The effects of environmental perturbation and measurement error on estimates of the shape parameter in the theta-logistic model of population regulation

The theta-logistic is a widely used generalisation of the logistic model of regulated biological processes which is used in particular to model population regulation. Then the parameter theta gives the shape of the relationship between per-capita population growth rate and population size. Estimation of theta from population counts is however subject to bias, particularly when there are measurement errors. Here we identify factors disposing towards accurate estimation of theta by simulation of populations regulated according to the theta-logistic model. Factors investigated were measurement error, environmental perturbation and length of time series. Large measurement errors bias estimates of theta towards zero. Where estimated theta is close to zero, the estimated annual return rate may help resolve whether this is due to bias. Environmental perturbations help yield unbiased estimates of theta. Where environmental perturbations are large, estimates of theta are likely to be reliable even when measurement errors are also large. By contrast where the environment is relatively constant, unbiased estimates of theta can only be obtained if populations are counted precisely Our results have practical conclusions for the design of long-term population surveys. Estimation of the precision of population counts would be valuable, and could be achieved in practice by repeating counts in at least some years. Increasing the length of time series beyond ten or 20 years yields only small benefits. if populations are measured with appropriate accuracy, given the level of environmental perturbation, unbiased estimates can be obtained from relatively short censuses. These conclusions are optimistic for estimation of theta. (C) 2008 Elsevier B.V All rights reserved.

ExtSym: a program to aid space-group determination from powder diffraction data

Once unit-cell dimensions have been determined from a powder diffraction data set and therefore the crystal system is known (e.g. orthorhombic), the method presented by Markvardsen, David, Johnson & Shankland [Acta Cryst. (2001), A57, 47-54] can be used to generate a table ranking the extinction symbols of the given crystal system according to probability. Markvardsen et al. tested a computer program (ExtSym) implementing the method against Pawley refinement outputs generated using the TF12LS program [David, Ibberson & Matthewman (1992). Report RAL-92-032. Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, UK]. Here, it is shown that ExtSym can be used successfully with many well known powder diffraction analysis packages, namely DASH [David, Shankland, van de Streek, Pidcock, Motherwell & Cole (2006). J. Appl. Cryst. 39, 910-915], FullProf [Rodriguez-Carvajal (1993). Physica B, 192, 55-69], GSAS [Larson & Von Dreele (1994). Report LAUR 86-748. Los Alamos National Laboratory, New Mexico, USA], PRODD [Wright (2004). Z. Kristallogr. 219, 1-11] and TOPAS [Coelho (2003). Bruker AXS GmbH, Karlsruhe, Germany]. In addition, a precise description of the optimal input for ExtSym is given to enable other software packages to interface with ExtSym and to allow the improvement/modification of existing interfacing scripts. ExtSym takes as input the powder data in the form of integrated intensities and error estimates for these intensities. The output returned by ExtSym is demonstrated to be strongly dependent on the accuracy of these error estimates and the reason for this is explained. ExtSym is tested against a wide range of data sets, confirming the algorithm to be very successful at ranking the published extinction symbol as the most likely. (C) 2008 International Union of Crystallography Printed in Singapore - all rights reserved.

Polymer protected gold nanoparticles: synthesis, characterization and application in catalysis

This review discusses the stabilization of gold nanoparticles (AuNPs) by nonionic, anionic, cationic and amphoteric polymers. The protocols used for synthesis of AuNPs in aqueous and organic solvents are described. Size, shape and morphology of AuNPs are characterized by various physicochemical methods. Application aspects of polymer-protected AuNPs in catalysis are outlined.

Time course of gag protein assembly in HIV-1-infected cells: a study by immunoelectron microscopy

Recent biochemical studies have identified high molecular complexes of the HIV Gag precursor in the cytosol of infected cells. Using immunoelectron microscopy we studied the time course of the synthesis and assembly of a HIV Gag precursor protein (pr55gag) in Sf9 cells infected with recombinant baculovirus expressing the HIV gag gene. We also immunolabeled for pr55gag human T4 cells acutely or chronically infected with HIV-1. In Sf9 cells, the time course study showed that the first Gag protein appeared in the cytoplasm at 28-30 h p.i. and that budding started 6-8 h later. Colloidal gold particles, used to visualize the Gag protein, were first scattered randomly throughout the cytoplasm, but soon clusters representing 100 to 1000 copies of pr55gag were also observed. By contrast, in cells with budding or released virus-like particles the cytoplasm was virtually free of gold particles while the released virus-like particles were heavily labeled. Statistical analysis showed that between 80 and 90% of the gold particles in the cytoplasm were seen as singles, as doublets, or in small groups of up to five particles probably representing small oligomers. Clusters of gold particles were also observed in acutely infected lymphocytes as well as in multinuclear cells of chronically infected cultures of T4 cells. In a few cases small aggregates of gold particles were found in the nuclei of T4 lymphocytes. These observations suggest that the Gag polyprotein forms small oligomers in the cytoplasm of expressing cells but that assembly into multimeric complexes takes place predominantly at the plasma membrane. Large accumulations of Gag protein in the cytoplasm may represent misfolded molecules destined for degradation.

Regioselective synthesis of mannobiose and mannotriose by reverse hydrolysis using a novel 1,6-alpha-D-mannosidase from Aspergillus phoenicis

A novel 1,6-alpha-D-mannosidase was produced by Aspergillus phoenicis grown on a commercial manno-oligosaccharide preparation in liquid culture. The enzyme hydrolysed only alpha-D-Manp-(1 --> 6)-D-Manp and did not act on alpha-D-Manp-(1 --> 2)-D-Manp, or alpha-D-Manp-(1 --> 3)-D-Manp. The 1,6-alpha-D-mannosidase was used for synthesis of manno-oligosaccharides by reverse hydrolysis reaction. The highest yields, expressed as percentages (w/w) of total sugar, were similar to21% mannobiose and similar to5% mannotriose, and they were obtained with 45% (w/w) initial mannose concentration at pH 4.5 after 12 days incubation at 55 degreesC. The disaccharide and trisaccharide products were separated and their structures determined by methylation analysis. Only 1-6 linkages were found in both of them. (C) 2003 Elsevier B.V. All rights reserved.

Spectroscopic properties of alkenylferrocenes; crystal and molecular structure of (Z)-(1,2-diphenylethenyl)ferrocene

Studies of the 1H n.m.r. and electronic spectra of a series of alkenylferrocenes including (E) and (Z) stereoisomers of various styrylferrocenes, have provided methods of structure elucidation. Crystals of the title compound are monoclinic, space group P21/c with Z= 4 in a unit cell of dimensions a= 17.603(2), b= 10.218(2), c= 10.072 Å, β= 103.27(2)°. The structure has been determined by the heavy-atom method from diffractometer data and refind by full-matrix least-squares techniques to R= 0.043 for 2 219 unique reflections.