Josiah Manson
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Streaming Surface Reconstruction Using Wavelets Abstract: We present a streaming method for reconstructing surfaces from large data sets generated by a laser range scanner using wavelets. Wavelets provide a localized, multiresolution representation of functions and this makes them ideal candidates for streaming surface reconstruction algorithms. We show how wavelets can be used to reconstruct the indicator function of a shape from a cloud of points with associated normals. Our method proceeds in several steps. We first compute a low-resolution approximation of the indicator function using an octree followed by a second pass that incrementally adds fine resolution details. The indicator function is then smoothed using a modified octree convolution step and contoured to produce the final surface. Due to the local, multiresolution nature of wavelets, our approach results in an algorithm over 10 times faster than previous methods and can process extremely large data sets in the order of several hundred million points in only an hour. |
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Effects of Nerve Injury and Segmental Regeneration on the Cellular Correlates of Neural Morphallaxis Abstract: Functional recovery of neural networks after injury requires a series of signaling events similar to the embryonic processes that governed initial network construction. Neural morphallaxis, a form of nervous system regeneration, involves reorganization of adult neural connectivity patterns. Neural morphallaxis in the worm, Lumbriculus variegatus, occurs during asexual reproduction and segmental regeneration, as body fragments acquire new positional identities along the anterior–posterior axis. Ectopic head (EH) formation, induced by ventral nerve cord lesion, generated morphallactic plasticity including the reorganization of interneuronal sensory fields and the induction of a molecular marker of neural morphallaxis. Morphallactic changes occurred only in segments posterior to an EH. Neither EH formation, nor neural morphallaxis was observed after dorsal body lesions, indicating a role for nerve cord injury in morphallaxis induction. Furthermore, a hierarchical system of neurobehavioral control was observed, where anterior heads were dominant and an EH controlled body movements only in the absence of the anterior head. Both suppression of segmental regeneration and blockade of asexual fission, after treatment with boric acid, disrupted the maintenance of neural morphallaxis, but did not block its induction. Therefore, segmental regeneration (i.e., epimorphosis) may not be required for the induction of morphallactic remodeling of neural networks. However, on-going epimorphosis appears necessary for the longterm consolidation of cellular and molecular mechanisms underlying the morphallaxis of neural circuitry. |
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Cooperative signaling among bacterial chemoreceptors Abstract: Four chemoreceptors in Escherichia coli mediate responses to chemicals in the environment. The receptors self-associate and localize to the cell poles. This aggregation implies that interactions among receptors are important parameters of signal processing during chemotaxis. We examined this phenomenon using a receptor-coupled in vitro assay of CheA kinase activity. The ability of homogeneous populations of the serine receptor Tsr and the aspartate receptor Tar to stimulate CheA was directly proportional to the ratio of the receptor to total protein in cell membranes up to a fraction of 50%. Membranes containing mixed populations of Tar and Tsr supported an up to 4-fold greater stimulation of CheA than expected on the basis of the contributions of the individual receptors. Peak activity was seen at a Tar:Tsr ratio of 1:4. This synergy was observed only when the two proteins were expressed simultaneously, suggesting that, under our conditions, the fundamental "cooperative receptor unit" is relatively static, even in the absence of CheA and CheW. Finally, we observed that inhibition of receptor-stimulated CheA activity by serine or aspartate required significantly higher concentrations of ligand for membranes containing mixed Tsr and Tar populations than for membranes containing only Tsr (up to 10(2)-fold more serine) or Tar (up to 10(4)-fold more aspartate). Together with recent analyses of the interactions of Tsr and Tar in vivo, our results reveal the emergent properties of mixed receptor populations and emphasize their importance in the integrated signal processing that underlies bacterial chemotaxis. |
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Bimetallic Cluster Complexes: The Synthesis, Structures, and Bonding of Ruthenium Carbonyl Cluster Complexes Containing Palladium and Platinum with the Bulky Tri-tert-butyl-phosphine Ligand Abstract: The bis-phosphine compounds M(PBut3)2, M = Pd and Pt, readily eliminate one PBut3 ligand and transfer MPBut3 groups to the ruthenium-ruthenium bonds in the compounds Ru3(CO)12, Ru6(CO)17(6-C), and Ru6(CO)14(6-C6H6)(6-C) without displacement of any of the ligands on the ruthenium complexes. The new compounds, Ru3(CO)12[Pd(PBut3)]3, 10, and Ru6(CO)17(6-C)[Pd(PBut3)]2, 11, Ru6(CO)17(6-C)[Pt(PBut3)]n, n = 1 (12), n = 2 (13), and Ru6(CO)14(6-C6H6)(6-C)[Pd(PBut3)]n, n = 1 (15), n = 2 (16), have been prepared and structurally characterized. In most cases the MPBut3 groups bridge a pair of mutually bonded ruthenium atoms, and the associated Ru-Ru bond distance increases in length. Fenske-Hall calculations were performed on 10 and 11 to develop an understanding of the electron deficient metal-metal bonding. 10 undergoes a Jahn-Teller distortion to increase bonding interactions between neighboring Ru(CO)4 and Pd(PBut3) fragments. 11 has seven molecular orbitals important to cluster bonding in accord with cluster electron-counting rules. |
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Nickel-Manganese Sulfido Carbonyl Cluster Complexes. Synthesis, Structure, and Properties of the Unusual Paramagnetic Complexes Cp2Ni2Mn(CO)3(m3-E)2, E = S, Se Abstract: The reaction of Mn2(CO)7(-S2) with [CpNi(CO)]2 yielded the paramagnetic new compound Cp2Ni2Mn(CO)3(3-S)2 (1) and a new hexanuclear metal product Cp2Ni2Mn4(CO)14(6-S2)(3-S)2 (2). Structurally, compound 1 contains two triply bridging sulfido ligands on opposite sides of an open Ni2Mn triangular cluster. EPR and temperature-dependent magnetic susceptibility measurements of 1 show that it contains one unpaired electron. The electronic structure of 1 was determined by Fenske-Hall molecular orbital calculations which show that the unpaired electron occupies a low lying antibonding orbital delocalized unequally across the three metal atoms. The selenium homologue Cp2Ni2Mn(CO)3(3-Se)2 (3) was obtained from the reaction of a mixture of Mn2(CO)10 and [CpNi(CO)]2 with elemental selenium and Me3NO·2H2O. It also has one unpaired electron. Compound 1 reacted with elemental sulfur to yield the dinickeldimanganese compound, Cp2Ni2Mn2(CO)6(4-S2)(4-S5), 4, which can also be made from the reaction of Mn2(CO)7(-S2) with [CpNi(CO)]2 and sulfur. Compound 4 was converted back to 1 by sulfur abstraction using PPh3. The reaction of Mn2(CO)10 with [CpNi(CO)]2 in the presence of thiirane yielded the ethanedithiolato compound CpNiMn(CO)3(-SCH2CH2S) (5), which was also obtained from the reaction of Mn4(CO)15(3-S2)(4-S2) with [CpNi(CO)]2 in the presence of thiirane. Compound 5 reacted with additional quantities of thiirane to yield the new compound CpNiMn(CO)3[-S(CH2CH2S)2], 6, which contains a 3-thiapentanedithiolato ligand that bridges the two metal atoms. Compound 6 was also obtained from the reaction of Mn2(CO)10 with [CpNi(CO)]2 and thiirane. The molecular structures of the new compounds 1-6 were established by single-crystal X-ray diffraction analyses. |
July 5, 2008