11,220 research outputs found
Comment on ``Validity of Feynman's prescription of disregarding the Pauli principle in intermediate states''
In a recent paper Coutinho, Nogami and Tomio [Phys. Rev. A 59, 2624 (1999);
quant-ph/9812073] presented an example in which, they claim, Feynman's
prescription of disregarding the Pauli principle in intermediate states of
perturbation theory fails. We show that, contrary to their claim, Feynman's
prescription is consistent with the exact solution of their example.Comment: 1 pag
Conservation of Orbital Angular Momentum in Stimulated Down-Conversion
We report on an experiment demonstrating the conservation of orbital angular
momentum in stimulated down-conversion. The orbital angular momentum is not
transferred to the individual beams of the spontaneous down-conversion, but it
is conserved when twin photons are taken individually. We observe the
conservation law for an individual beam of the down-conversion through
cavity-free stimulated emission.Comment: Submitted for publication in Phys. Rev. Let
Recent results in vine improvement regarding its resistance to downy and powdery mildews
Resistance to mildew remains very important under Portuguese conditions and induced mutations present a great interest to obtain it. γ -rays irradiation was applied to woody material and X-ray was used on in vitro cultures of stem apexes and leaf explants. After doses up to 2000 rad, two resistant mutants were selected: with γ -rays, the resistance being expressed as surrounding necrotic patches; with X-rays, the plant coming out from in vitro culture presented small necrotic patches. Concerning powdery mildew, we are developing new techniques for laboratorial inoculation, including a technique for sowing isolated spores. By using these, we can conduct studies on morphology and biology of the isolated colony, variability of the resistance within the rids varieties and genetic variability of the parasite. Some results concerning these areas are presented
Insights into the Limitations of Parameter Transferability in Heteronuclear SAFT-type Equations of State
The use of heteronuclear models are often viewed as ways to improve the
predictive ability and parameter transferability of advanced association
models, such as those derived from the Statistical Associating Fluid Theory
(SAFT). Indeed, several results in the literature have suggested that this
approach can be useful to accurately describe a given family/series of
homologous compounds and their mixtures, with accuracies competitive (and in
some cases better) than those obtained using the more traditional SAFT
variants. However, the parameter transferability of the different groups, i.e.
between different families of compounds, without the introduction of new groups
or refitting existing ones, is seldom reported, and often overlooked, making an
accurate evaluation of the heteronuclear models difficult. This work analyzes
whether the increased complexity of a heteronuclear treatment of a SAFT-type
EoS, namely the SAFT-{\gamma}-Mie EoS, results in a significant increase on
both the predictive ability and parameter transferability of the model, across
different families of compounds. This is done by using a case study involving
some different (yet related) families of compounds, containing a small number
of common functional groups. The results obtained show that the transferability
of group parameters, across different families of compounds, in a heteronuclear
SAFT-type EoS does not allow an adequate description of the phase equilibria of
these systems. Therefore, to achieve a reasonable accuracy in the description
of these systems, a specific refitting of group parameters is required for a
given family, or even for a particular system, destroying the predictive
capability of these models. Moreover, this increases the number of adjustable
parameters to numbers similar to those used in homonuclear approaches, further
reducing the advantages of using heteronuclear models
Entropy production and Kullback-Leibler divergence between stationary trajectories of discrete systems
The irreversibility of a stationary time series can be quantified using the
Kullback-Leibler divergence (KLD) between the probability to observe the series
and the probability to observe the time-reversed series. Moreover, this KLD is
a tool to estimate entropy production from stationary trajectories since it
gives a lower bound to the entropy production of the physical process
generating the series. In this paper we introduce analytical and numerical
techniques to estimate the KLD between time series generated by several
stochastic dynamics with a finite number of states. We examine the accuracy of
our estimators for a specific example, a discrete flashing ratchet, and
investigate how close is the KLD to the entropy production depending on the
number of degrees of freedom of the system that are sampled in the
trajectories.Comment: 14 pages, 7 figure
Oxidative DNA damage protection and repair by polyphenolic compounds in PC12 cells
Biological systems are frequently exposed to excessive reactive oxygen species, causing a disturbance in the
cells natural antioxidant defence systems and resulting in damage to all biomolecules, including nucleic
acids. In fact, oxidative DNA damage is described as the type of damage most likely to occur in neuronal cells.
In this study, three polyphenolic compounds, luteolin, quercetin and rosmarinic acid, were investigated for
their protective effects against oxidative DNA damage induced in PC12 cells, a neuronal cell model. Although
luteolin and quercetin prevented the formation of strand breaks to a greater extent than rosmarinic acid, this
last one presented the highest capacity to repair strand breaks formation. In addition, rosmarinic acid was
the only compound tested that increased the repair of oxidized nucleotidic bases induced with the
photosensitizer compound [R]-1-[(10-chloro-4-oxo-3-phenyl-4H-benzo[a]quinolizin-1-yl) carbonyl]-2-pyrrolidine-
methanol (Ro 19-8022). The activity of repair enzymes was indicated by the in vitro base excision
repair assay, using a cell-free extract obtained from cells previously treated with the compounds to incise
DNA. The protective effect of rosmarinic acid was further confirmed by the increased expression of OGG1
repair gene, observed through real time RT-PCR. The data obtained is indicative that rosmarinic acid seems to
act on the intracellular mechanisms responsible for DNA repair, rather than by a direct effect on reactive
oxygen species scavenging, as deducted from the effects observed for luteolin and quercetin. Therefore, these
results suggest the importance of these polyphenols, and in particular rosmarinic acid, as protectors of
oxidative stress-induced DNA damage that commonly occurs in several pathological conditions, such as
neurodegenerative diseases.Fundação para a Ciência e a Tecnologia (FCT) - bolsa SFRH/BD/17174/2004
The motion of two masses coupled to a massive spring
We discuss the classical motion of a spring of arbitrary mass coupled to two
arbitrary massive blocks attached at its ends. A general approach to the
problem is presented and some general results are obtained. Examples for which
a simple elastic function can be inferred are discussed and the normal modes
and normal frequencies obtained. An approximation procedure to the evaluation
of the normel frequencies in the case of uniform elastic function and mass
density is also discussed.Comment: Standard Latex file plus three eps figure
Monoolein-based lipoplexes (DODAB/MO/DNA) as non-viral vector for transfection- from physicochemical characterization to biological application
Cationic liposomes/DNA (lipoplexes) have been widely used as non-viral vectors for
transfection, the role of the neutral lipid in liposome formulation being determinant for the
efficiency of this process [1,2]. In this work, we studied the potential of monoolein (MO)
as helper lipid for cellular transfection. Lipoplexes composed of pDNA and
dioctadecyldimethylammonium bromide (DODAB)/1-monooleoyl-rac-glycerol (MO) at
different molar ratios (4:1, 2:1 and 1:1) were investigated, as well as different cationic
lipid/DNA ratios. The physicochemical properties of the lipoplexes (size and charge), the
formation of the lipoplexes, the effect of MO on pDNA condensation and the effect of
heparin on percentage of pDNA release from the lipoplexes were also studied by
Ethidium Bromide (EtBr) exclusion assays, Dynamic Light Scattering (DLS), Zeta
Potential (æ) and electrophoresis. The cytotoxicity, transfection efficiency, as well as the
intracellular localization of labeled DNA were evaluated on 293T cells. It was found that
the presence of MO not only increases the efficiency of pDNA compactation, but also
affects the physicochemical properties of lipoplexes, which could possibly interfere with
lipoplex-cell interactions. The DODAB:MO (2:1) and (4:1) formulations were capable of
efficiently mediate in vitro cell transfection. These results were consistent with
fluorescence microscopy studies, which illustrated that lipoplexes were able to gain entry
into the cytosol and deliver pDNA to the nucleus. Understanding the structure–activity
relationship of MO based lipoplexes will give direction toward the development of safe
and efficient gene delivery systems.Portuguese Foundation for Science and Technology (FCT) for financial support to Center of
Physics and Center of Molecular & Environmental Biology and funding through projects PTDC/QUI/69795/2006 and SFRH/BD/46968/2009
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