Showing papers by "University of Bath published in 1998"

Photonic Band Gap Guidance in Optical Fibers

Abstract: A fundamentally different type of optical waveguide structure is demonstrated, in which light is confined to the vicinity of a low-index region by a two-dimensional photonic band gap crystal. The waveguide consists of an extra air hole in an otherwise regular honeycomb pattern of holes running down the length of a fine silica glass fiber. Optical fibers based on this waveguide mechanism support guided modes with extraordinary properties.

Vertebrate genome evolution and the zebrafish gene map.

Abstract: In chordate phylogeny, changes in the nervous system, jaws, and appendages transformed meek filter feeders into fearsome predators. Gene duplication is thought to promote such innovation. Vertebrate ancestors probably had single copies of genes now found in multiple copies in vertebrates and gene maps suggest that this occurred by polyploidization. It has been suggested that one genome duplication event occurred before, and one after the divergence of ray-finned and lobe-finned fishes. Holland et al., however, have argued that because various vertebrates have several HOX clusters, two rounds of duplication occurred before the origin of jawed fishes. Such gene-number data, however, do not distinguish between tandem duplications and polyploidization events, nor whether independent duplications occurred in different lineages. To investigate these matters, we mapped 144 zebrafish genes and compared the resulting map with mammalian maps. Comparison revealed large conserved chromosome segments. Because duplicated chromosome segments in zebrafish often correspond with specific chromosome segments in mammals, it is likely that two polyploidization events occurred prior to the divergence of fish and mammal lineages. This zebrafish gene map will facilitate molecular identification of mutated zebrafish genes, which can suggest functions for human genes known only by sequence.

Vascular pericytes express osteogenic potential in vitro and in vivo.

Abstract: At postconfluence, cultured bovine pericytes isolated from retinal capillaries form three-dimensional nodule-like structures that mineralize. Using a combination of Northern and Southern blotting, in situ hybridization, and immunofluorescence we have demonstrated that this process is associated with the stage-specific expression of markers of primitive clonogenic marrow stromal cells (STRO-1) and markers of cells of the osteoblast lineage (bone sialoprotein, osteocalcin, osteonectin, and osteopontin). To demonstrate that the formation of nodules and the expression of these proteins were indicative of true osteogenic potential, vascular pericytes were also inoculated into diffusion chambers and implanted into athymic mice. When recovered from the host, chambers containing pericytes were found reproducibly to contain a tissue comprised of cartilage and bone, as well as soft fibrous connective tissue and cells resembling adipocytes. This is the first study to provide direct evidence of the osteogenic potential of microvascular pericytes in vivo. Our results are also consistent with the possibility that the pericyte population in situ serves as a reservoir of primitive precursor cells capable of giving rise to cells of multiple lineages including osteoblasts, chondrocytes, adipocytes, and fibroblasts.

Large mode area photonic crystal fibre

Abstract: The authors report the realisation of a new design for a large mode area monomode optical fibre. This photonic crystal fibre will guide only a single mode, no matter how large the fibre diameter, provided the shape is kept constant. This is demonstrated with a fibre which has a core diameter equal to approximately 50 free-space wavelengths.

The Genetic Code Is One in a Million

Abstract: Statistical and biochemical studies of the genetic code have found evidence of nonrandom patterns in the distribution of codon assignments. It has, for example, been shown that the code minimizes the effects of point mutation or mistranslation: erroneous codons are either synonymous or code for an amino acid with chemical properties very similar to those of the one that would have been present had the error not occurred. This work has suggested that the second base of codons is less efficient in this respect, by about three orders of magnitude, than the first and third bases. These results are based on the assumption that all forms of error at all bases are equally likely. We extend this work to investigate (1) the effect of weighting transition errors differently from transversion errors and (2) the effect of weighting each base differently, depending on reported mistranslation biases. We find that if the bias affects all codon positions equally, as might be expected were the code adapted to a mutational environment with transition/transversion bias, then any reasonable transition/transversion bias increases the relative efficiency of the second base by an order of magnitude. In addition, if we employ weightings to allow for biases in translation, then only 1 in every million random alternative codes generated is more efficient than the natural code. We thus conclude not only that the natural genetic code is extremely efficient at minimizing the effects of errors, but also that its structure reflects biases in these errors, as might be expected were the code the product of selection.

Surface Structure and Morphology of Calcium Carbonate Polymorphs Calcite, Aragonite, and Vaterite: An Atomistic Approach

Abstract: Atomistic simulation techniques have been employed to investigate the effect of molecular adsorption of water on the low-index surfaces of calcite, aragonite, and vaterite. Calculated surface and hydration energies agree with experiment and previous calculations where available. Known experimental surface features are reproduced, i.e., 1 × 1 symmetry and structural features of the calcite {1014} surface and bulk termination of the {1011} and {1120} surfaces. Surface carbonate groups tend to rotate to lie flat in the surface. The morphologies of the hydrated crystals agree with experimentally found morphologies. The bulk lattice energies of the polymorphs reflect their thermodynamic stability.

Properties of photonic crystal fiber and the effective index model

Abstract: We report on the waveguiding properties of a new type of low-loss optical waveguide. The photonic crystal fiber can be engineered to support only the fundamental guided mode at every wavelength within the transparency window of silica. Experimentally, a robust single mode has been observed over a wavelength range from 337nm to beyond 1550nm (restricted only by available wavelength sources). By studying the number of guided modes for fibers with different parameters and the use of an effective index model we are able to quantify the requirements for monomode operation. The requirements are independent of the scale of the fiber for sufficiently short wavelengths. Further support for the predictions of the effective index model is given by the variation of the spot size with wavelength,

When somatic information threatens, catastrophic thinking enhances attentional interference

Abstract: Styles of catastrophic thinking about pain have been related to an inability to divert attention away from pain. We investigated whether pain catastrophizers displayed high attentional interference during a threatening low-intensity electrocutaneous stimulus (ES). In Experiment 1, 44 undergraduates performed a tone discrimination task whilst experiencing several times an ES on the left or right arms. Tones were also presented 250 ms and 750 ms after ES onset. Participants were threatened that a high-intensity painful stimulus would occur at one site. As predicted, pain catastrophizers displayed pronounced task interference immediately after threat stimulus onset. In Experiment 2, threat was induced in 36 undergraduates by informing them that an ES excites pain fibres. Again, catastrophizers had marked interference immediately after onset. The results are discussed in terms of how catastrophizing amplifies somatosensory information and primes fear mechanisms.

Group-velocity dispersion in photonic crystal fibers.

Abstract: The dispersion properties of photonic crystal fibers are calculated by expression of the modal field as a sum of localized orthogonal functions. Even simple designs of these fibers can yield zero dispersion at wavelengths shorter than 1.27 µm when the fibers are single mode, or a large normal dispersion that is suitable for dispersion compensation at 1.55 µm.

An Analysis of Corporate Donations: United Kingdom Evidence

Abstract: Drawing a framework from stakeholder theory, this study uses 1994 data drawn from 100 United Kingdom listed companies to test empirically whether the level of discretionary donations made by companies to charitable, social and political causes is related to four company-specific factors, namely leverage, company size, profitability and ownership structure Consistent with our hypotheses, the results indicate that the decision to contribute funds to charities and other bodies is positively related to company size and profitability and negatively related to leverage However, the study provides no support for the view that there is a link between discretionary donations and a company's ownership structure

Robust hedging of the lookback option

Abstract: The aim of this article is to find bounds on the prices of exotic derivatives, and in particular the lookback option, in terms of the (market) prices of call options. This is achieved without making explicit assumptions about the dynamics of the price process of the underlying asset, but rather by inferring information about the potential distribution of asset prices from the call prices. Thus the bounds we obtain and the associated hedging strategies are model independent. The appeal and significance of the hedging strategies arises from their universality and robustness to model mis-specification.

Complete Models with Stochastic Volatility

Abstract: The paper proposes an original class of models for the continuous-time price process of a financial security with nonconstant volatility. The idea is to define instantaneous volatility in terms of exponentially weighted moments of historic log-price. The instantaneous volatility is therefore driven by the same stochastic factors as the price process, so that, unlike many other models of nonconstant volatility, it is not necessary to introduce additional sources of randomness. Thus the market is complete and there are unique, preference-independent options prices. We find a partial differential equation for the price of a European call option. Smiles and skews are found in the resulting plots of implied volatility.

Investigating the relationship between pain and discomfort and quality of life, using the WHOQOL.

Abstract: The aim of this study was to examine the impact of pain on quality of life and its components in a representative sample of 320 well people, and patients selected from all major categories of illness. Quality of life was assessed using a new, multidimensional, multilingual, generic profile designed for cross-cultural use in health care, i.e. the WHOQOL. Within the WHOQOL, pain and discomfort is one of 29 areas or facets of quality of life, grouped into six domains. It was found that pain and discomfort made a significant impact on perceptions of general quality of life related to health. Furthermore, the presence of pain affected perceptions of five of the six domains of quality of life; the domain of spirituality, religion and personal beliefs being the exception. When quality of life is assessed, negative feelings are most closely associated with reports of pain and discomfort than any other facet. But quality of life surrounding pain and discomfort is more fully explained by the inclusion of six additional facets; the availability of social care, mobility, activities of daily living, positive mood and to a lesser extent, sleep and dependence on medication. Together, these seven facets represent criteria against which the success of pain treatments may be evaluated. As predicted, those who were pain-free had significantly better quality of life than those in pain. A longer duration of pain is associated with increasingly poorer quality of life. Intense affective pain is particularly detrimental to a good quality of life. The psychometric properties of the pain and discomfort facet of the WHOQOL and WHOQOL-100 were assessed. Internal consistency (reliability), discriminant and criterion/concurrent validity were found to be good to excellent, justifying the use of this instrument with a range of chronic and acute pain patients.

Regulation of Hox gene expression and posterior development by the Xenopus caudal homologue Xcad3.

Abstract: The caudal gene codes for a homeodomain transcription factor that is required for normal posterior development in Drosophila. In this study the biological activities of the Xenopus caudal (Cdx) family member Xcad3 are examined. A series of domain-swapping experiments demonstrate that the N-terminus of Xcad3 is necessary for it to activate Hox gene expression and that this function can be replaced by the activation domain from the viral protein VP16. In addition, experiments using an Xcad3 repressor mutant (XcadEn-R), which potently blocks the activity of wild-type Xcad3, are reported. Overexpression of XcadEn-R in embryos inhibits the activation of the same subset of Hox genes that are activated by wild-type Xcad3 and leads to a dramatic disruption of posterior development. We show that Xcad3 is an immediate early target of the FGF signalling pathway and that Xcad3 posteriorizes anterior neural tissue in a similar way to FGF. Furthermore, Xcad3 is required for the activation of Hox genes by FGFs. These data provide strong evidence that Xcad3 is required for normal posterior development and that it regulates the expression of the Hox genes downstream of FGF signalling.

Molecular-dynamics simulation of MgO surfaces in liquid water using a shell-model potential for water

Abstract: A shell-model water potential, derived to be compatible with existing potential models for inorganic solids, is introduced. It reproduces experimental data of the water monomer and dimer such as structure, dipole moment, and binding energy. Properties of liquid water, especially ordering and energetics, are in adequate agreement with experiment. The polarizable water potential model is used to model the interaction of liquid water with MgO surfaces. Adsorption of the first monolayer to the surfaces is shown to disrupt ordering in the next layers, leading to decreased water density near the solid surface.

Chemokines: understanding their role in T-lymphocyte biology

Abstract: The chemokines are a complex superfamily of small, secreted proteins that were initially characterized through their chemotactic effects on a variety of leucocytes. The superfamily is divided into families based on structural and genetic considerations and have been termed the CXC, CC, C and CX3C families. Chemokines from these families have a key role in the recruitment and function of T lymphocytes. Moreover, T lymphocytes have also been identified as a source of a number of chemokines. T lymphocytes also express most of the known CXC and CC chemokine receptors to an extent that depends on their state of activation/differentiation and/or the activating stimuli. The expression of two chemokine receptors, namely CXCR4 and CCR5, together with the regulated production of their respective ligands, appears to be extremely important in determining sensitivity of T cells to HIV-1 infection. The intracellular events which mediate the effects of chemokines, particularly those elicited by the CC chemokine RANTES, include activation of both G-protein- and protein tyrosine kinase-coupled signalling pathways. The present review describes our current understanding of the structure and expression of chemokines and their receptors, the effects of chemokines on T-cell function(s), the intracellular signalling pathways activated by chemokines and the role of certain chemokines and chemokine receptors in determining sensitivity to HIV-1 infection.

Electron confinement to nanoscale ag islands on ag(111) : a quantitative study

Abstract: Lateral confinement has been suggested as a possible mechanism for depopulation of surface state levels and concomitant modification of metal surface related properties. Studies to date give conflicting accounts of the effectiveness of confinement due to scattering at step edges. We present a quantitative study of surface states on nanoscale Ag islands on Ag(111), using low-temperature scanning tunneling microscopy and electronic structure calculations. These results confirm the validity of the confinement picture down to the smallest of island sizes.

Structural basis for the recognition of carbohydrates by human galectin-7.

Abstract: Knowledge about carbohydrate recognition domains of galectins, formerly known as S-type animal lectins, is important in understanding their role(s) in cell-cell interactions. Here we report the crystal structure of human galectin-7 (hGal-7), in free form and in the presence of galactose, galactosamine, lactose, and N-acetyl-lactosamine at high resolution. This is the first structure of a galectin determined in both free and carbohydrate-bound forms. The structure shows a fold similar to that of the prototype galectins -1 and -2, but has greater similarity to a related galectin molecule, Gal-10. Even though the carbohydrate-binding residues are conserved, there are significant changes in this pocket due to shortening of a loop structure. The monomeric hGal-7 molecule exists as a dimer in the crystals, but adopts a packing arrangement considerably different from that of Gal-1 and Gal-2, which has implications for carbohydrate recognition.