: correlation with the extinction

We present a new database of 5780.5 and 6283.8 {\AA} DIB measurements and also study their correlation with the reddening. The database is based on high-resolution, high-quality spectra of early-type nearby stars located in the southern hemisphere at an average distance of 300 pc. Equivalent widths of the two DIBs were determined by means of a realistic continuum fitting and synthetic atmospheric transmissions. For all stars that possess a precise measurement of their color excess, we compare the DIBs and the extinction. We find average linear relationships of the DIBS and the color excess that agree well with those of a previous survey of northern hemisphere stars closer than 550 pc. This similarity shows that there is no significant spatial dependence of the average relationship in the solar neighborhood within $\simeq$ 600 pc. A noticeably different result is our higher degree of correlation of the two DIBs with the extinction. We demonstrate that it is simply due to the lower temperature and intrinsic luminosity of our targets. Using cooler target stars reduces the number of outliers, especially for nearby stars, confirming that the radiation field of UV bright stars has a significant influence on the DIB strength. We have used the cleanest data to compute updated DIB shapes.Comment: Astronomy & Astrophysics (in press

Far-infrared study of the Jahn-Teller distorted C60 monoanion in C60 tetraphenylphosphoniumiodide

We report high-resolution far-infrared transmission measurements on C(60)-tetraphenylphosphoniumiodide as a function of temperature. In the spectral region investigated (20-650 cm(-1)), we assign intramolecular modes of the C(60) monoanion and identify low-frequency combination modes. The well-known F(1u)(1) and F(1u)(2) modes are split into doublers at room temperature, indicating a D(5d) or D(3d) distorted ball. This result is consistent with a dynamic Jahn-Teller effect in the strong-coupling limit or with a static distortion stabilized by low-symmetry perturbations. The appearance of silent odd modes is in keeping with symmetry reduction of the hall, while activation of even modes is attributed to interband electron-phonon coupling and orientational disorder in the fulleride salt. Temperature dependences reveal a weak transition in the region 125-150 K in both C(60)(-) and counterion modes, indicating a bulk, rather than solely molecular, effect. Anomalous softening (with decreasing temperature) in several modes may correlate with the radial character of those vibrations. [S0163-1829(98)03245-7]

Geometric and electronic structure of the Cs-doped Bi2Se3(0001) surface

Using surface x-ray diffraction and scanning tunneling microscopy in combination with first-principles calculations, we have studied the geometric and electronic structure of Cs-deposited Bi2Se3(0001) surface kept at room temperature. Two samples were investigated: a single Bi2Se3 crystal, whose surface was Ar sputtered and then annealed at ∼500∘C for several minutes prior to Cs deposition, and a 13-nm-thick epitaxial Bi2Se3 film that was not subject to sputtering and was annealed only at ∼350∘C. In the first case, a considerable fraction of Cs atoms occupy top layer Se atoms sites both on the terraces and along the upper step edges where they form one-dimensional-like structures parallel to the step. In the second case, Cs atoms occupy the fcc hollow site positions. First-principles calculations reveal that Cs atoms prefer to occupy Se positions on the Bi2Se3(0001) surface only if vacancies are present, which might be created during the crystal growth or during the surface preparation process. Otherwise, Cs atoms prefer to be located in fcc hollow sites in agreement with the experimental finding for the MBE-grown sample

A search for diffuse bands in the circumstellar envelopes of post-AGB stars

In this work we present the results of a systematic search for diffuse bands (DBs, hereafter) in the circumstellar envelopes of a carefully selected sample of post-AGB stars. We concentrated on the analysis of 9 of the DBs most commonly found in the interstellar medium. The strength of these features is determined using high resolution optical spectroscopy and the results obtained are compared with literature data on field stars affected only by interstellar reddening. Based on the weak features observed in the subsample of post-AGB stars dominated by circumstellar reddening we conclude that the carrier(s) of these DBs must not be present in the circumstellar environment of these sources, or at least not under the excitation conditions in which DBs are formed. The conclusion is applicable to all the post-AGB stars studied, irrespective of the dominant chemistry or the spectral type of the star considered. A detailed radial velocity analysis of the features observed in individual sources confirms this result, as the Doppler shifts measured are found to be consistent with an interstellar origin.Comment: Accepted for A&