1,634 research outputs found
Experimental Synthetic Aperture Radar with Dynamic Metasurfaces
We investigate the use of a dynamic metasurface as the transmitting antenna
for a synthetic aperture radar (SAR) imaging system. The dynamic metasurface
consists of a one-dimensional microstrip waveguide with complementary electric
resonator (cELC) elements patterned into the upper conductor. Integrated into
each of the cELCs are two diodes that can be used to shift each cELC resonance
out of band with an applied voltage. The aperture is designed to operate at K
band frequencies (17.5 to 20.3 GHz), with a bandwidth of 2.8 GHz. We
experimentally demonstrate imaging with a fabricated metasurface aperture using
existing SAR modalities, showing image quality comparable to traditional
antennas. The agility of this aperture allows it to operate in spotlight and
stripmap SAR modes, as well as in a third modality inspired by computational
imaging strategies. We describe its operation in detail, demonstrate
high-quality imaging in both 2D and 3D, and examine various trade-offs
governing the integration of dynamic metasurfaces in future SAR imaging
platforms
Toward the synthesis of an acetal-free Tn antigen anti-cancer vaccine candidate
Cancer vaccines are a promising approach to cancer treatment by activating the immune system towards cancer cells. The challenge, however, comes with the identification of biomarkers correlated with cancer. Unfortunately, many oncotargets are simply upregulated in cancer; these are inappropriate vaccine targets as they are also present on healthy cells and therefore would ultimately initiate a very dangerous systemic immune response. In contrast, Tumor-Associated Carbohydrate Antigens (TACAs) are not found in healthy adult tissue but are found on over 90% of biopsied carcinomas. When incorporated into glycopeptides or other immunogenic scaffolds, these carbohydrate antigens have formed the basis of the development of anti-tumor immunotherapies through the induction of a specific immune response against cancer cells. However, despite promising preliminary data, none of these candidates have reached the clinic. Our hypothesis is that the carbohydrates on the vaccines may not survive antigen processing, and so more stable versions of these materials are required to create viable vaccines.
Our approach is to incorporate TACAs lacking the labile glycosidic bond: acetal-free carbohydrates (AFCs). This project aims to remove the unstable acetal functional group to yield a more robust carbohydrate structure that can then be incorporated into a vaccine candidate. In this presentation, two synthetic pathways will be described: conversion of a carbohydrate to a carbasugar through a rearrangement, and de novo synthesis from noncarbohydrate materials
A concise synthesis of a methyl ester 2-resorcinarene: A chair-conformation macrocycle
Anions are important hydrogen bond acceptors in a range of biological, chemical, environ-mental and medical molecular recognition processes. These interactions have been exploited for the design and synthesis of ditopic resorcinarenes as the hydrogen bond strength can be tuned through the modification of the substituent at the 2-position. However, many potentially useful compounds, especially those incorporating electron-withdrawing functionalities, have not been prepared due to the challenge of their synthesis: their incorporation slows resorcinarene formation that is accessed by electrophilic aromatic substitution. As part of our broader campaign to employ resorcinarenes as selective recognition elements, we need access to these specialized materials. In this article, we report a straightforward synthetic pathway for obtaining a 2-(carboxymethyl)-resorcinarene, and resorcinarene esters in general. We discuss the unusual conformation it adopts and propose that this arises from the electron-withdrawing nature of the ester substituents that renders them better hydrogen bond acceptors than the phenols, ensuring that each of them acts as a donor only. Density Functional Theory (DFT) calculations show that this conformation arises as a consequence of the unusual configurational isomerism of this compound and interruption of the archetypal hydrogen bonding by the ester functionality
Resolved Spectroscopy of the Narrow-Line Region in NGC 1068. II. Physical Conditions Near the NGC 1068 ``Hot-Spot''
The physical conditions near the optical continuum peak (``hot spot'') in the
inner narrow line region (NLR) of the Seyfert 2 galaxy, NGC 1068. Spectra were
taken with HST/STIS through the 0.1X52 arcsec slit, covering the full STIS 1200
to 10000 Angstrom waveband, and are from a region that includes the hot spot,
extending 0.2, or ~ 14 pc (for H= 75 km/sec/Mpc). Perhaps the most striking
feature of these spectra is the presence of strong coronal emission lines,
including [S XII] 7611 which has hitherto only been identified in spectra of
the solar corona. There is an apparent correlation between ionization energy
and velocity of the emission lines with respect to the systemic velocity of the
host galaxy, with the coronal lines blueshifted, most other high excitation
lines near systemic, and some of the low ionization lines redshifted. From the
results of our modeling, we find that the emission-line gas consists of three
principal components: 1) one in which most of the strong emission-lines, such
as [O III] 5007, [Ne V] 3426, C IV 1550, arise, 2) a more tenuous, highly
ionized component, which is the source of the coronal-line emission, and 3) a
component, which is not co-planar with the other two, in which the low
ionization and neutral lines, such as [N II] 6548 and [O I] 6300, are formed.
The first two components are directly ionized by the EUV-Xray continuum emitted
by the central source, while the low ionization gas is ionized by a combination
of highly absorbed continuum radiation and a small fraction of unabsorbed
continuum scattered by free electrons associated with the hot spot. The
combination of covering factor and Thomson optical depth of the high ionization
components is insufficient to scatter the observed fraction of continuum
radiation into our line-of-sight.Comment: 42 pages, Latex, includes 5 figures (postscript), to appear in the
Astrophysical Journa
A new measure based on degree distribution that links information theory and network graph analysis
BACKGROUND: Detailed connection maps of human and nonhuman brains are being generated with new technologies, and graph metrics have been instrumental in understanding the general organizational features of these structures. Neural networks appear to have small world properties: they have clustered regions, while maintaining integrative features such as short average pathlengths. RESULTS: We captured the structural characteristics of clustered networks with short average pathlengths through our own variable, System Difference (SD), which is computationally simple and calculable for larger graph systems. SD is a Jaccardian measure generated by averaging all of the differences in the connection patterns between any two nodes of a system. We calculated SD over large random samples of matrices and found that high SD matrices have a low average pathlength and a larger number of clustered structures. SD is a measure of degree distribution with high SD matrices maximizing entropic properties. Phi (Ί), an information theory metric that assesses a systemâs capacity to integrate information, correlated well with SD - with SD explaining over 90% of the variance in systems above 11 nodes (tested for 4 to 13 nodes). However, newer versions of Ί do not correlate well with the SD metric. CONCLUSIONS: The new network measure, SD, provides a link between high entropic structures and degree distributions as related to small world properties
Characterization of metal ion-induced [3H]inositol hexakisphosphate binding to rat cerebellar membranes
The binding of [3H]inositol hexakisphosphate ([3H] InsP6) to rat cerebellar membranes has been characterized with the objective of establishing the role, if any, of a membrane protein receptor. In the presence of EDTA, we have previously identified an InsP6-binding site with a capacity of approximately 20 pmol/mg protein (Hawkins, P. T., Reynolds, D. J. M., Poyner, D. R., and Hanley, M. R. (1990) Biochem. Biophys. Res. Commun. 167, 819-827). However, in the presence of 1 mM Mg2+, the capacity of [3H]InsP6 binding to membranes was increased approximately 9-fold. This enhancing effect of Mg2+ was reversed by addition of 10 microM of several cation chelators, suggesting that the increased binding required trace quantities of other metal cations. This is supported by experiments where it was possible to saturate binding by addition of excess membranes, despite not significantly depleting radioligand, pointing to removal of some other factor. Removal of endogenous cations from the binding assay by pretreatment with chelex resin also prevents the Mg(2+)-induced potentiation. Consideration of the specificity of the chelators able to abolish this potentiation suggested involvement of Fe3+ or Al3+. Both these ions (but not several others) were able to increase [3H]InsP6 binding to chelex-pretreated membranes at concentrations of 1 microM. It is possible to demonstrate synergy between Fe3+ and Mg2+ under these conditions. We propose that [3H]InsP6 may interact with membranes through non-protein recognition possibly via phospholipids, in a manner dependent upon trace metals. The implications of this for InsP6 biology are considered
Intrinsic Absorption Lines in Seyfert 1 Galaxies. I. Ultraviolet Spectra from the Hubble Space Telescope
We present a study of the intrinsic absorption lines in the ultraviolet
spectra of Seyfert 1 galaxies. We find that the fraction of Seyfert 1 galaxies
that show absorption associated with their active nuclei is more than one-half
(10/17), which is much higher than previous estimates (3 - 10%) . There is a
one-to-one correspondence between Seyferts that show intrinsic UV absorption
and X-ray ``warm absorbers''. The intrinsic UV absorption is generally
characterized by high ionization: C IV and N V are seen in all 10 Seyferts with
detected absorption (in addition to Ly-alpha), whereas Si IV is present in only
four of these Seyferts, and Mg II absorption is only detected in NGC 4151. The
absorption lines are blueshifted (or in a few cases at rest) with respect to
the narrow emission lines, indicating that the absorbing gas is undergoing net
radial outflow. At high resolution, the absorption often splits into distinct
kinematic components that show a wide range in widths (20 - 400 km/s FWHM),
indicating macroscopic motions (e.g., radial velocity subcomponents or
turbulence) within a component. The strong absorption components have cores
that are much deeper than the continuum flux levels, indicating that the
regions responsible for these components lie completely outside of the broad
emission-line regions. The covering factor of the absorbing gas in the line of
sight, relative to the total underlying emission, is C > 0.86, on average. The
global covering factor, which is the fraction of emission intercepted by the
absorber averaged over all lines of sight, is C > 0.5.Comment: 56 pages, Latex, includes 4 figures (encapsulated postscript), Fig. 1
has 2 parts and Fig. 2 has 3 parts, to appear in the Astrophysical Journa
An X-Ray View of the Jet-Cycle in the Radio Loud AGN 3C120
We present a study of the central engine in the broad-line radio galaxy 3C120
using a multi-epoch analysis of a deep XMM-Newton observation and two deep
Suzaku pointings (in 2012). In order to place our spectral data into the
context of the disk-disruption/jet-ejection cycles displayed by this object, we
monitor the source in the UV/X-ray bands, and in the radio band. We find three
statistically acceptable spectral models, a disk-reflection model, a jet-model
and a jet+disk model. Despite being good descriptions of the data, the
disk-reflection model violates the radio constraints on the inclination, and
the jet-model has a fine-tuning problem, requiring a jet contribution exceeding
that expected. Thus, we argue for a composite jet+disk model. Within the
context of this model, we verify the basic predictions of the jet-cycle
paradigm, finding a truncated/refilling disk during the Suzaku observations and
a complete disk extending down to the innermost stable circular orbit (ISCO)
during the XMM-Newton observation. The idea of a refilling disk is further
supported by the detection of the ejection of a new jet knot approximately one
month after the Suzaku pointings. We also discover a step-like event in one of
the Suzaku pointings in which the soft band lags the hard band. We suggest that
we are witnessing the propagation of a disturbance from the disk into the jet
on a timescale set by the magnetic field.Comment: 14 pages, 10 figures, accepted for publication in Ap
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