The effect of solidification rate on the structure of magnesium-aluminium eutectic grains

A magnesium-aluminium alloy of eutectic composition was solidified under two different cooling conditions, producing a low and a high growth rate of the eutectic solid-liquid interface. The high growth rate specimen contained smaller eutectic grains and cells, with a smaller interphase spacing compared with the low growth rate specimen. The high growth rate specimen also contained some primary Mg17Al12 dendrites, suggesting that the coupled zone is skewed towards the Mg phase with increased undercooling, A lamellar eutectic morphology was observed in the low growth rate specimen, while the morphology was fibrous in the high growth rate specimen.

Integrating attribute and space characteristics in choropleth display and spatial data mining

This paper develops an interactive approach for exploratory spatial data analysis. Measures of attribute similarity and spatial proximity are combined in a clustering model to support the identification of patterns in spatial information. Relationships between the developed clustering approach, spatial data mining and choropleth display are discussed. Analysis of property crime rates in Brisbane, Australia is presented. A surprising finding in this research is that there are substantial inconsistencies in standard choropleth display options found in two widely used commercial geographical information systems, both in terms of definition and performance. The comparative results demonstrate the usefulness and appeal of the developed approach in a geographical information system environment for exploratory spatial data analysis.

The fine structure of (v, 3) directed triple systems: v equivalent to 2 (mod 3)

The fine structure of a directed triple system of index lambda is the vector (c(1), c(2),...,C-lambda), where c(i) is the number of directed triples appearing precisely i times in the system. We determine necessary and sufficient conditions for a vector to be the fine structure of a directed triple system of index 3 for upsilon = 2 (mod 3).

Three-dimensional structure of RK-1: A novel alpha-defensin peptide

NMR spectroscopy and simulated annealing calculations have been used to determine the three-dimensional structure of RK-1, an antimicrobial peptide from rabbit kidney recently discovered from homology screening based on the distinctive physicochemical properties of the corticostatins/defensins. RK-1 consists of 32 residues, including six cysteines arranged into three disulfide bonds. It exhibits antimicrobial activity against Escherichia coli and activates Ca2+ channels in vitro. Through its physicochemical similarity, identical cysteine spacing, and linkage to the corticostatins/defensins, it was presumed to be a member of this family. However, RK-1 lacks both a large number of arginines in the primary sequence and a high overall positive charge, which are characteristic of this family of peptides. The three-dimensional solution structure, determined by NMR, consists of a triple-stranded antiparallel beta -sheet and a series of turns and is similar to the known structures of other alpha -defensins. This has enabled the definitive classification of RK-1 as a member of this family of antimicrobial peptides. Ultracentrifuge measurements confirmed that like rabbit neutrophil defensins, RK-1 is monomeric in solution, in contrast to human neutrophil defensins, which are dimeric.

Th structure of the P-II-ATP complex

P-II is a signal transduction protein that is part of the cellular machinery used by many bacteria to regulate the activity of glutamine synthetase and the transcription of its gene. The structure of P-II was solved using a hexagonal crystal form (form I). The more physiologically relevant form of P-II is a complex with small molecule effecters. We describe the structure of P-II with ATP obtained by analysis of two different crystal forms (forms II and III) that were obtained by co-crystallization of P-II with ATP. Both structures have a disordered recognition (T) loop and show differences at their C termini. Comparison of these structures with the form I protein reveals changes that occur on binding ATP. Surprisingly, the structure of the P-II/ATP complex differs with that of GlnK, a functional homologue. The two proteins bind the base and sugar of ATP in a similar manner but show differences in the way that they interact with the phosphates. The differences in structure could account for the differences in their activities, and these have been attributed to a difference in sequence at position 82. It has been demonstrated recently that P-II and GlnK form functional heterotrimers in vivo. We construct models of the heterotrimers and examine the junction between the subunits.

Structure revision and assignment of absolute stereochemistry of a marine C-21 bisfuranoterpene

The C-21 bisfuranoterpene (-)-isotetradehydrofurospongin-1 (6), previously isolated from a Western Australian Spongia sp., has been reisolated from a specimen of Spirastrella papilosa collected during scientific trawling operations in the Great Australian Eight. A 2D NMR analysis of 6 has prompted reassignment of the published structure 5, while degradation and chiral HPLC analysis have allowed determination of the absolute stereochemistry.

Modelling solute transport in structured soils: Performance evaluation of the ADR and TRM models

The movement of chemicals through the soil to the groundwater or discharged to surface waters represents a degradation of these resources. In many cases, serious human and stock health implications are associated with this form of pollution. The chemicals of interest include nutrients, pesticides, salts, and industrial wastes. Recent studies have shown that current models and methods do not adequately describe the leaching of nutrients through soil, often underestimating the risk of groundwater contamination by surface-applied chemicals, and overestimating the concentration of resident solutes. This inaccuracy results primarily from ignoring soil structure and nonequilibrium between soil constituents, water, and solutes. A multiple sample percolation system (MSPS), consisting of 25 individual collection wells, was constructed to study the effects of localized soil heterogeneities on the transport of nutrients (NO3-, Cl-, PO43-) in the vadose zone of an agricultural soil predominantly dominated by clay. Very significant variations in drainage patterns across a small spatial scale were observed tone-way ANOVA, p < 0.001) indicating considerable heterogeneity in water flow patterns and nutrient leaching. Using data collected from the multiple sample percolation experiments, this paper compares the performance of two mathematical models for predicting solute transport, the advective-dispersion model with a reaction term (ADR), and a two-region preferential flow model (TRM) suitable for modelling nonequilibrium transport. These results have implications for modelling solute transport and predicting nutrient loading on a larger scale. (C) 2001 Elsevier Science Ltd. All rights reserved.

Circular proteins in plants - Solution structure of a novel macrocyclic trypsin inhibitor from Momordica cochinchinensis

Much interest has been generated by recent reports on the discovery of circular (i.e. head-to-tail cyclized) proteins in plants. Here we report the three-dimensional structure of one of the newest such circular proteins, MCoTI-II, a novel trypsin inhibitor from Momordica cochinchinensis, a member of the Cucurbitaceae plant family. The structure consists of a small beta -sheet, several turns, and a cystine knot arrangement of the three disulfide bonds. Interestingly, the molecular topology is similar to that of the plant cyclotides (Craik, D. J., Daly, N. L., Bond, T., and Waine, C. (1999) J. Mol. Biol, 294, 1327-1336), which derive from the Rubiaceae and Violaceae plant families, have antimicrobial activities, and exemplify the cyclic cystine knot structural motif as part of their circular backbone. The sequence, biological activity, and plant family of MCoTI-II are all different from known cyclotides. However, given the structural similarity, cyclic backbone, and plant origin of MCoTI-II, we propose that MCoTI-II can be classified as a new member of the cyclotide class of proteins. The expansion of the cyclotides to include trypsin inhibitory activity and a new plant family highlights the importance and functional variability of circular proteins and the fact that they are more common than has previously been believed, Insights into the possible roles of backbone cyclization have been gained by a comparison of the structure of MCoTI-II with the homologous acyclic trypsin inhibitors CMTI-I and EETI-II from the Cucurbitaceae plant family.

Crystal structure of a heptameric Sm-like protein complex from archaea: Implications for the structure and evolution of snRNPs

The Sm/Lsm proteins associate with small nuclear RNA to form the core of small nuclear ribonucleoproteins, required for processes as diverse as pre-mRNA splicing, mRNA degradation and telomere formation. The Lsm proteins from archaea are likely to represent the ancestral Sm/Lsm domain. Here, we present the crystal structure of the Lsm alpha protein from the thermophilic archaeon Methanobacterium thermoautrophicum at 2.0 Angstrom resolution. The Lsm alpha protein crystallizes as a heptameric ring comprised of seven identical subunits interacting via beta -strand pairing and hydrophobic interactions. The heptamer can be viewed as a propeller-like structure in which each blade consists of a seven-stranded antiparallel beta -sheet formed from neighbouring subunits. There are seven slots on the inner surface of the heptamer ring, each of which is lined by Asp, Asn and Arg residues that are highly conserved in the Sm/Lsm sequences. These conserved slots are likely to form the RNA-binding site. In archaea, the gene encoding Lsm alpha is located next to the L37e ribosomal protein gene in a putative operon, suggesting a role for the Lsm alpha complex in ribosome function or biogenesis. (C) 2001 Academic Press.

Clarification of the Crystal Structure of [(HC(Ph2PO)3)2Cu](ClO4)2·2H2O

EXAFS spectra of [(HC(Ph2PO)(3))(2)Cu](ClO4)(2). 2H(2)O have been measured at room temperature. These show that the CuO6 unit is tetragonally elongated, rather than having the compressed tetragonal geometry previously inferred from the X-ray crystal structure determination. [GRAPHICS]

Disaggregation of polygons of surficial geology and soil maps using spatial modelling and legacy data

Examples from the Murray-Darling basin in Australia are used to illustrate different methods of disaggregation of reconnaissance-scale maps. One approach for disaggregation revolves around the de-convolution of the soil-landscape paradigm elaborated during a soil survey. The descriptions of soil ma units and block diagrams in a soil survey report detail soil-landscape relationships or soil toposequences that can be used to disaggregate map units into component landscape elements. Toposequences can be visualised on a computer by combining soil maps with digital elevation data. Expert knowledge or statistics can be used to implement the disaggregation. Use of a restructuring element and k-means clustering are illustrated. Another approach to disaggregation uses training areas to develop rules to extrapolate detailed mapping into other, larger areas where detailed mapping is unavailable. A two-level decision tree example is presented. At one level, the decision tree method is used to capture mapping rules from the training area; at another level, it is used to define the domain over which those rules can be extrapolated. (C) 2001 Elsevier Science B.V. All rights reserved.

Community structure of Quaternary coral reefs compared with Recent life and death assemblages

This paper assesses the reliability with which fossil reefs record the diversity and community structure of adjacent Recent reefs. The diversity and taxonomic composition of Holocene raised fossil reefs was compared with those of modern reef coral life and death assemblages in adjacent moderate and low-energy shallow reef habitats Of Madang Lagoon, Papua New Guinea. Species richness per sample area and Shannon-Wiener diversity (H') were highest in the fossil reefs, intermediate in the life assemblages, and lowest in the death assemblages. The taxonomic composition of the fossil reefs was most similar to the combination of the life and death assemblages from the modern reefs adjacent to the two fossil reefs. Depth zonation was recorded accurately in the fossil reefs. The Madang fossil reefs represent time-averaged composites of the combined life and death assemblages as they existed at the time the reef was uplifted. Because fossil reefs include overlapping cohorts from the life and death assemblages, lagoonal facies of fossil reefs are dominated by the dominant sediment-producing taxa, which are not necessarily the most abundant in the life assemblage. Rare or slow-growing taxa accumulate more slowly than the encasing sediments and are underrepresented in fossil reef lagoons. Time-averaging dilutes the contribution of rare taxa, rather than concentrating their contribution. Consequently, fidelity indices developed for mollusks in sediments yield low values in coral reef death and fossil assemblages. Branching corals dominate lagoonal facies of fossil reefs because they are abundant, they grow and produce sediment rapidly, and most of the sediment they produce is not exported. Fossil reefs distinguished kilometer-scale variations in community structure more clearly than did the modern life assemblages. This difference implies that fossil,reefs may provide a better long-term record of community structure than modern reefs. This difference also suggests that modern kilometer-scale variation in coral reef community structure may have been reduced by anthropogenic degradation, even in the relatively unimpacted reefs of Madang Lagoon. Holocene and Pleistocene fossil reefs provide a time-integrated historical record of community composition and may be used as long-term benchmarks for comparison with modern, degraded, nearshore reefs. Comparisons between fossil reefs and degraded modern reefs display gross changes in community structure more effectively than they demonstrate local extinction of rare taxa.

Social structures, genetic structures and dispersal strategies in Australian rabbit (Oryctolagus cuniculus) populations

In this study, the pattern of movement of young male and female rabbits and the genetic structures present in adult male and female populations in four habitats was examined. The level of philopatry in young animals was found to vary between 18-90% for males and 32-95% for females in different populations. It was skewed, with more males dispersing than females in some but not all populations. Analysis of allozyme data using spatial autocorrelation showed that adult females from the same social group, unlike males, were significantly related in four of the five populations studied. Changes in genetic structure and rate of dispersal were measured before and during the recovery of a population that was artificially reduced in size. There were changes in the rate and distance of dispersal with density and sex. Subadults of both sexes moved further in the first year post crash (low density) than in the following years. While the level of dispersal for females was lower than that of the males for the first 3 years, thereafter (high density) both sexes showed similar, low levels of dispersal (20%). The density at which young animals switch behaviour between dispersal and philopatry differed for males and females. The level of genetic structuring in adult females was high in the precrash population, reduced in the first year post crash and undetectable in the second year. Dispersal behaviour of rabbits both affects the genetic structure of the population and changes with conditions. Over a wide range of levels of philopatry, genetic structuring is present in the adult female, but not the male population. Consequently, though genetic structuring is present, it does not lead to inbreeding. More long-distance movements are found in low-density populations, even though vacant warrens are available near birth warrens. The distances moved decreased as density increased. Calculation of the effective population size (N-e) shows that changes in dispersal distance offset changes in density, so that N-e remains constant.

Heterogeneity in inoculum potential and effectiveness of arbuscular mycorrhizal fungi

Arbuscular mycorrhizae are symbiotic associations among glomalean fungi and plant roots that often lead to enhanced water and nutrient uptake and plant growth. We describe experiments to test whether inoculum potential of arbuscular mycorrhizal (AM) fungal communities varies spatially within a broadleaf temperate forest, and also whether there is variability in the effectiveness of AM fungal communities in enhancing seedling growth. Inoculum potential of arbuscular mycorrhizal fungi in a temperate broad-leaved forest did not vary significantly among sites. Inoculum potential, measured as the extent to which the roots of red maple seedlings that had been germinated on sterile sand and then transplanted into the forest, were colonized by AM fungi, was similar in floodplain and higher elevation sites. It was as similar under ectomycorrhizal oaks as it was under red maples and other AM tree species. It was also similar among sites with deciduous understory shrubs with arbuscular mycorrhizae (spicebush, Lindera benzoin) and those with evergreen vegetation with ericoid mycorrhizae (mountain laurel, Kalmia latifolia). Where spicebush was the dominant understory shrub, inoculum potential was greater under gaps in the canopy than within the understory. Survivorship of transplanted red maple seedlings varied significantly over sites but was not strongly correlated with measures of inoculum potential. In a greenhouse growth experiment, arbuscular mycorrhizal fungal communities obtained from tree roots from the forest had different effects on plant growth. Seedlings inoculated with roots of red maple had twice the leaf area after 10 wk of growth compared to the AM community obtained from roots of southern red oaks. Thus, although there appears to be little heterogeneity in inoculum potential in the forest, there are differences in the effectiveness of different inocula. These effects have the potential to affect tree species diversity in forests by modifying patterns of seedling recruitment.