29 research outputs found
Husain-Kuchar model as a constrained BF theory
The Husain-Kuchar theory is a four-dimensional background-independent model
that has long been viewed as a useful model for addressing several conceptual
and technical problems appearing in the quantization of general relativity
mainly in the loop quantum gravity approach. The model was defined at
Lagrangian level in terms of a su(2)-valued connection one-form coupled
through its curvature to a su(2)-valued one-form field . We address here the
problem of writing a Lagrangian formulation for the Husain-Kuchar model as a
constrained BF theory motivated by the fact that spin foam models for quantum
gravity are related to action principles of the BF type. The Lagrangian action
principle for the Husain-Kuchar model reported here differs from a previous one
found by Barbero et al in that this description involves a single constrained
BF theory rather than two interacting BF theories. It is, essentially, the
Plebanski action with the condition on the trace of the Lagrange multipliers
removed. Moreover, it can be stated that the relationship between our BF-like
action and the original one for the Husain-Kuchar model is the same
relationship that exists between the Plebanski action and the self-dual
Palatini action for complex general relativity, first because the solution to
the constraint on the two-forms coming from the BF-like action leads
to the Husain-Kuchar action, and second because the Hamiltonian analysis of the
Husain-Kuchar model is straightforward starting from the BF-like action
principle.Comment: new version, 5 pages, no figures, LaTeX fil
Design Considerations for the Implementation of a Mobile IP Telephony System in a Nuclear Power Plant
Guidelines for the use and interpretation of assays for monitoring autophagy (3rd edition)
In 2008 we published the first set of guidelines for standardizing research in autophagy. Since then, research on this topic has continued to accelerate, and many new scientists have entered the field. Our knowledge base and relevant new technologies have also been expanding. Accordingly, it is important to update these guidelines for monitoring autophagy in different organisms. Various reviews have described the range of assays that have been used for this purpose. Nevertheless, there continues to be confusion regarding acceptable methods to measure autophagy, especially in multicellular eukaryotes. For example, a key point that needs to be emphasized is that there is a difference between measurements that monitor the numbers or volume of autophagic elements (e.g., autophagosomes or autolysosomes) at any stage of the autophagic process versus those that measure fl ux through the autophagy pathway (i.e., the complete process including the amount and rate of cargo sequestered and degraded). In particular, a block in macroautophagy that results in autophagosome accumulation must be differentiated from stimuli that increase autophagic activity, defi ned as increased autophagy induction coupled with increased delivery to, and degradation within, lysosomes (inmost higher eukaryotes and some protists such as Dictyostelium ) or the vacuole (in plants and fungi). In other words, it is especially important that investigators new to the fi eld understand that the appearance of more autophagosomes does not necessarily equate with more autophagy. In fact, in many cases, autophagosomes accumulate because of a block in trafficking to lysosomes without a concomitant change in autophagosome biogenesis, whereas an increase in autolysosomes may reflect a reduction in degradative activity. It is worth emphasizing here that lysosomal digestion is a stage of autophagy and evaluating its competence is a crucial part of the evaluation of autophagic flux, or complete autophagy. Here, we present a set of guidelines for the selection and interpretation of methods for use by investigators who aim to examine macroautophagy and related processes, as well as for reviewers who need to provide realistic and reasonable critiques of papers that are focused on these processes. These guidelines are not meant to be a formulaic set of rules, because the appropriate assays depend in part on the question being asked and the system being used. In addition, we emphasize that no individual assay is guaranteed to be the most appropriate one in every situation, and we strongly recommend the use of multiple assays to monitor autophagy. Along these lines, because of the potential for pleiotropic effects due to blocking autophagy through genetic manipulation it is imperative to delete or knock down more than one autophagy-related gene. In addition, some individual Atg proteins, or groups of proteins, are involved in other cellular pathways so not all Atg proteins can be used as a specific marker for an autophagic process. In these guidelines, we consider these various methods of assessing autophagy and what information can, or cannot, be obtained from them. Finally, by discussing the merits and limits of particular autophagy assays, we hope to encourage technical innovation in the field
Nanoparticle Self-Assembly Assisted by Polymers: The Role of Shear Stress in the Nanoparticle Arrangement of Langmuir and Langmuir–Blodgett Films
We
propose to use the self-assembly ability of a block copolymer combined
with compression–expansion cycles to obtain CdSe quantum dots
(QDs) structures of different morphology. The methodology proposed
consists in transferring onto mica mixed Langmuir monolayers of QDs
and the polymer poly(styrene-<i>co</i>-maleic anhydride)
partial 2-butoxyethyl ester cumene terminated, PS-MA-BEE, previously
sheared by 50 compression–expansion cycles. Results indicate
that the shear stress takes out nanoparticles at the air–water
interface from metastable states and promotes a new equilibrium state
of the Langmuir monolayer. This new state was transferred onto mica
by the Langmuir–Blodgett (LB) methodology, and the morphology
of the LB films was analyzed by atomic force microscopy and transmission
electron microscopy measurements. Our results show that when the amplitude
strain increases, the QDs domain size decreases and the QDs LB film
arrangement becomes more ordered. The dynamics of the monolayer relaxation
after cycling involves at least three time scales which are related
to the damping of surface fluctuation, raft rearrangement, and component
movements inside each raft. Brewster angle microscopy allowed visualizing <i>in situ</i> the raft rearrangement at the air–water interface
Adsorption of Quantum Dots onto Polymer and Gemini Surfactant Films: A Quartz Crystal Microbalance Study
We
used quartz crystal microbalance with dissipation to study the
mechanical properties, the kinetics of adsorption, and the amount
of CdSe quantum dots (QDs) adsorbed onto a SiO<sub>2</sub> sensor,
referred as bare sensor, onto the sensor modified with a film of the
polymer poly(maleic anhydride-<i>alt</i>-1-octadecene),
PMAO, or with a film of the Gemini surfactant ethyl-bis(dimethyl octadecyl
ammonium bromide), abbreviated as 18-2-18. Results showed that when
the sensor is coated with polymer or surfactant molecules, the coverage
increases compared with that obtained for the bare sensor. On the
other hand, rheological properties and kinetics of adsorption of QDs
are driven by QD nanoparticles. Thus, the QD films present elastic
behavior, and the elasticity values are independent of the molecule
used as coating and similar to the elasticity value obtained for QDs
films on the bare sensor. The QD adsorption is a two-step mechanism
in which the fastest process is attributed to the QD adsorption onto
the solid substrate and the slowest one is ascribed to rearrangement
movements of the nanoparticles adsorbed at the surface
Graphene Oxide Thin Films: Influence of Chemical Structure and Deposition Methodology
We synthesized graphene oxide sheets
of different functionalization
by oxidation of two different starting materials, graphite and GANF
nanofibers, followed by purification based on alkaline washing. The
chemical structure of graphene oxide materials was determined by X-ray
photoelectron spectroscopy (XPS), and the nanoplatelets were characterized
by ζ potential and dynamic light scattering (DLS) measurements.
The XPS results indicated that the chemical structure depends on the
starting material. Two different deposition methodologies, Langmuir–Blodgett
(LB) and Langmuir–Schaefer (LS), were employed to build the
graphene oxide thin films. The film morphology was analyzed by scanning
electron microscopy (SEM). The SEM images allow us to conclude that
the LB methodology provides the highest coverage. This coverage is
almost independent of the chemical composition of sheets. Conversely,
the coverage obtained by the LS methodology increases with the percentage
of C–O groups attached to sheets. Surface–pressure isotherms
of these materials were interpreted according to the Volmer model
Arginine Kinase Peptide Mass Fingerprinting as a Proteomic Approach for Species Identification and Taxonomic Analysis of Commercially Relevant Shrimp Species
Lauro panegyrico de el evangelista San Marcos, gratitud festiva, con que celebrè su iglesia el triunfo, que el dia veinte y cinco de Abril, dedicado à los cultos de este Marcial Glorioso, ganaron las armas catholicas de el Sr. D. Felipe Quinto ...
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