Dephasing of a particle in a dissipative environment

The motion of a particle in a ring of length L is influenced by a dirty metal environment whose fluctuations are characterized by a short correlation distance $\ell << L$. We analyze the induced decoherence process, and compare the results with those obtained in the opposing Caldeira-Leggett limit ($\ell >> L$). A proper definition of the dephasing factor that does not depend on a vague semiclassical picture is employed. Some recent Monte-Carlo results about the effect of finite temperatures on "mass renormalization" in this system are illuminated.Comment: 18 pages, 2 figures, some textual improvements, to be published in JP

Nonlinear level crossing models

We examine the effect of nonlinearity at a level crossing on the probability for nonadiabatic transitions $P$. By using the Dykhne-Davis-Pechukas formula, we derive simple analytic estimates for $P$ for two types of nonlinear crossings. In the first type, the nonlinearity in the detuning appears as a {\it perturbative} correction to the dominant linear time dependence. Then appreciable deviations from the Landau-Zener probability $P_{LZ}$ are found to appear for large couplings only, when $P$ is very small; this explains why the Landau-Zener model is often seen to provide more accurate results than expected. In the second type of nonlinearity, called {\it essential} nonlinearity, the detuning is proportional to an odd power of time. Then the nonadiabatic probability $P$ is qualitatively and quantitatively different from $P_{LZ}$ because on the one hand, it vanishes in an oscillatory manner as the coupling increases, and on the other, it is much larger than $P_{LZ}$. We suggest an experimental situation when this deviation can be observed.Comment: 9 pages final postscript file, two-column revtex style, 5 figure

General solution to the band-broadening problem in polymer molecular weight distributions

A method is developed for overcoming the problem of band broadening ( where a sample which is monodisperse in molecularweight elutes over a range of elution volumes) in order to obtain accurate molecularweight distributions of polymers using size exclusion chromatography ( SEC). It is proved that the SEC signal from an exponential number distribution ( as obtained from free-radical polymerization under certain conditions) has the same functional form at the maximum, irrespective of band broadening. This leads to a method for quantifying the band broadening from any SEC trace, and hence for deconvoluting the trace to obtain the true distribution - by free- radical polymerization one should synthesize 'standards' which have exponential distributions and then carry out least-squares fitting to find the corresponding broadening function. The new method opens the way for mechanistic understanding and rate parameters to be obtained from the full detail that has hitherto been inaccessible in molecular weight distributions

Unique scorpion toxin with a putative ancestral fold provides insight into evolution of the inhibitor cystine knot motif

The three-disulfide inhibitor cystine knot (ICK) motif is a fold common to venom peptides from spiders, scorpions, and aquatic cone snails. Over a decade ago it was proposed that the ICK motif is an elaboration of an ancestral two-disulfide fold coined the disulfide-directed β-hairpin (DDH). Here we report the isolation, characterization, and structure of a novel toxin [U1-liotoxin-Lw1a (U1-LITX-Lw1a)] from the venom of the scorpion Liocheles waigiensis that is the first example of a native peptide that adopts the DDH fold. U1-LITX-Lw1a not only represents the discovery of a missing link in venom protein evolution, it is the first member of a fourth structural fold to be adopted by scorpion-venom peptides. Additionally, we show that U1-LITX-Lw1a has potent insecticidal activity across a broad range of insect pest species, thereby providing a unique structural scaffold for bioinsecticide development

Facile synthesis of site-specifically acetylated and methylated histone proteins: Reagents for evaluation of the histone code hypothesis

The functional capacity of genetically encoded histone proteins can be powerfully expanded by posttranslational modification. A growing body of biochemical and genetic evidence clearly links the unique combinatorial patterning of side chain acetylation, methylation, and phosphorylation mainly within the highly conserved N termini of histones H2A, H2B, H3, and H4 with the regulation of gene expression and chromatin assembly and remodeling, in effect constituting a “histone code” for epigenetic signaling. Deconvoluting this code has proved challenging given the inherent posttranslational heterogeneity of histone proteins isolated from biological sources. Here we describe the application of native chemical ligation to the preparation of full-length histone proteins containing site-specific acetylation and methylation modifications. Peptide thioesters corresponding to histone N termini were prepared by solid phase peptide synthesis using an acid labile Boc/HF assembly strategy, then subsequently ligated to recombinantly produced histone C-terminal globular domains containing an engineered N-terminal cysteine residue. The ligation site is then rendered traceless by hydrogenolytic desulfurization, generating a native histone protein sequence. Synthetic histones generated by this method are fully functional, as evidenced by their self-assembly into a higher order H3/H4 heterotetramer, their deposition into nucleosomes by human ISWI-containing (Imitation of Switch) factor RSF (Remodeling and Spacing Factor), and by enzymatic modification by human Sirt1 deacetylase and G9a methyltransferase. Site-specifically modified histone proteins generated by this method will prove invaluable as novel reagents for the evaluation of the histone code hypothesis and analysis of epigenetic signaling mechanisms

Solar parks can enhance bird diversity in agricultural landscape

Abstract: Solar photovoltaic power parks are a relatively new anthropogenic habitat that will become more widespread in the future. The greatest potential for solar photovoltaic power production is on arable land and grassland. Knowledge on the impacts of solar parks on biodiversity is scarce and spatially limited. We investigated the impact of ground-mounted solar parks on species richness, abundance, Shannon diversity and composition of bird communities in Slovakia (Central Europe), taking into account pre-construction land cover, elevation and landscape context. We recorded breeding, foraging or perching birds on 32 solar park plots and 32 adjacent control plots (two hectares each) during single breeding season. We found that solar parks supported higher total bird species richness and diversity, and richness and abundance of invertebrate-eaters, and that the abundance of ground-foragers was higher in solar parks developed on grassland than in grassland control plots. Ordination analysis showed that solar parks had a different composition of bird communities and thus increased overall species diversity and beta diversity in the agricultural landscapes studied. Plot type and landscape context accounted for most of the variation in bird community composition. Black redstart, European stonechat, white wagtail and Eurasian tree sparrow were identified as indicator species for solar parks. The observed pattern could be due to the higher structural diversity of solar parks. The solar parks studied were designed and managed exclusively for electricity production. It can therefore be assumed that solar parks designed and managed in synergy with a stronger focus on wildlife would have an even greater positive impact on bird diversity in an agricultural landscape