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At least 1,153 records · Page 64Linked to original sources
PubMed1998

Dielectrophoretic ratchets.

We have experimentally applied some concepts of "force-free" motion to micron size particles (latex beads). The coupling of dissipation and local spatial asymmetry of the potential experienced by the beads can put them into motion. The potentials used in these experiments are of dielectrophoretic nature. To that end, electrodes of particular shapes were used in order to submit the considered suspensions to inhomogeneous ac electric fields. Two regimes were explored: i-the Brownian ratchet case in which a Brownian particle is successively trapped in a factory roof-like potential and left free to diffuse. ii-the shifted ratchets case in which two potentials exhibiting similar characteristics are applied successively, one of them being shifted by a fraction of their common period relatively to the other. In both cases, a good agreement with the theoretical predictions was observed. In particular, particles of different sizes were characterized by different macroscopic velocities leading to the prospect of promising separation techniques. (c) 1998 American Institute of Physics.

Journal Article↗
PubMed2004

Scaled opposite-spin second order Møller-Plesset correlation energy: an economical electronic structure method.

A simplified approach to treating the electron correlation energy is suggested in which only the alpha-beta component of the second order Møller-Plesset energy is evaluated, and then scaled by an empirical factor which is suggested to be 1.3. This scaled opposite-spin second order energy (SOS-MP2), where MP2 is Møller-Plesset theory, yields results for relative energies and derivative properties that are statistically improved over the conventional MP2 method. Furthermore, the SOS-MP2 energy can be evaluated without the fifth order computational steps associated with MP2 theory, even without exploiting any spatial locality. A fourth order algorithm is given for evaluating the opposite spin MP2 energy using auxiliary basis expansions, and a Laplace approach, and timing comparisons are given.

Algorithms↗
PubMed2005

Non-Born-Oppenheimer treatment of the H2 Hookean molecule.

We show that the exact non-Born-Oppenheimer Schrodinger equation for the Hookean diatomic molecule H2 (a two-proton, two-electron system where the electron-proton interaction is harmonic while the proton-proton and electron-electron interactions are Coulombic) can be decoupled into equations describing the relative motion of the electrons, the relative motion of nuclei, the motion of a collective mode representing a three-dimensional harmonic oscillator, and the motion of a free particle expressed as a linear combination of the individual center-of-mass coordinates of the nuclei and electrons. Analytic solutions to the relative motion of electrons can be readily obtained for the given values of the harmonic coupling constant. However, exact analytic solutions to the equation for the relative motion of the nuclei cannot be obtained simultaneously due to the fact that the harmonic constants in these two equations are coupled. For this reason, we present for the relative nuclear motion approximate analytic wave functions, one of them obtained variationally and the other by a series solution where the coefficients are determined recursively. We also explore a variational solution to the Taylor-series expansion of the nuclear interaction potential. Properties of the electronic and nuclear intracule densities are examined at different values of the coupling constant. An interesting result of the present non-Born-Oppenheimer treatment of this harmonic model is the fact that the relative nuclear motion occurs in a highly correlated regime. This leads in a natural way to a spatial localization of the nuclei akin to Wigner electronic crystallization.

Journal Article↗
PubMed2006

Embedding wave function theory in density functional theory.

We present a framework for embedding a highly accurate coupled-cluster calculation within a larger density functional calculation. We use a perturbative buffer to help insulate the coupled-cluster region from the rest of the system. Regions are defined, not in real space, but in Hilbert space, though connection between the two can be made by spatial localization of single-particle orbitals. Relations between our embedding approach and some similar techniques are discussed. We present results for small sample systems for which we can extract essentially exact results, demonstrating that our approach seems to work quite well and is generally more reliable than some of the related approaches due to the introduction of additional interaction terms.

Journal Article↗
PubMed2006

The nature of the hydrogen bond: a synthesis from the interacting quantum atoms picture.

The interacting quantum atoms approach [IQA, as presented by Blanco et al., J. Chem. Theory Comput. 1, 1096 (2005)] is applied to standard hydrogen bonded dimers. IQA is an interpretation tool based on a real space energy decomposition scheme fully consistent with the quantum theory of atoms in molecules. It provides a partition of every physical term present in the Hamiltonian into atomic and interatomic contributions. The procedure is orbital-free and self-contained, needing neither external references nor artificial intermediate states. Binding is the result of a competition between the destabilizing deformations suffered by the interacting fragments upon interaction and the stabilizing interaction energy itself. According to IQA, there is no incompatibility between the prevalent electrostatic image of hydrogen bonded systems and that favoring important covalent contributions. Depending on how we gather the different energetic terms, we may recover electrostatic or covalent pictures from the same underlying quantum mechanical description. Our results show that the nonclassical contributions to hydrogen bonding are spatially localized, involving only the H atom and its two nearest neighbors. IQA is well suited as a comparative tool. Its thin energetic decomposition allows us to recover exactly (or to a very good approximation) the quantities of the most widely used energy decomposition schemes. Such a comparison sheds light on the virtues and faults of the different methods and on the origin of the 50 years old debate regarding the covalent/electrostatic nature of the hydrogen bond.

Journal Article↗
PubMed2003

NADPH oxidase activation in fibronectin adherent human neutrophils: A potential role for beta1 integrin ligation.

BACKGROUND: Hydrogen peroxide production by human neutrophils is tightly coupled to integrin ligation. This provides a means for spatial localization of oxidant production. However, integrin specificity and consequent signaling mechanisms for this process remain undefined. In the present study we demonstrate that otherwise unstimulated neutrophils adherent to fibronectin, a beta(1) ligand, but not fibrinogen, a beta(2) ligand, produce hydrogen peroxide in a time-dependent fashion. We hypothesized that signaling proceeded through focal, adhesionlike structures. METHODS: Triton-X insoluble (actin cytoskeleton) fractions from suspension phase cells and cells adherent to the beta(1) integrin ligand fibronectin were assessed for the presence of the integrin-associated kinase Pyk2, the scaffolding protein paxillin, and the downstream Src family kinase Lyn. Lysates were subjected to immunoprecipitation with anti-phosphotyrosine and probed for Pyk2, paxillin, and Lyn. Associations between focal adhesion kinase (FAK) and paxillin were determined by immunoprecipitation with anti-FAK and probing with anti-paxillin antibody. Activation of NADPH oxidase was determined by demonstration of redistribution of p47(phox) in Triton-X insoluble fractions. RESULTS: NADPH oxidase activation, as judged by H(2)O(2) production, occurred with fibronectin-, but not in suspension or fibrinogen-adherent cells. Cells adherent to fibronectin for 20 minutes demonstrated marked increases in Pyk2, paxillin, and Lyn activation in comparison to fibronectin-adherent and suspension phase cells. CONCLUSIONS: This study demonstrates that fibronectin adherence is a significant initiating factor for NADPH oxidase assembly in human neutrophils. This appears to be mediated by beta(1) integrins. We demonstrate formation of focal adhesions containing Pyk2/FAK, paxillin, and Lyn, and translocation of p47(phox).

Cell Adhesion↗
PubMed1990

Stereo and motion cues in preattentive vision processing--some experiments with random-dot stereographic image sequences.

Low-level preattentive vision processing is of special interest since it seems the logical starting point of all vision processing. Exploration of the human visual processing system at this level is, however, extremely difficult, but can be facilitated by the use of stroboscopic presentation of sequences of random-dot stereograms, which contain only local spatial and temporal information and therefore limit the processing of these images to the low level. Four experiments are described in which such sequences were used to explore the relationships between various cues (optical flow, stereo disparity, and accretion and deletion of image points) at the low level. To study these relationships in more depth, especially the resolution of conflicting information among the cues, some of the image sequences presented information not usually encountered in 'natural' scenes. The results indicate that the processing of these cues is undertaken as a set of cooperative processes.

Attention↗
PubMed1992

A case of a dual frame of reference.

It is demonstrated that observers may relate to two parts of the same object by using two different frames of reference. Subjects were asked to indicate directions within a model of a hallway in which signs were posted on a single prism. The majority of subjects interpreted a sign frontally facing them as indicating the direction which is ahead of them, yet they interpreted an adjacent sign that was slanted with respect to the frontal plane as indicating a direction which is at the same side of the line of sight as the sign is. This manifests a mixture of an egocentric and an object-centred frame of reference, that is reminiscent of the mixture of local spatial interpretations in impossible pictures. It is suggested that frames of reference are not necessarily unique in a given percept, and that they are not derived from a global computation.

Cognition↗
PubMed1998

Selective attention to specific location cues: the peak and center of a patch are equally accessible as location cues.

Asymmetric patterns have several spatially distinct cues for spatial localization. These cues include the peak of the luminance distribution, the centroid of the contrast distribution, zero-crossings in the second derivative of the luminance profile, and the midpoint of the visible area. If these cues are represented as primitives in the visual system, the observer should be able to access them at will. To examine whether observers can selectively attend to particular cues, we measured perceived alignment for an asymmetric pattern with two distinct instructions: "align the peak", and "align the center". We found that observers could align the patterns in accord with the instructions with identical precision, suggesting that the peak and the center cues were equally accessible by the observer. We conclude that multiple localization cues are represented in and can be selectively accessed by the visual system.

Computer Graphics↗
PubMed2001

Immunolocalization of retinol-binding protein, cellular retinoic acid-binding protein I and retinoid X receptor beta in the porcine reproductive tract during the oestrous cycle.

Retinoid-binding proteins and nuclear receptors are expressed in the reproductive tissues of different species and their expression is hormonally regulated. In the present study, we demonstrated immunocytochemically the temporal and spatial localization of retinol-binding protein (RBP), cellular retinoic acid-binding protein I (CRABPI) and retinoid X receptor beta (RXRbeta) in porcine ovary, oviduct and uterus during the oestrous cycle. RBP and CRABPI were localized in the cytoplasm, whereas RXRbeta occurred in the nucleus. RBP was not detected in either the ovary or the oviduct at any stage of the oestrous cycle. CRABPI was present in luteal cells of the ovary only during dioestrus and in glandular and ciliated cells of the oviduct during oestrus. In the ovary, RXRbeta was always present in granulosa cells and germinal epithelium, with highest levels observed during oestrus. In the uterus, RXRbeta was present throughout the cycle in both the endometrium and the myometrium. However, changes in RXRbeta were observed in the endometrium, with highest levels observed during dioestrus. RBP and CRABPI could be observed in the endometrium only during dioestrus. The results show that the occurrence of retinoid-binding proteins and nuclear receptors in individual tissues of the reproductive tract are strongly dependent on the stage of the oestrous cycle. In the oviduct, the expression of CRABPI seems to be dependent on oestrogen, whereas in the uterus the expression of RBP and CRABPI is influenced by progesterone. The association of expression in different sections of the reproductive tissues investigated shows that the presence of specific proteins involved in retinoid metabolism was dependent on events associated with ovulation, the migration of the oocyte through the oviduct and the possible implantation of the blastocyst into the uterus.

Animals↗
PubMed2006

Imaging peptidoglycan biosynthesis in Bacillus subtilis with fluorescent antibiotics.

The peptidoglycan (PG) layers surrounding bacterial cells play an important role in determining cell shape. The machinery controlling when and where new PG is made is not understood, but is proposed to involve interactions between bacterial actin homologs such as Mbl, which forms helical cables within cells, and extracellular multiprotein complexes that include penicillin-binding proteins. It has been suggested that labeled antibiotics that bind to PG precursors may be useful for imaging PG to help determine the genes that control the biosynthesis of this polymer. Here, we compare the staining patterns observed in Bacillus subtilis using fluorescent derivatives of two PG-binding antibiotics, vancomycin and ramoplanin. The staining patterns for both probes exhibit a strong dependence on probe concentration, suggesting antibiotic-induced perturbations in PG synthesis. Ramoplanin probes may be better imaging agents than vancomycin probes because they yield clear staining patterns at concentrations well below their minimum inhibitory concentrations. Under some conditions, both ramoplanin and vancomycin probes produce helicoid staining patterns along the cylindrical walls of B. subtilis cells. This sidewall staining is observed in the absence of the cytoskeletal protein Mbl. Although Mbl plays an important role in cell shape determination, our data indicate that other proteins control the spatial localization of the biosynthetic complexes responsible for new PG synthesis along the walls of B. subtilis cells.

Actins↗
PubMed2006

Visual saliency and texture segregation without feature gradient.

A central notion in the study of texture segregation is that of feature gradient (or feature contrast). In orientation-based texture segregation, orientation gradients have indeed played a fundamental role in explaining behavioral results. Here, however, we show that general, smoothly varying, orientation-defined textures (ODTs) exhibit striking perceptual singularities that are completely unpredictable from orientation gradients. These singularities defy not only popular texture segregation theories but also virtually all computational segmentation methods, and they confound previous behavioral studies with smoothly varying ODTs. We provide psychophysical evidence that perceptual singularities in smooth ODTs are salient visual features consistent across observers and with significant effect on the perception and segregation of oriented textures. We further show that, although orientation gradients cannot predict them, perceptual singularities in smooth ODTs emerge directly from, and can be spatially localized by, two ODT curvatures. Given the traditional role of feature gradients in early vision, the significance of these findings extends well beyond orientation-based texture segregation to issues ranging from curve integration and fragment grouping, through the perception of 3D shape, to the functional organization of the primary visual cortex.

Depth Perception↗
PubMed2002

Imaging cells and extracellular matrix in vivo by using second-harmonic generation and two-photon excited fluorescence.

Multiphoton microscopy relies on nonlinear light-matter interactions to provide contrast and optical sectioning capability for high-resolution imaging. Most multiphoton microscopy studies in biological systems have relied on two-photon excited fluorescence (TPEF) to produce images. With increasing applications of multiphoton microscopy to thick-tissue "intravital" imaging, second-harmonic generation (SHG) from structural proteins has emerged as a potentially important new contrast mechanism. However, SHG is typically detected in transmission mode, thus limiting TPEF/SHG coregistration and its practical utility for in vivo thick-tissue applications. In this study, we use a broad range of excitation wavelengths (730-880 nm) to demonstrate that TPEF/SHG coregistration can easily be achieved in unstained tissues by using a simple backscattering geometry. The combined TPEF/SHG technique was applied to imaging a three-dimensional organotypic tissue model (RAFT). The structural and molecular origin of the image-forming signal from the various tissue constituents was determined by simultaneous spectroscopic measurements and confirming immunofluorescence staining. Our results show that at shorter excitation wavelengths (<800 nm), the signal emitted from the extracellular matrix (ECM) is a combination of SHG and TPEF from collagen, whereas at longer excitation wavelengths the ECM signal is exclusively due to SHG. Endogenous cellular signals are consistent with TPEF spectra of cofactors NAD(P)H and FAD at all excitation wavelengths. The reflected SHG intensity follows a quadratic dependence on the excitation power, decays exponentially with depth, and exhibits a spectral dependence in accordance with previous theoretical studies. The use of SHG and TPEF in combination provides complementary information that allows noninvasive, spatially localized in vivo characterization of cell-ECM interactions in unstained thick tissues.

Animals↗
PubMed2026

Ultrasound-driven mechanophore activation in living plants.

This study presents a biocompatible, ultrasound-responsive platform for remotely activating mechanochemical reactions within live plant tissue. Fluorogenic Mechanophore-embedded silica NanoParticles (FMNPs) that are thermally stable were engineered to emit blue fluorescence at 440 nm upon mechanical activation. In Solanum lycopersicum (tomato) leaves, activation was achieved through the synergistic combination of gas vesicles (GVs) and high-frequency focused ultrasound (FUS, 550 kHz), enabling spatially localized and minimally invasive stimulation. Low-frequency ultrasound (25 kHz) triggered activation but caused extensive tissue damage, while high-frequency FUS alone was biocompatible yet insufficient to activate FMNPs. Incorporation of GVs as a cavitation amplifier significantly boosted activation efficiency under mild acoustic conditions without observable tissue disruption. In planta fluorescence imaging confirmed that FMNPs retained their functionality after injection into leaf vasculature, and only the combination of GV and FUS produced a statistically significant fluorescence increase, indicating successful mechanochemical activation. This represents a demonstration of noninvasive and biocompatible ultrasound-induced mechanophore activation in live plants. This modular and noninvasive strategy opens possibilities for programmable release of regulatory and metabolic chemicals, biosensing, and synthetic molecular control in plant systems.

Plant Leaves↗
PubMed1980

Endogenous electrical currents in the water mold Blastocladiella emersonii during growth and sporulation.

We have explored the pattern of electrical currents generated by single cells of the water mold Blastocladiella emersonii at several stages of its life cycle. Extracellular currents were measured with a vibrating probe constructed after the design of Jaffe and Nuccitelli [Jaffe, L. F. & Nuccitelli, R. (1974) J. Cell Biol. 63, 614-628]. In growing cells positive current, of the order of 1 microA/cm2, enters the rhizoid and leaves from the thallus; circumstantial evidence suggests that protons carry much of the current. Sporulation is associated with reversal of the current pattern, such that positive current enters the thallus and leaves from the rhizoidal region; the ions that carry the current have not been identified. These current patterns appear to play a role in the spatial localization of fungal growth and development.

Blastocladiella↗
PubMed1980

Compositional domain structure in phosphatidylcholine--cholesterol and sphingomyelin--cholesterol bilayers.

the lateral distribution of cholesterol in phospholipid bilayers has been investigated through a method of Monte Carlo calculations, using interaction energies deduced from calorimetric results for cholesterol-phospholipid mixtures. Analysis of computer-generated bilayer configurations allows calculation of the spatial localization and relative abundance of distinct regions of varying cholesterol content along the plane of the bilayer. An interfacial phospholipid region between cholesterol-bound and cholesterol-free domains is found to extend one lipid beyond the cholesterol-bound domain for mixtures of cholesterol with palmitoyl sphingomyelin, lignoceroyl sphingomyelin, and dipalmitoyl phosphatidylcholine. The results indicate that the degree of nonideality in the mixing of cholesterol is dependent on fatty acid chain length and that cholesterol mixes more ideally in sphingomyelins than in phosphatidylcholines of equal chain length. It is found that at approximately 20 mol % cholesterol the cholesterol-rich areas suddenly become connected, forming a network that extends over the entire bilayer. This change in the lateral connectivity of the cholesterol-rich domains occurs over a narrow concentration interval and is presumably responsible for the abrupt change in the lateral diffusion coefficient observed at this concentration.

Cholesterol↗
PubMed1987

Single-channel recordings demonstrate that cGMP opens the light-sensitive ion channel of the rod photoreceptor.

Patch-clamp recordings were made from outer segments of single dark-adapted rod photoreceptors from toad retina. When patch pipettes were filled with a solution free of divalent cations, inward current through individual light-sensitive ion channels was sufficiently large to allow single-channel recording. Channel activity was suppressed by illumination within the normal response range of the dark-adapted rod. When illumination was restricted to a portion of the outer segment, the effect of light on channel activity was spatially localized to the illuminated region. Hyperpolarization of the recorded patch did not reduce the frequency of channel opening, indicating that the suppression of channel activity by light was due to illumination itself and not to light-induced hyperpolarization of the rod. After recording single light-sensitive channel activity in the intact cell, the patch of membrane was detached to form an excised, inside-out patch. Cyclic GMP applied to the intracellular face of the excised patch then opened a channel that appeared to be the same as the light-sensitive channel recorded earlier from the same membrane in the intact, functioning rod. This provides direct evidence that cyclic GMP acts as an internal transmitter that opens the light-sensitive channel of the vertebrate photoreceptor.

Animals↗
PubMed1987

Activation of a potassium current by rapid photochemically generated step increases of intracellular calcium in rat sympathetic neurons.

Although Ca2+ is a well-established intracellular messenger, there are many questions concerning the kinetics and spatial localization of its effects. Such problems may now be approached with the photosensitive Ca2+ chelator nitr-5. The Ca2+ affinity of this molecule decreases by a factor of 40 after absorption of near-UV light; Ca2+ is liberated with a time constant of approximately equal to 300 microseconds. Nitr-5 or the related compounds nitr-2 and nitr-7, complexed with Ca2+, were introduced into rat sympathetic ganglion cells by dialysis from a patch pipette electrode operating in the whole-cell, voltage-clamp mode. Light flashes released Ca2+ and activated a K+ current. Flash-induced current relaxations followed a simple exponential time course with time constants as brief as 5 ms. Comparison of the kinetics among the chelators, which photolyze at different rates, suggests that release of Ca2+ from nitr-5 is too fast to limit the relaxation. Thus we confirm directly that Ca2+ can modulate membrane properties within a few milliseconds after entering a cell. A preliminary kinetic description of K+ current activation by Ca2+ in rat sympathetic neurons is presented; Ca2+ appears to bind to the channel with a rate constant of at least 2 X 10(7) M-1 X s-1.

Animals↗