Bibliothek

Notizen: We describe the design and operation of a tunable, picosecond laser system for use in time-resolved spectroscopic measurements in the visible and ultraviolet (UV) spectral region. The laser is designed for fine tuning and high wavelength stability. A Nd:YAG-pumped distributed-feedback dye laser (DFDL) generates pulses that are ∼100 ps in duration with a nearly transform-limited linewidth (∼5 GHz) at a 20 Hz repetition rate. The DFDL pulses are amplified in two bow-tie amplifiers, providing pulse energies of up to 3.0 mJ; the amplified pulses may be frequency doubled to the UV spectral region, providing up to 1.0 mJ. The DFDL wavelength is computer stabilized to within ±0.8 pm (±0.7 GHz, two standard deviations), allowing the wavelength to be stationed on a narrow atomic or molecular transition or permitting nearly continuous spectral scans. Application of the laser system to studies of OH energy transfer has been demonstrated; both laser-induced-fluorescence and degenerate-four-wave-mixing spectra have been recorded. © 2000 American Institute of Physics.

Notizen: In 1980 the State of Connecticut began a tidal marsh restoration program targeting systems degraded by tidal restrictions and impoundments. Such marshes become dominated by common reed grass (Phragmites australis) and cattail (Typha angustifolia and T. latifolia), with little ecological connection to Long Island Sound. The management and scientific hypothesis was that returning tidal action, reconnecting marshes to Long Island Sound, would set these systems on a recovery trajectory. Specific restoration targets (i.e., pre-disturbance conditions or particular reference marshes) were considered unrealistic. However, it was expected that with time restored tides would return ecological functions and attributes characteristic of fully functioning tidal salt marshes. Here we report results of this program at nine separate sites within six marsh systems along 110 km of Long Island Sound shoreline, with restoration times of 5 to 21 years. Biotic parameters assessed include vegetation, macroinvertebrates, and use by fish and birds. Abiotic factors studied were soil salinity, elevation and tidal flooding, and soil water table depth. Sites fell into two categories of vegetation recovery: slow, ca. 0.5%, or fast, more than 5% of total area per year. Although total cover and frequency of salt marsh angiosperms was positively related to soil salinity, and reed grass stand parameters negatively so, fast versus slow recovery rates could not be attributed to salinity. Instead, rates appear to reflect differences in tidal flooding. Rapid recovery was characterized by lower elevations, greater hydroperiods, and higher soil water tables. Recovery of other biotic attributes and functions does not necessarily parallel those for vegetation. At the longest studied system (rapid vegetation recovery) the high marsh snail Melampus bidentatus took two decades to reach densities comparable with a nearby reference marsh, whereas the amphipod Orchestia grillus was well established on a slow-recovery marsh, reed grass dominated after 9 years. Typical fish species assemblages were found in restoration site creeks and ditches within 5 years. Gut contents of fish in ditches and on the high marsh suggest that use of restored marsh as foraging areas may require up to 15 years to reach equivalence with reference sites. Bird species that specialize in salt marshes require appropriate vegetation; on the oldest restoration site, breeding populations comparable with reference marshland had become established after 15 years. Use of restoration sites by birds considered marsh generalists was initially high and was still nearly twice that of reference areas even after 20 years. Herons, egrets, and migratory shorebirds used restoration areas extensively. These results support our prediction that returning tides will set degraded marshes on trajectories that can bring essentially full restoration of ecological functions. This can occur within two decades, although reduced tidal action can delay restoration of some functions. With this success, Connecticut's Department of Environmental Protection established a dedicated Wetland Restoration Unit. As of 1999 tides have been restored at 57 separate sites along the Connecticut coast.

Notizen: Nicastrin, a type-I transmembrane glycoprotein, is a necessary component of the high molecular weight presenilin (PS) complexes that mediate intramembranous cleavage of β-amyloid precursor protein (βAPP) and Notch. Nicastrin undergoes trafficking-dependent glycosylation maturation, and PS1 interacts preferentially with these maturely glycosylated forms of nicastrin. We investigated the effects of differing levels of the immature and mature endoglycosidase-H-resistant forms of nicastrin on Aβ40- and Aβ42-peptide secretion in several cell lines stably expressing a mutant nicastrin (D336A/Y337A) that increases Aβ secretion. There was no correlation between Aβ secretion and the level of over-expression of the immature forms of nicastrin. The total level of mature nicastrin remained constant, but mutant nicastrin replaced endogenous mature nicastrin in varying degrees. Differences in the levels of mature mutant nicastrin positively correlated with Aβ secretion, but did not influence either βAPP trafficking or processing by α- and β-secretases. Proper trafficking and terminal maturation of nicastrin is therefore a necessary event for the regulated intramembranous proteolysis of βAPP.

Notizen: A mechanical activation technique has been used to form composites of alumina with titanium carbide, nitride, or carbonitride, both with and without elemental iron. The composites were formed by reacting elemental aluminum with either ilmenite (FeTiO3) or rutile (TiO2) concentrates in the presence of carbon and/or nitrogen in a ball-mill at ambient temperature. The reaction was complete for the ilmenite samples after milling but was completed only for rutile under hot pressing conditions. Microhardness measurements indicated that the composites had hardnesses in the range 19–30 GPa (1740–2750 VHN), with only a small variation within each sample. Elemental mapping of the pressed pellets indicated that titanium and aluminum were evenly distributed on a submicrometer level whereas iron tended to coalesce into 〈20 μm particles in the presence of TiC. The coalescence decreased with the carbon content of the hard material until iron was evenly distributed with TiN. A superstoichiometric amount of aluminum led to the formation of iron–aluminum phases which decreased the iron coalescence. The XRD crystallite size of the alumina was 30–50 nm and was 25–50 nm for the titanium phases, confirming the extremely fine microstructure.