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At least 721 records · Page 40Linked to original sources
PubMed2005

Molecular view of water dynamics near model peptides.

Incoherent quasi-elastic neutron scattering (QENS) has been used to measure the dynamics of water molecules in solutions of a model protein backbone, N-acetyl-glycine-methylamide (NAGMA), as a function of concentration, for comparison with results for water dynamics in aqueous solutions of the N-acetyl-leucine-methylamide (NALMA) hydrophobic peptide at comparable concentrations. From the analysis of the elastic incoherent structure factor, we find significant fractions of elastic intensity at high and low concentrations for both solutes, which corresponds to a greater population of protons with rotational time scales outside the experimental resolution (>13 ps). The higher-concentration solutions show a component of the elastic fraction that we propose is due to water motions that are strongly coupled to the solute motions, while for low-concentration solutions an additional component is activated due to dynamic coupling between inner and outer hydration layers. An important difference between the solute types at the highest concentration studied is found from stretched exponential fits to their experimental intermediate scattering functions, showing more pronounced anomalous diffusion signatures for NALMA, including a smaller stretched exponent beta and a longer structural relaxation time tau than those found for NAGMA. The more normal water diffusion exhibited near the hydrophilic NAGMA provides experimental support for an explanation of the origin of the anomalous diffusion behavior of NALMA as arising from frustrated interactions between water molecules when a chemical interface is formed upon addition of a hydrophobic side chain, inducing spatial heterogeneity in the hydration dynamics in the two types of regions of the NALMA peptide. We place our QENS measurements on model biological solutes in the context of other spectroscopic techniques and provide both confirming as well as complementary dynamic information that attempts to give a unifying molecular view of hydration dynamics signatures near peptides and proteins.

Diffusion↗
PubMed2006

Self-diffusion of supercooled o-terphenyl near the glass transition temperature.

Self-diffusion coefficients for the low molecular weight glass former o-terphenyl have been measured near Tg by isothermally desorbing thin film bilayers of deuterio and protio o-terphenyl in a vacuum chamber. We observe translational diffusion that is about 100 times faster at Tg + 3 K than the Stokes-Einstein prediction. Predictions from random first order transition theory and a dynamic facilitation approach are in reasonable agreement with our results; in these approaches, enhanced translational diffusion is associated with spatially heterogeneous dynamics. Self-diffusion controls crystallization in o-terphenyl for most of the supercooled liquid regime, but at temperatures below Tg + 10 K, the reported crystallization rate increases suddenly while the self-diffusion coefficient does not. This work and previous work on trisnaphthylbenzene both find a self-diffusion-controlled crystal growth regime and an enhancement in self-diffusion near Tg, suggesting that these phenomena are general characteristics of fragile low molecular weight glass formers. We discuss the width of the relaxation time distributions of o-terphenyl and trisnaphthylbenzene as they relate to the observation of enhanced translational diffusion.

Journal Article↗
PubMed2006

Tail aggregation and domain diffusion in ionic liquids.

An extended multiscale coarse-graining model for ionic liquids is used to investigate the liquid crystal-like phase in certain ionic liquids. The tail groups of the cations with a sufficient side-chain length are found to aggregate, forming spatially heterogeneous domains, due to the competition between the electrostatic interactions between the charged head groups and the anions and the collective short-range interactions between the neutral tail groups. With a sufficiently long alkyl chain at a low enough temperature, the tail domains remain relatively stable, despite the diffusion of individual ions in the liquid phase. With increasing temperature, the average tail domains begin to diffuse, while beyond a transition temperature, their average density has an almost uniform distribution, although the tail groups still form instantaneous domains.

Carbon↗
PubMed2005

Metal speciation dynamics in colloidal ligand dispersions.

In this work we propose a dynamic metal speciation theory for colloidal systems in which the complexing ligands are localized on the surface of the particles; i.e., there is spatial heterogeneity of binding sites within the sample volume. The differences between the complex formation and dissociation rate constants of complexes in colloidal dispersions and those in homogeneous solutions originate from the differences in kinetic and mass transport conditions. In colloidal systems, when the effective rate of dissociation of the surface complexes becomes fully diffusion controlled, its value is defined via the geometrical parameters of the particle. We assess the extent to which the conventional approach of assuming a homogeneously smeared-out ligand distribution overestimates the lability of surface complexes in colloidal ligand dispersions. The validity of the theory is illustrated by application to binding of lead and cadmium by carboxyl modified latex particles: our approach correctly predicts the formation/dissociation rate constants, which differ by several orders of magnitude from their homogeneous solution counterparts.

Journal Article↗
PubMed1998

A two-patch model of Gambian sleeping sickness: application to vector control strategies in a village and plantations.

A compartmental model is described for the spread of Gambian sleeping sickness in a spatially heterogeneous environment in which vector and human populations migrate between two "patches": the village and the plantations. The number of equilibrium points depends on two "summary parameters": gr the proportion removed among human infections, and R0, the basic reproduction number. The origin is stable for R0 < 1 and unstable for R0 > 1. Control strategies are assessed by studying the mix of vector control between the two patches that bring R0 below 1. The results demonstrate the importance of vector control in the plantations. For example if 20 percent of flies are in the village and the blood meal rate in the village is 10 percent, then a 20 percent added vector mortality in the village must be combined with a 9 percent added mortality in the plantations in order to bring R0 below 1. The results are quite insensitive to the blood meal rate in the village. Optimal strategies (that minimize the total number of flies trapped in both patches) are briefly discussed.

Communicable Disease Control↗
PubMed2002

Multiple stressor effects on benthic biodiversity of Chesapeake Bay: implications for ecological risk assessment.

In natural aquatic systems, there is frequently overlap in the spatial distribution of both natural and anthropogenic stressors, particularly at regional geographic scales. Yet the proportional risk associated with individual stressors, their cumulative effects and the manner in which they interact to affect aquatic ecology is frequently unknown, limiting the robustness of multiple-stressor ecological risk assessments (ERA). The current study used historical environmental monitoring data (1984-1999) to identify a combination of natural and anthropogenic stressors that best accounted for observed patterns of benthic biodiversity in Chesapeake Bay. Geographic information systems (GIS) were used to geographically link spatially heterogeneous databases for benthic biodiversity, water quality and sediment toxicant concentrations. Single and multiple variable regression techniques were subsequently used to develop a statistical model to explain observed patterns of benthic biodiversity. Combinations of natural stressors alone accounted for as much as 34% of the variation in benthic biodiversity, and combinations of anthropogenic toxicants accounted for as much as 48% of the variation. The consideration of both natural and anthropogenic stressors resulted in a statistical model that accounted for approximately 73% of the observed variation in benthic biodiversity of Chesapeake Bay. These results suggest that benthic biodiversity in Chesapeake Bay is a function of complex interactions among water quality characteristics and anthropogenic toxicants. Therefore, new risk assessment methodologies are required to assess the risk of multiple stressors at regional scales.

Animals↗
PubMed1997

Microsatellites in the sand lizard (Lacerta agilis): description, variation, inheritance, and applicability.

We developed microsatellite markers for the sand lizard (Lacerta agilis) to enable investigations of the genetic variability within and among populations with a heterogeneous spatial distribution in Sweden. The populations, which could not be characterized by variation in allozymes or mitochondrial DNA, had a substantial level of variability in microsatellite loci. However, the variability in Swedish populations was limited compared to a large, outbred Hungarian population. In the sand lizard, the number of (GT/CA)n repeats was approximately three times higher than that for (CT/GA)n. The number of repeats and the frequency of microsatellites were within the range reported for other species. Three of nine microsatellite loci showed alleles that could not be amplified, which is in agreement with recent reports describing microsatellite "null alleles" as a common occurrence. We discuss the caution which this calls for when calculating paternity probabilities and when estimating between-population allelic differentiation. A potential problem with different mutation rates for alleles within the same locus is discussed.

Alleles↗
PubMed2004

Temporal and spatial genetic variability in white-tailed deer (Odocoileus virginianus).

Starch-gel electrophoresis was used to assess temporal and spatial genetic variation in populations of white-tailed deer (Odocoileus virginianus) in western Tennessee. Samples of liver and kidney obtained from animals at five localities during 1985-1992 were analyzed at 11 loci known to be polymorphic in white-tailed deer. There were minimal significant differences in allelic frequencies between sexes and among age groups for each year at each locality and among years at each locality with ages and sexes combined. No significant difference occurred among mean values of heterozygosity. A heterozygote deficiency was detected in white-tailed deer populations from western Tennessee and could reflect limited inbreeding but was more likely due to Wahlund effects. Limited temporal variation occurred among years within a locality and indicated a stasis in terms of minimal genetic drift and harvest effects on genetic structure of the populations. Herd origin, stocking history of populations, and gene flow were probable causes of spatial heterogeneity in deer populations in western Tennessee.

Animals↗
PubMed1993

Modification of tumour blood flow using the hypertensive agent, angiotensin II.

The effects of different doses of angiotensin II (0.02 to 0.5 microgram kg-1 min-1 on mean arterial blood pressure, tissue blood flow and tissue vascular resistance were investigated in BD9 rats. Blood flow was measured using the uptake of 125I- or 14C-labelled iodoantipyrine (125I-IAP and 14C-IAP). Spatial heterogeneity of blood flow within tumours, before and after angiotensin II infusion, was also measured using 14C-IAP and an autoradiographic procedure. Mean arterial blood pressure rose steeply with angiotensin II dose. Blood flow to skeletal muscle, skin overlying the tumour, contralateral skin, small intestine and kidney tended to decline in a dose-dependent manner. Blood flow to the tumour was also reduced (to 80% of control values) but there was no dose response. Blood flow to the heart was slightly increased and blood flow to the brain was unaffected by angiotensin II. Vascular resistance, in all tissues, was increased by angiotensin II infusion. The increase in tumour tissue was similar to that found in skeletal muscle and small intestine and is likely to be caused by a direct vasoconstricting effect of the drug rather than autoregulation of tumour blood flow in the face of an increase in perfusion pressure. The reduction in overall blood flow at the highest perfusion pressure was due to a preferential effect of angiotensin II at the tumour periphery. These results show that some tumours, at least, can respond directly to the effects of vasoactive agents.

Angiotensin II↗
PubMed1976

Enzyme variability in natural populations of Daphnia magna. IV. Ecological differentiation and frequency changes of genotypes at Audley End.

Genotypic frequencies were analysed for two years in a permanent population of the cladoceran crustacean, Daphnia magna, which was polymorphic for an esterase and for malate dehydrogenase. Large temporal changes in genotypic frequencies occurred at both loci. There was no evidence of a seasonal pattern in the frequency changes. In most samples, genotypes at the two enzyme loci were non-randomly associated; these associations showed temporal changes. On some occasions marked spatial heterogeneity in genotypic frequencies existed within the population. Genotypic differences in parthenogenetic and sexual egg production were observed. In a primarily parthenogenetically reproducing population, non-random associations between genotypes of structural and regulatory loci will be the rule. The allozyme variants themselves may or may not be under selection. The relevance of these observations to ecological studies on Daphnia is considered.

Animals↗
PubMed1993

Dynamic high-resolution MR imaging of brain deoxygenation during transient anoxia in the anesthetized rat.

Transient alterations in brain oxygenation during 60-s periods of anoxia were visualized at high spatial resolution (voxel size < or = 0.15 microliter) with the use of serial long echo time FLASH (fast low-angle shot) magnetic resonance images (measuring time > or = 6 s) of halothane-anesthetized rats in vivo. Difference images from normoxia and anoxia exploit the signal decrease associated with increased levels of paramagnetic deoxyhemoglobin in the arterial and venous blood pool. Insights into the spatial heterogeneity of oxygen deprivation are complemented by physiologic information from the time course of pertinent signal changes in different regions of the brain.

Animals↗
PubMed1995

Commentary and opinion: I. Principal component analysis, variance partitioning, and "functional connectivity".

We briefly review the need for careful study of "variance partitioning" and "optimal model selection" in functional positron emission tomography (PET) data analysis, emphasizing the use of principal component analysis (PCA) and the importance of data analytic techniques that allow for heterogeneous spatial covariance structures. Using an [15O]water dataset, we demonstrate that--even after data processing--the intrasubject signal component of primary interest in baseline activation studies constitutes a very small fraction of the intersubject variance. This small intrasubject variance component is subtly but significantly changed by using analysis of covariance instead of scaled subprofile model processing before applying PCA. Finally, we argue that the concept of "functional connectivity" should be interpreted very generally until the relative roles of inter- and intrasubject variability in both disease and normal PET datasets are better understood.

Analysis of Variance↗
PubMed2003

Volcanically emitted sodium chloride as a source for Io's neutral clouds and plasma torus.

The atmosphere of Jupiter's satellite Io is extremely tenuous, time variable and spatially heterogeneous. Only a few molecules--SO2, SO and S2--have previously been identified as constituents of this atmosphere, and possible sources include frost sublimation, surface sputtering and active volcanism. Io has been known for almost 30 years to be surrounded by a cloud of Na, which requires an as yet unidentified atmospheric source of sodium. Sodium chloride has been recently proposed as an important atmospheric constituent, based on the detection of chlorine in Io's plasma torus and models of Io's volcanic gases. Here we report the detection of NaCl in Io's atmosphere; it constitutes only approximately 0.3% when averaged over the entire disk, but is probably restricted to smaller regions than SO2 because of its rapid photolysis and surface condensation. Although the inferred abundance of NaCl in volcanic gases is lower than predicted, those volcanic emissions provide an important source of Na and Cl in Io's neutral clouds and plasma torus.

Journal Article↗
PubMed2003

Evolution of cooperation and conflict in experimental bacterial populations.

A fundamental problem in biology is the evolutionary transition from single cells to multicellular life forms. During this transition the unit of selection shifts from individual cells to groups of cooperating cells. Although there is much theory, there are few empirical studies. Here we describe an evolutionary transition that occurs in experimental populations of Pseudomonas fluorescens propagated in a spatially heterogeneous environment. Cooperating groups are formed by over-production of an adhesive polymer, which causes the interests of individuals to align with those of the group. The costs and benefits of cooperation, plus evolutionary susceptibility to defecting genotypes, were analysed to determine conformation to theory. Cooperation was costly to individuals, but beneficial to the group. Defecting genotypes evolved in populations founded by the cooperating type and were fitter in the presence of this type than in its absence. In the short term, defectors sabotaged the viability of the group; but these findings nevertheless show that transitions to higher orders of complexity are readily achievable, provide insights into the selective conditions, and facilitate experimental analysis of the evolution of individuality.

Bacterial Adhesion↗
PubMed2005

Non-equilibration of hydrostatic pressure in blebbing cells.

Current models for protrusive motility in animal cells focus on cytoskeleton-based mechanisms, where localized protrusion is driven by local regulation of actin biochemistry. In plants and fungi, protrusion is driven primarily by hydrostatic pressure. For hydrostatic pressure to drive localized protrusion in animal cells, it would have to be locally regulated, but current models treating cytoplasm as an incompressible viscoelastic continuum or viscous liquid require that hydrostatic pressure equilibrates essentially instantaneously over the whole cell. Here, we use cell blebs as reporters of local pressure in the cytoplasm. When we locally perfuse blebbing cells with cortex-relaxing drugs to dissipate pressure on one side, blebbing continues on the untreated side, implying non-equilibration of pressure on scales of approximately 10 microm and 10 s. We can account for localization of pressure by considering the cytoplasm as a contractile, elastic network infiltrated by cytosol. Motion of the fluid relative to the network generates spatially heterogeneous transients in the pressure field, and can be described in the framework of poroelasticity.

Actins↗
PubMed2005

Plant communities: ecosystem stability in Inner Mongolia.

Bai et al. suggest that in China's Inner Mongolia steppe, community-level stability arises from compensatory effects among the principal components at both the species and plant functional group (PFG) levels. By analysing a consistent 19-year data set (1980-98), we show here that their analysis of a 24-year field data set (1980-2003) is called into question by inconsistencies in sampling location and numbers after 1998; the authors' findings are further undermined because they do not distinguish temporal variation from spatial heterogeneity in analysing compensatory effects among species or PFGs. We believe that rigorous reanalysis is needed for a better understanding of grassland stability.

Biodiversity↗
PubMed2002

BDNF release from single cells elicits local dendritic growth in nearby neurons.

In cultured neurons, the exogenous application of neurotrophins (in homogenous concentrations) alters many features of axonal and dendritic arbors. In vivo, however, release of endogenous neurotrophins from neuronal processes creates spatially heterogeneous neurotrophin distributions. To probe the consequences of such endogenous neurotrophin distribution, we produced 'donor neurons' in ferret cortex brain slices that co-expressed brain-derived neurotrophic factor (BDNF) and red fluorescent protein (RFP). Using two-photon microscopy, we analyzed their effects on 'recipient neurons' that expressed green fluorescent protein (GFP) alone. BDNF released from dendrites and cell bodies acted directly on nearby recipient neurons to increase dendritic branching in a distance-dependent manner. Three-dimensional analysis of donor and recipient dendrites indicated that the BDNF source had to be within 4.5 microm to induce dendritic growth in the recipient neuron. Thus, BDNF released from an individual cell alters the structure of nearby dendrites on an exquisitely local scale.

Animals↗