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At least 1,099 records · Page 61Linked to original sources
PubMed2002

Spatially resolved spectroscopy and structurally encoded imaging by magnetic resonance force microscopy of quadrupolar spin systems.

Enhancement of the sensitivity of magnetic resonance force microscopy allowed the extension of the technique to observe half-integer quadrupolar nuclei. This is demonstrated for various different compounds and nuclei with different spin numbers. The possibility of obtaining spatially localized spectral information through the quadrupole interaction is implemented by using nutation NMR. This enables us to superimpose a contrast function on the image of materials depending on their local lattice structure. This opens up new possibilities for both surface and subsurface studies in materials chemistry.

Journal Article↗
PubMed2003

Purely site-specific chemisorption and conformation of trimethylamine on Si(100)c(4 x 2).

One of the fundamental points of interest on the Si(100) surface is how the spatial localization of electron density on the buckled silicon dimer controls the site-specific reaction toward different Lewis acid and Lewis base molecules. We have investigated the adsorption of trimethylamine (TMA) on Si(100)c(4x2) using scanning tunneling microscopy (STM) at 80 K. The adsorbed TMA appears as a triangle-shaped bright protrusion in the occupied-state STM image. The triangle-shaped protrusion is ascribed to three methyl groups in the adsorbed TMA. The center of the protrusion is located on the down atom site, which indicates that the adsorption of TMA occurs only on the down dimer atom. Thus, TMA adsorption on Si(100)c(4x2) is found to be purely site-specific on the down dimer atom and can be categorized in Lewis acid-base reaction.

Journal Article↗
PubMed2004

Computational elucidation of the transition state shape selectivity phenomenon.

The most commonly cited example of a transition state shape selective reaction, m-xylene disproportionation in zeolites, is examined to determine if the local spatial environment of a reaction can significantly alter selectivity. In the studied reaction, ZPE-corrected rate limiting energy barriers are 136 kJ/mol for the methoxide-mediated pathway and 109 to 145 kJ/mol for the diphenylmethane-mediated pathway. Both pathways are likely to contribute to selectivity and disfavor one product isomer (1,3,5-trimethylbenzene), but relative selectivity to the other two isomers varies with pore geometry, mechanistic pathway, and inclusion of entropic effects. Most importantly, study of one pathway in three different common zeolite framework types (FAU, MFI, and MOR) allows explicit and practically oriented consideration of pore shape. Variation of the environment shape at the critical transition states is thus shown to affect the course of reaction. Barrier height shifts on the order of 10-20 kJ/mol are achievable. Observed selectivities do not agree with the transition state characteristics calculated here and, hence, are most likely due to product shape selectivity. Further examination of the pathways highlights the importance of mechanistic steps that do not result in isomer-defining bonds and leads to a more robust definition of transition state shape selectivity.

Journal Article↗
PubMed2003

Chemical imaging of microstructures of plant tissues within cellular dimension using synchrotron infrared microspectroscopy.

Synchrotron radiation-based Fourier transform infrared microspectroscopy (SR-FTIR) is an advanced bioanalytical technique capable of exploring the chemistry within microstructures of plant and animal tissues with a high signal to noise ratio at high ultraspatial resolutions (3-10 microm) without destruction of the intrinsic structures of a tissue. This technique is able to provide information relating to the quantity, composition, structure, and distribution of chemical constituents and functional groups in a tissue. The objective of this study was to illustrate how the SR-FTIR technique can be used to image inherent structures of plant tissues on a cellular level (pixel size, approximately 10 microm x 10 microm). The results showed that with the extremely bright synchrotron light, spectra with high signal to noise ratios were obtained from areas as small as 10 microm x 10 microm in the plant tissue, which allowed us to "see" plant tissue in a chemical sense on a cellular level. The ultraspatial resolved imaging of plant tissues by stepping in pixel-sized increments was obtained. Chemical distributions of plant tissues such as lignin, cellulose, protein, lipid, and total carbohydrate could be mapped. These images revealed the chemical information of plant intrinsic structure. In conclusion, SR-FTIR can provide chemical and functional characteristics of plant tissue at high ultraspatial resolutions. The SR-FTIR microspectroscopic images can generate spatially localized functional group and chemical information within cellular dimensions.

Hordeum↗
PubMed2004

Using synchrotron transmission FTIR microspectroscopy as a rapid, direct, and nondestructive analytical technique to reveal molecular microstructural-chemical features within tissue in grain barley.

The objective of this study was to use synchrotron transmission FTIR microspectroscopy as a rapid, direct, and nondestructive analytical technique to reveal molecular microstructural-chemical features within tissue in grain barley. The results showed that synchrotron transmission FTIR microspectroscopy could provide spectral, chemical, and functional group characteristics of grain barley tissue at ultrahigh spatial resolutions. The spatially localized structural-chemical distributions of biological components (lignin, cellulose, protein, lipid, and carbohydrates) and biological component ratios could be imaged. Such information on molecular microstructural-chemical features within the tissue can be used for plant breeding programs for selecting superior varieties of barley for special purposes and for prediction of grain barley quality and nutritive value for humans and animals.

Carbohydrates↗
PubMed2005

Realization of a particle-in-a-box: electron in an atomic Pd chain.

Well-defined Pd chains were assembled from single atoms on a NiAl(110) surface with the tip of a scanning tunneling microscope. The electronic properties of the chains were determined by spatially resolved conductance measurements, revealing a series of quantum well states with parabolic dispersion. The particle-in-a-box states in Pd chains show higher onset energy and larger effective mass than those in Au chains investigated before, reflecting the influence of elemental composition on one-dimensional electronic systems. The intrinsic widths and spectral intensities of Pd induced states provide information on lifetime and spatial localization of states in the atomic chain.

Letter↗
PubMed2005

Self-assembled, mesoporous polymeric networks for patterned protein arrays.

A facile, self-assembly approach to the fabrication of a robust, mesoporous, biocompatible polymeric network for the spatial organization of proteins is described. Surface-deposited poly(styrene) (PS) beads that assemble into a two-dimensional (2-D) hexagonal array are used to template cross-linked poly(vinyl alcohol) (PVA), yielding an inverse opal structure. The porous, water insoluble network is used to entrain a model, soluble protein, green fluorescent protein (GFP). The polymeric network is characterized by atomic force microscopy (AFM) and optical microscopy, and the spatial localization of the incorporated GFP is determined by fluorescence microscopy. The results demonstrate that this system may constitute a versatile platform for the lateral organization of biomolecules.

Green Fluorescent Proteins↗
PubMed2006

Defect distribution along single GaN nanowhiskers.

In this letter we report on spectral photoconductivity (PC) on different sections of single MBE-grown GaN nanowhiskers of diameters ranging on the order of 100 nm. The photoconductivity spectra show, besides the band-gap related transition, deep-levels corresponding to the yellow, green, and blue bands. A strong spatial localization of specific photocurrent peaks has been observed, indicating that the defects responsible for such transitions are distributed inhomogeneously along the column growth direction.

Electric Conductivity↗
PubMedDate not supplied

MALDI mass spectrometry for direct tissue analysis: a new tool for biomarker discovery.

The direct analysis of tissue sections by MALDI mass spectrometry holds tremendous potential for biomarker discovery. This technology routinely allows many hundreds of proteins to be detected over a mass range of approximately 2000-70 000 Da while maintaining the spatial localization of the proteins detected. This technology has been applied to a wide range of tissue samples, including human glioma tissue and human lung tumor tissue. In many cases, biostatistical analyses of the resulting protein profiles revealed patterns that correlated with disease state and/or clinical endpoints. This work serves as a review of recent applications and summarizes the current state of technology.

Animals↗
PubMed2005

The influence of cue type on backward inhibition.

Backward inhibition is proposed as a process of lateral inhibition that operates during response selection in task switching, reducing interference caused by the most recently abandoned task set. The effect has been observed across a wide range of contexts but is eliminated by using spatial location to cue tasks (K. D. Arbuthnott & T. S. Woodward, 2002). The present studies replicated this finding, showing that spatial cues are also associated with greater response congruity than verbal cues, consistent with the lateral inhibition model. Spatial cues may introduce greater discriminability between competing category-response rules, reducing the need for lateral inhibition. However, when participants named the task before target presentation, backward inhibition was observed with spatial cues, suggesting that verbalization increased competition between sets, despite spatial localization.

Adolescent↗
PubMed1994

Subjective landmarks in perception and memory for spatial location.

Four experiments investigated the use of cognitive strategies for encoding spatial location in visual figures. Subjects reproduced the position of a dot in a square figure that had distance markers placed along two sides. Subjects' responses were biased toward imaginary points of intersection formed by the distance markers when subjects responded from memory (Experiment 1) or while viewing the figures (Experiment 2). Dots located at an intersection point were reproduced more accurately than those located off an intersection. These findings demonstrate that empty regions of a figure can serve as subjective landmarks for spatial localization. In Experiment 3, dot relocation was found to be similarly distorted toward physical cross marks placed at the intersections of distance markers, supporting the landmark hypothesis. The attraction of dots to intersection points depends on the viewer employing a strategy of mentally projecting from distance markers to form imaginary intersections, which makes intersection points salient landmarks for coding location of nearby stimulus dots. In Experiment 4, attraction toward intersection points was observed only when subjects employed the projection strategy and not when instructed to use a different encoding strategy.

Adult↗
PubMed1999

The arrangement of the three cone classes in the living human eye.

Human colour vision depends on three classes of receptor, the short- (S), medium- (M), and long- (L) wavelength-sensitive cones. These cone classes are interleaved in a single mosaic so that, at each point in the retina, only a single class of cone samples the retinal image. As a consequence, observers with normal trichromatic colour vision are necessarily colour blind on a local spatial scale. The limits this places on vision depend on the relative numbers and arrangement of cones. Although the topography of human S cones is known, the human L- and M-cone submosaics have resisted analysis. Adaptive optics, a technique used to overcome blur in ground-based telescopes, can also overcome blur in the eye, allowing the sharpest images ever taken of the living retina. Here we combine adaptive optics and retinal densitometry to obtain what are, to our knowledge, the first images of the arrangement of S, M and L cones in the living human eye. The proportion of L to M cones is strikingly different in two male subjects, each of whom has normal colour vision. The mosaics of both subjects have large patches in which either M or L cones are missing. This arrangement reduces the eye's ability to recover colour variations of high spatial frequency in the environment but may improve the recovery of luminance variations of high spatial frequency.

Cell Count↗
PubMed1983

Caulobacter flagellin mRNA segregates asymmetrically at cell division.

Molecular processes which promote the spatial localization of subcellular components are fundamental to cell development and differentiation. At various stages in development unequal segregation of molecular information must occur to result in the differentiated characteristics which distinguish cell progeny. Biological attributes of the dimorphic bacterium, Caulobacter crescentus, provide an experimental system permitting examination of the generation of asymmetry at the molecular level. When a Caulobacter cell divides, two different daughter cells are produced--a motile swarmer cell with a polar flagellum and a non-motile cell with a static appendage referred to as a stalk. The two cell types are distinct with respect to surface morphology, developmental potential, protein composition and biosynthetic capabilities. One of the more conspicuous manifestations of asymmetric expression of macromolecules in this system, the flagellum, has been studied extensively. We have cloned the flagellin genes of Caulobacter and report here the use of these sequences as probes to demonstrate that (1) the level of flagellin mRNA is regulated during the cell cycle in a pattern coincident with flagellum polypeptide synthesis and (2) flagellin mRNA synthesized before cell division is segregated with progeny swarmer cells. This provides molecular evidence of specific partitioning of an mRNA at the time of cell division.

Bacteria↗
PubMedDate not supplied

Electrophysiological correlates of hyperacuity in the human visual cortex.

The human visual system is capable of detecting a vernier misalignment with extraordinary accuracy. Since this remarkable precision in spatial localization is better than can be naively predicted by simple optical or anatomical considerations it has been termed a hyperacuity. So far no single neurone model seems capable of accounting for hyperacuity, and the retinal image might require reconstitution in a finer grained from in the visual cortex. We report here an electrophysiological correlate of hyperacuity recorded from the human visual cortex. The amplitude of the visually evoked potentials (v.e.ps) elicited by the appearance of a vernier offset varied systematically with the magnitude of the offset. Extrapolation of the function relating v.e.p. amplitude and log offset to zero voltage resulted in an electrophysiological estimate of vernier acuity that was similar to the observer's psychophysical threshold.

Electroencephalography↗
PubMedDate not supplied

Electrogenic glutamate uptake is a major current carrier in the membrane of axolotl retinal glial cells.

Glutamate is taken up avidly by glial cells in the central nervous system. Glutamate uptake may terminate the transmitter action of glutamate released from neurons, and keep extracellular glutamate at concentrations below those which are neurotoxic. We report here that glutamate evokes a large inward current in retinal glial cells which have their membrane potential and intracellular ion concentrations controlled by the whole-cell patch-clamp technique. This current seems to be due to an electrogenic glutamate uptake carrier, which transports at least two sodium ions with every glutamate anion carried into the cell. Glutamate uptake is strongly voltage-dependent, decreasing at depolarized potentials: when fully activated, it contributes almost half of the conductance in the part of the glial cell membrane facing the retinal neurons. The spatial localization, glutamate affinity and magnitude of the uptake are appropriate for terminating the synaptic action of glutamate released from photoreceptors and bipolar cells. These data challenge present explanations of how the b-wave of the electroretinogram is generated, and suggest a mechanism for non-vesicular voltage-dependent release of glutamate from neurons.

Ambystoma↗
PubMed1988

Molecular manipulation using a tunnelling microscope.

In a very short time the scanning tunnelling microscope has become an important tool in surface science, and physics in general. Its primary use has been to obtain atomic-resolution images of surfaces, but recently, efforts have been made to use it to manipulate materials as well as image them. One may now reasonably ask if it is possible to move and alter matter predictably on an atomic scale. Here we report the accomplishment of the smallest yet, purposeful, spatially localized changes in matter, effected on a graphite surface. We believe that the changes result from the pinning of individual organic molecules to the graphite. The reverse manipulation, the removal of pinned molecules, has also been demonstrated. Finally, we have evidence that we can remove a portion of a pinned molecule, effectively performing transformations on single molecules using the tunnelling microscope.

Microscopy, Electron, Scanning↗
PubMed1989

Alpha-adrenergic inhibition of sympathetic neurotransmitter release mediated by modulation of N-type calcium-channel gating.

In sympathetic neurons, catecholamines interact with prejunctional alpha-adrenergic receptors to reduce delivery of transmitter to postjunctional target organs. This autoinhibitory feedback is a general phenomenon seen in diverse neurons containing a variety of transmitters. The underlying mechanisms of alpha-adrenergic inhibition are not clear, although decreases in cyclic AMP and cAMP-mediated phosphorylation have been implicated. We have studied depolarization-induced catecholamine release and calcium-channel currents in frog sympathetic neurons. Here we show that alpha-adrenergic inhibition of transmitter release can be explained by inhibition of Ca2+-channel currents and not by modulation of intracellular proteins. Noradrenaline strongly reduces the activity of N-type Ca2+ channels, the dominant calcium entry pathway triggering sympathetic transmitter release, whereas L-type Ca2+ channels are not significantly inhibited. The down-modulation of N-type channels involves changes in rapid gating kinetics but not in unitary flux. This is the first detailed description of inhibition of a high-voltage activated neuronal Ca2+ channel at the single-channel level. The coupling between alpha-adrenergic receptors and N-type channels involves a G protein, but not a readily diffusible cytoplasmic messenger or protein kinase C, and may be well suited for rapid and spatially localized feedback-control of transmitter release.

Animals↗
PubMed1990

Postsynaptic NMDA receptor-mediated calcium accumulation in hippocampal CA1 pyramidal cell dendrites.

In the CA1 hippocampal region, intracellular calcium is a putative second messenger for the induction of long-term potentiation (LTP), a persistent increase of synaptic transmission produced by high frequency afferent fibre stimulation. Because LTP in this region is blocked by the NMDA (N-methyl-D-aspartate) receptor antagonist AP5 (DL-2-amino-5-phosphonovaleric acid) and the calcium permeability of NMDA receptors is controlled by a voltage-dependent magnesium block, a model has emerged that suggests that the calcium permeability of NMDA receptor-coupled ion channels is the biophysical basis for LTP induction. We have performed microfluorometric measurements in individual CA1 pyramidal cells during stimulus trains that induce LTP. In addition to a widespread component of postsynaptic calcium accumulation previously described, we now report that brief high frequency stimulus trains produce a transient component spatially localized to dendritic areas near activated afferents. This localized component is blocked by the NMDA receptor antagonist AP5. The results directly confirm the calcium rise predicted by NMDA receptor models of LTP induction.

2-Amino-5-phosphonovalerate↗