			Home About us Contact

Relative Locations (relative + locations)

Distribution by Scientific Domains

Chemistry	50%

Selected Abstracts

Elements of a neurobiological theory of hippocampal function: the role of synaptic plasticity, synaptic tagging and schemas

EUROPEAN JOURNAL OF NEUROSCIENCE, Issue 11 2006
R. G. M. MorrisArticle first published online: 8 JUN 200
Abstract The 2004 EJN Lecture was an attempt to lay out further aspects of a developing neurobiological theory of hippocampal function [Morris, R.G.M., Moser, E.I., Riedel, G., Martin, S.J., Sandin, J., Day, M. & O'Carroll, C. (2003) Phil. Trans. R. Soc. Lond. B Biol. Sci., 358, 773,786.] These are that (i) activity-dependent synaptic plasticity plays a key role in the automatic encoding and initial storage of attended experience; (ii) the persistence of hippocampal synaptic potentiation over time can be influenced by other independent neural events happening closely in time, an idea with behavioural implications for memory; and (iii) that systems-level consolidation of memory traces within neocortex is guided both by hippocampal traces that have been subject to cellular consolidation and by the presence of organized schema in neocortex into which relevant newly encoded information might be stored. Hippocampal memory is associative and, to study it more effectively than with previous paradigms, a new learning task is described which is unusual in requiring the incidental encoding of flavour,place paired associates, with the readout of successful storage being successful recall of a place given the flavour with which it was paired. NMDA receptor-dependent synaptic plasticity is shown to be critical for the encoding and intermediate storage of memory traces in this task, while AMPA receptor-mediated fast synaptic transmission is necessary for memory retrieval. Typically, these rapidly encoded traces decay quite rapidly over time. Synaptic potentiation also decays rapidly, but can be rendered more persistent by a process of cellular consolidation in which synaptic tagging and capture play a key part in determining whether or not it will be persistent. Synaptic tags set at the time of an event, even many trivial events, can capture the products of the synthesis of plasticity proteins set in train by events before, during or even after an event to be remembered. Tag,protein interactions stabilize synaptic potentiation and, by implication, memory. The behavioural implications of tagging are explored. Finally, using a different protocol for flavour,place paired associate learning, it is shown that rats can develop a spatial schema which represents the relative locations of several different flavours of food hidden at places within a familiar space. This schema is learned gradually but, once acquired, enables new paired associates to be encoded and stored in one trial. Their incorporation into the schema prevents rapid forgetting and suggests that schema play a key and hitherto unappreciated role in systems-level memory consolidation. The elements of what may eventually mature into a more formal neurobiological theory of hippocampal memory are laid out as specific propositions with detailed conceptual discussion and reference to recent data. [source]

New insights from reactive transport modelling: the formation of the sericitic vein envelopes during early hydrothermal alteration at Butte, Montana

GEOFLUIDS (ELECTRONIC), Issue 3 2002
S. Geiger
Abstract A reactive transport computer code has been employed to model hydrothermal alteration of a granitoid rock bordering a discrete vein channel. The model suggests that the grey sericitic and sericitic with remnant biotite alteration envelopes at the porphyry copper deposit at Butte, Montana, can be formed by a reducing, low pH, and low salinity fluid under constant temperature and pressure conditions of approximately 400 °C and less than 100 MPa during a time span of approximately 100 years or less. Hydrothermal alteration has little effect on the porosity of the host rock (Butte Quartz Monzonite), and the diffusivity of the aqueous species also changes little. A sequence of mineral reaction fronts characterizes the alteration envelopes. The biotite dissolution front occurs closest to the vein channel and marks the transition from the grey sericitic to sericitic with remnant biotite envelope. The plagioclase dissolution front occurs farthest into the matrix and marks the edge of relatively fresh Butte Quartz Monzonite. From the properties of the quasi-stationary state approximation (Lichtner 1988; Lichtner 1991), it follows that once the sequence of reaction fronts is fully established, their relative locations remain constant and the widths of the reaction zones increase with the square root of time. [source]

A statistically derived parameterization for the collagen triple-helix

PROTEIN SCIENCE, Issue 11 2002
Jan K. Rainey
Abstract The triple-helix is a unique secondary structural motif found primarily within the collagens. In collagen, it is a homo- or hetero-tripeptide with a repeating primary sequence of (Gly-X-Y)n, displaying characteristic peptide backbone dihedral angles. Studies of bulk collagen fibrils indicate that the triple-helix must be a highly repetitive secondary structure, with very specific constraints. Primary sequence analysis shows that most collagen molecules are primarily triple-helical; however, no high-resolution structure of any entire protein is yet available. Given the drastic morphological differences in self-assembled collagen structures with subtle changes in assembly conditions, a detailed knowledge of the relative locations of charged and sterically bulky residues in collagen is desirable. Its repetitive primary sequence and highly conserved secondary structure make collagen, and the triple-helix in general, an ideal candidate for a general parameterization for prediction of residue locations and for the use of a helical wheel in the prediction of residue orientation. Herein, a statistical analysis of the currently available high-resolution X-ray crystal structures of model triple-helical peptides is performed to produce an experimentally based parameter set for predicting peptide backbone and C, atom locations for the triple-helix. Unlike existing homology models, this allows easy prediction of an entire triple-helix structure based on all existing high-resolution triple-helix structures, rather than only on a single structure or on idealized parameters. Furthermore, regional differences based on the helical propensity of residues may be readily incorporated. The parameter set is validated in terms of the predicted bond lengths, backbone dihedral angles, and interchain hydrogen bonding. [source]

Inclusion Behavior of ,-Cyclodextrin with Bipyridine Molecules: Factors Governing Host-Guest Inclusion Geometries

CHEMISTRY - AN ASIAN JOURNAL, Issue 3 2009
Yan-Li Zhao Dr.
Abstract Guest Effect: The differences of nitrogen atom positions and the bridge bonds linked to two pyridine rings of some bipyridine guests can significantly affect the binding abilities and inclusion geometries of ,-cyclodextrin with the guests in both the solution and solid states. The 1:1 complexation of ,-cyclodextrin (,-CD) with structurally similar bipyridine guests which lead to the formation of six inclusion complexes (1,6) of ,-CD with 4,4,-vinylenedipyridine, 2,2,-vinylenedipyridine, 1-(2-pyridyl)-2-(4-pyridyl)ethylene, 4,4,-ethylene-dipyridine, 4,4,-dithiodipyridine, and 2,2,-dithiodipyridine has been investigated comprehensively by X-ray crystallography in the solid state and by 1H,NMR spectroscopy and microcalorimetric titration in aqueous solution. The complex formation constants (KS) for the stoichiometric 1:1 host,guest inclusion complexation of ,-CD with the bipyridine derivatives were determined in aqueous solution by microcalorimetry and the host,guest inclusion geometries of the complexes were deduced from 1H ROESY NMR spectroscopy. It transpires that the guest bipyridine molecules are included in the ,-CD cavity with a range of different inclusion geometries. In the solid state, the crystal superstructures for the ,-CD complexes 1, 4, and 5 are characterized by the triclinic crystal system (space group P1) commensurate with AAAA type supramolecular aggregation. By contrast, the ,-CD complexes 2, 3, and 6 display either monoclinic (space group P21) or orthorhombic (space group C2221) crystal systems, characteristic of ABAB type supramolecular aggregation. The results demonstrate that the relative locations of the nitrogen atom positions and the bridge-bond links between the two pyridine rings in these bipyridine guests, not only lead to distinct crystal systems and space groups, but also to different binding geometries and thermodynamical parameters on complexation of the bipyridines with ,-CD. The knowledge obtained from this research improves our understanding of the molecular recognition and self-assembly processes exhibited by ,-CD, both in the solid state and in aqueous solution. [source]