The Ultrastructure of Cryo-Sections and Intact Vitrified Cells - The Effects of Cryoprotectants and Acceleration Voltage on Beam Induced Bubbling

Chemically fixed pancreas was infiltrated with various cryoprotectants to obtain homogeneously vitrified samples upon cooling. The suitability of these samples for cryo-ultramicrotomy was tested. Contrast was hardly detectable initially in thin cryo-sections but increased upon irradiation, irrespective of the cryoprotectant (glycerol, propylene glycol, methanol) used. Contrast and beam damage were analyzed in vitrified thin films from collagen, phospholipid vesicles and various concentrations of glycerol. Glycerol increased the beam sensitivity of both collagen and phospholipid vesicles, but diminished the contrast between matrix and lipid vesicles or collagen fibers. The effects of glycerol as observed in thin films explain some of the effects of cryoprotectants in thin cryo-sections. To reduce beam damage in vitrified specimens two approaches are proposed. Firstly, when vitrified films are prepared, dilute suspensions should be used without cryoprotectant. In some cases, such as (thin) intact cells, the composition of the suspended material can only be marginally influenced. Then a second approach can be used involving the application of higher accelerating voltages (e.g. 300 kV). This has two advantages; the increase in mean free path-length of the electrons causes less beam damage on one hand and allows better resolution of thick specimens on the other hand. Micrographs from E. coli bacteria vitrified from suspension illustrate some of the potentials of intermediate voltage cryo-electron microscopy

GaBoDS: The Garching-Bonn Deep Survey - III. Lyman-Break Galaxies in the Chandra Deep Field South

We present first results of our search for high-redshift galaxies in deep CCD mosaic images. As a pilot study for a larger survey, very deep images of the Chandra Deep Field South (CDFS), taken withWFI@MPG/ESO2.2m, are used to select large samples of 1070 U-band and 565 B-band dropouts with the Lyman-break method. The data of these Lyman-break galaxies are made public as an electronic table. These objects are good candidates for galaxies at z~3 and z~4 which is supported by their photometric redshifts. The distributions of apparent magnitudes and the clustering properties of the two populations are analysed, and they show good agreement to earlier studies. We see no evolution in the comoving clustering scale length from z~3 to z~4. The techniques presented here will be applied to a much larger sample of U-dropouts from the whole survey in near future.Comment: 11 pages, 11 figures, replaced with version accepted by A&A. Minor changes and tabular appendix with LBG catalogues. Version with full resolution figures available at http://www.astro.uni-bonn.de/~hendrik/2544.pd

Controlling the melting transition of semi-crystalline self-assembled block copolymer aggregates: controlling release rates of ibuprofen

Bicontinuous nanospheres and multi-lamellar micelles were self-assembled from poly[ethylene oxide]-block-(poly[octadecyl methacrylate]-random-poly[docosyl methacrylate]), (PEO-b-[PODMA-co-PDSMA]) where PEO is the hydrophilic block (25 wt%) and PODMA/PDSMA is the semi-crystalline hydrophobic block (75 wt%) that gives a thermoresponsive component to the self-assembled aggregates. By varying the relative molar proportion of DSMA to ODMA (from 0 : 1 to 1 : 0) in the synthesis of the copolymers by atom transfer radical polymerisation, the melting transition Tm of the hydrophobic block could be varied from 21.5 to 41.1 °C in the solid state. When self-assembled in aqueous dispersions the Tm range was 23.4 to 41.3 °C, closely matching that of the solid samples. Preliminary analysis of the rate of release of ibuprofen from three of the block copolymer aggregates demonstrated that the rate of release was correlated with the degree of crystallinity of the hydrophobic block and that increasing temperature causes melting and a significantly enhanced release rate

Highly Ionized High Velocity Clouds: Intergalactic Gas in the Local Group or Distant Gas in the Galactic Halo?

We have recently identified several high velocity (V < -100 km/s) clouds in the directions of Mrk 509 and PKS 2155-304 that have unusual ionization properties. The clouds exhibit strong C IV absorption with little or no detectable low ion (C II, Si II) absorption or H I 21cm emission. As the closest known analog to the outer diffuse halos of damped Ly-alpha absorbers and the low H I column density metal line absorption systems seen in the spectra of high redshift quasars, these "C IV-HVCs" present unique opportunities for relating the conditions within the Milky Way halo and nearby intergalactic gas to the properties of galactic halos at higher redshift. The C IV-HVCs have ionization properties consistent with photoionization by extragalactic background radiation, though some contribution by collisional ionization within a hot plasma cannot be ruled out. The clouds are probably low density [n(H) ~ 10^-4 cm^-3], large [greater than several kiloparsecs], and mostly ionized [n(HI)/n(H) ~ 10^-3] regions located well beyond the neutral gas layer of the Galaxy. The presence of weak H I-HVCs detected through 21cm emission near both sight lines indicates that the C IV-HVCs trace the extended, ionized, low density regions of the H I-HVCs. Several lines of evidence, including very low thermal pressures (P/k ~ 2 cm^-3 K), favor a location for the C IV-HVCs in the Local Group or very distant Galactic halo.Comment: AASTEX manuscript and tables, 55 pages, 8 postscript figures. Astrophysical Journal, in pres

Supramolecular Double Helices from Small C-3-Symmetrical Molecules Aggregated in Water

Supramolecular fibers in water, micrometers long and several nanometers in width, are among the most studied nanostructures for biomedical applications. These supramolecular polymers are formed through a spontaneous self-assembly process of small amphiphilic molecules by specific secondary interactions. Although many compounds do not possess a stereocenter, recent studies suggest the (co)existence of helical structures, albeit in racemic form. Here, we disclose a series of supramolecular (co)polymers based on water-soluble benzene-1,3,5-tricarboxamides (BTAs) that form double helices, fibers that were long thought to be chains of single molecules stacked in one dimension (1D). Detailed cryogenic transmission electron microscopy (cryo-TEM) studies and subsequent three-dimensional-volume reconstructions unveiled helical repeats, ranging from 15 to 30 nm. Most remarkable, the pitch can be tuned through the composition of the copolymers, where two different monomers with the same core but different peripheries are mixed in various ratios. Like in lipid bilayers, the hydrophobic shielding in the aggregates of these disc-shaped molecules is proposed to be best obtained by dimer formation, promoting supramolecular double helices. It is anticipated that many of the supramolecular polymers in water will have a thermodynamic stable structure, such as a double helix, although small structural changes can yield single stacks as well. Hence, it is essential to perform detailed analyses prior to sketching a molecular picture of these 1D fibers

Models of X-ray Photoionization in LMC X-4: Slices of a Stellar Wind

We show that the orbital variation in the UV P Cygni lines of the X-ray binary LMC X-4 results when X-rays photoionize nearly the entire region outside of the X-ray shadow of the normal star. We fit models to HST GHRS observations of N V and C IV P Cygni line profiles. Analytic methods assuming a spherically symmetric wind show that the wind velocity law is well-fit by v~(1-1/r)^beta, where beta is likely 1.4-1.6 and definitely <2.5. Escape probability models can fit the observed P Cygni profiles, and provide measurements of the stellar wind parameters. The fits determine Lx/Mdot=2.6+/-0.1 x10^43 erg/s/Msun yr, where Lx is the X-ray luminosity and Mdot is the mass-loss rate of the star. Allowing an inhomogeneous wind improves the fits. IUE spectra show greater P Cygni absorption during the second half of the orbit than during the first. We discuss possible causes of this effect.Comment: 56 pages, 12 figures, to be published in the Astrophysical Journa

The Recent Evolution of the Dwarf Starburst Galaxy NGC 625 from Hubble Space Telescope Imaging

New HST/WFPC2 imaging of the dwarf starburst galaxy NGC 625 is presented. These data, 80% complete to V and I magnitudes of 26.0 and 25.3, respectively, allow us to study the recent star formation history of NGC 625. We derive a tip of the red giant branch (TRGB) distance modulus of 27.95+/-0.07, corresponding to a distance of 3.89+/-0.22 Mpc, and a location on the far side of the Sculptor Group. NGC 625 has a well-defined radial stellar population gradient, evidenced by a central concentration of young MS stars and an RGB/AGB ratio that increases with galactocentric distance. The prominent AGB is very red, and RGB stars are detected far from the central star forming regions. Using H Alpha and H Beta narrow band imaging and previous optical spectroscopy we identify substantial and varying internal extinction (A_V = 0.0 to 0.6 mag) associated with the central active star formation regions. To better understand the effects of internal extinction on the analysis of young stellar populations, synthetic models are presented which, for the first time, examine and account for this effect. Using the luminous blue helium burning stars, we construct a simple model of the recent (< 100 Myr) star formation in which an elevated but declining star formation rate has been present over this entire period. This is at odds with the presence of spectroscopic W-R features in the major star formation region which imply a short duration (<= 5 Myr) for the recent starburst. This suggests that starbursts displaying W-R features are not necessarily all of a short duration. Finally, we speculate on the possible causes of the present burst of star formation in this apparently isolated galaxy, and compare it to other nearby, well-studied dwarf starburst systems.Comment: 56 pages, including 15 figures (2 in color). Scheduled to appear in AJ, December, 2003. Full-resolution version may be obtained at http://www.astro.umn.edu/~Cannon/n625.p

A Region of Violent Star Formation in the Irr Galaxy IC 10: Structure and Kinematics of Ionized and Neutral Gas

We have used observations of the galaxy IC 10 at the 6-m telescope of the Special Astrophysical Observatory with the SCORPIO focal reducer in the Fabry-Perot interferometer mode and with the MPFS spectrograph to study the structure and kinematics of ionized gas in the central region of current intense star formation. Archive VLA 21-cm observations are used to analyze the structure and kinematics of neutral gas in this region. High-velocity wings of the H-alpha and [SII] emission lines were revealed in the inner cavity of the nebula HL 111 and in other parts of the complex of violent star formation. We have discovered local expanding neutral-gas shells around the nebulae HL 111 and HL 106.Comment: 22 pages, 10 figures; accepted in Astronomy Report