1,006 research outputs found
External Inversion, Internal Inversion, and Reflection Invariance
Having in mind that physical systems have different levels of structure we
develop the concept of external, internal and total improper Lorentz
transformation (space inversion and time reversal). A particle obtained from
the ordinary one by the application of internal space inversion or time
reversal is generally a different particle. From this point of view the
intrinsic parity of a nuclear particle (`elementary particle') is in fact the
external intrinsic parity, if we take into account the internal structure of a
particle. We show that non-conservation of the external parity does not
necessarily imply non-invariance of nature under space inversion. The
conventional theory of beta-decay can be corrected by including the internal
degrees of freedom to become invariant under total space inversion, though not
under the external one.Comment: 15 pages. An early proposal of "mirror matter", published in 1974.
This is an exact copy of the published paper. I am posting it here because of
the increasing interest in the "exact parity models" and its experimental
consequence
Conformational Dependence of a Protein Kinase Phosphate Transfer Reaction
Atomic motions and energetics for a phosphate transfer reaction catalyzed by
the cAMP-dependent protein kinase (PKA) are calculated by plane-wave density
functional theory, starting from structures of proteins crystallized in both
the reactant conformation (RC) and the transition-state conformation (TC). In
the TC, we calculate that the reactants and products are nearly isoenergetic
with a 0.2 eV barrier; while phosphate transfer is unfavorable by over 1.2 eV
in the RC, with an even higher barrier. With the protein in the TC, the motions
involved in reaction are small, with only P and the catalytic proton
moving more than 0.5 \AA. Examination of the structures reveals that in the RC
the active site cleft is not completely closed and there is insufficient space
for the phosphorylated serine residue in the product state. Together, these
observations imply that the phosphate transfer reaction occurs rapidly and
reversibly in a particular conformation of the protein, and that the reaction
can be gated by changes of a few tenths of an \AA in the catalytic site.Comment: revtex4, 7 pages, 4 figures, to be submitted to Scienc
Structurally specific thermal fluctuations identify functional sites for DNA transcription
We report results showing that thermally-induced openings of double stranded
DNA coincide with the location of functionally relevant sites for
transcription. Investigating both viral and bacterial DNA gene promoter
segments, we found that the most probable opening occurs at the transcription
start site. Minor openings appear to be related to other regulatory sites. Our
results suggest that coherent thermal fluctuations play an important role in
the initiation of transcription. Essential elements of the dynamics, in
addition to sequence specificity, are nonlinearity and entropy, provided by
local base-pair constraints
Carnets de bord en sciences humaines : genĂšse, dynamique et fin d'une revue
Ce dernier numéro est l'occasion de faire un bilan et d'analyser les raisons qui nous ont conduits à décider de mettre un terme à l'existence d'une revue que nous avons animée pendant prÚs de dix ans. FidÚles à la ligne éditoriale, nous dévoilons les coulisses de la production de Carnets de bord. Chemin faisant, nous nous livrons à un exercice d'auto-analyse susceptible d'éclairer la fragilité de
ce genre d'initiatives intellectuelles dans un contexte universitaire oĂč les critĂšres managĂ©riaux d'Ă©valuation ont largement gagnĂ© du terrain
Picosecond fluctuating protein energy landscape mapped by pressureâtemperature molecular dynamics simulation
Microscopic statistical pressure fluctuations can, in principle, lead to corresponding fluctuations in the shape of a protein energy landscape. To examine this, nanosecond molecular dynamics simulations of lysozyme are performed covering a range of temperatures and pressures. The well known dynamical transition with temperature is found to be pressure-independent, indicating that the effective energy barriers separating conformational substates are not significantly influenced by pressure. In contrast, vibrations within substates stiffen with pressure, due to increased curvature of the local harmonic potential in which the atoms vibrate. The application of pressure is also shown to selectively increase the damping of the anharmonic, low-frequency collective modes in the protein, leaving the more local modes relatively unaffected. The critical damping frequency, i.e., the frequency at which energy is most efficiently dissipated, increases linearly with pressure. The results suggest that an invariant description of protein energy landscapes should be subsumed by a fluctuating picture and that this may have repercussions in, for example, mechanisms of energy dissipation accompanying functional, structural, and chemical relaxation
The postpneumonectomy syndrome: clinical presentation and treatment
Background: Postpneumonectomy syndrome (PPS) is a rare complication after pneumonectomy. It consists of an excessive mediastinal shift resulting in compression and stretching of the tracheobronchial tree and the esophagus. The aim of this study was to give a comprehensive overview of diagnosis, variety of symptoms and evaluation of surgical treatment of PPS. Methods: We retrospectively reviewed the charts of all our patients with PPS since 1994 with respect to symptomatology, treatment and outcome. Our results were compared with case reports and case series in the literature. Results: Six women with a median age of 56.5 years (range 49-65) developed PPS after pneumonectomy for the treatment of lung cancer. Four presented with a right PPS and two with a left PPS, respectively. Symptoms consisted of shortness of breath in all patients and dysphagia as well as heartburn in two patients. Correction of PPS required re-exploration of the pneumonectomy space, reposition of the mediastinum followed by the insertion of single silicone prosthesis in five patients or fixation of the mediastinum with a xenopericardial graft in one patient. We could observe an improvement of the FEV(1)/FVC ratio in all our patients and the clinical improvement of shortness of breath was better than we expected by changes of lung function. Four patients returned to their regular activities with a follow-up of four years. We found 73 cases of PPS in the literature, on the right side in 50 patients (68%) and on the left side in 23 patients (32%). Fifty-nine patients (81%) were treated surgically. Symptoms can be suspicious for cardiogenic origin and vary from heartburn to recurrent syncopes. Conclusion: PPS is rare and not predictable. It can occur after right or left pneumonectomy. Symptoms are manifold and result from a shift, leading to compression and stretching of the two conduits located within the mediastinum, the tracheobronchial tree and the esophagus and consists of shortness of breath, stridor and heartburn. Diagnosis must be made by exclusion. Implantation of prosthesis is the most commonly used and effective treatmen
Establishing the entatic state in folding metallated Pseudomonas aeruginosa azurin
Understanding how the folding of proteins establishes their functional characteristics at the molecular level challenges both theorists and experimentalists. The simplest test beds for confronting this issue are provided by electron transfer proteins. The environment provided by the folded protein to the cofactor tunes the metal's electron transport capabilities as envisioned in the entatic hypothesis. To see how the entatic state is achieved one must study how the folding landscape affects and in turn is affected by the metal. Here, we develop a coarse-grained functional to explicitly model how the coordination of the metal (which results in a so-called entatic or rack-induced state) modifies the folding of the metallated Pseudomonas aeruginosa azurin. Our free-energy functional-based approach directly yields the proper nonlinear extra-thermodynamic free energy relationships for the kinetics of folding the wild type and several point-mutated variants of the metallated protein. The results agree quite well with corresponding laboratory experiments. Moreover, our modified free-energy functional provides a sufficient level of detail to explicitly model how the geometric entatic state of the metal modifies the dynamic folding nucleus of azurin
Properties of the energy landscape of network models for covalent glasses
We investigate the energy landscape of two dimensional network models for
covalent glasses by means of the lid algorithm. For three different particle
densities and for a range of network sizes, we exhaustively analyse many
configuration space regions enclosing deep-lying energy minima. We extract the
local densities of states and of minima, and the number of states and minima
accessible below a certain energy barrier, the 'lid'. These quantities show on
average a close to exponential growth as a function of their respective
arguments. We calculate the configurational entropy for these pockets of states
and find that the excess specific heat exhibits a peak at a critical
temperature associated with the exponential growth in the local density of
states, a feature of the specific heat also observed in real glasses at the
glass transition.Comment: RevTeX, 19 pages, 7 figure
Use of Simulation to Visualize Healthcare Worker Exposure to Aerosol in the Operating Room
Simulation resources offer an opportunity to highlight aerosol dispersion within the operating room environment. We demonstrate our methodology with a supporting video that can offer operating room teams support in their practical understanding of aerosol exposure and the importance of personal protective equipment
On the origin of the Boson peak in globular proteins
We study the Boson Peak phenomenology experimentally observed in globular
proteins by means of elastic network models. These models are suitable for an
analytic treatment in the framework of Euclidean Random Matrix theory, whose
predictions can be numerically tested on real proteins structures. We find that
the emergence of the Boson Peak is strictly related to an intrinsic mechanical
instability of the protein, in close similarity to what is thought to happen in
glasses. The biological implications of this conclusion are also discussed by
focusing on a representative case study.Comment: Proceedings of the X International Workshop on Disordered Systems,
Molveno (2006
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