82 research outputs found
Action-derived molecular dynamics in the study of rare events
We present a practical method to generate classical trajectories with fixed
initial and final boundary conditions. Our method is based on the minimization
of a suitably defined discretized action. The method finds its most natural
application in the study of rare events. Its capabilities are illustrated by
non-trivial examples. The algorithm lends itself to straightforward
parallelization, and when combined with molecular dynamics (MD) it promises to
offer a powerful tool for the study of chemical reactions.Comment: 7 Pages, 4 Figures (3 in color), submitted to Phys. Rev. Let
Efficient Dynamic Importance Sampling of Rare Events in One Dimension
Exploiting stochastic path integral theory, we obtain \emph{by simulation}
substantial gains in efficiency for the computation of reaction rates in
one-dimensional, bistable, overdamped stochastic systems. Using a well-defined
measure of efficiency, we compare implementations of ``Dynamic Importance
Sampling'' (DIMS) methods to unbiased simulation. The best DIMS algorithms are
shown to increase efficiency by factors of approximately 20 for a
barrier height and 300 for , compared to unbiased simulation. The
gains result from close emulation of natural (unbiased), instanton-like
crossing events with artificially decreased waiting times between events that
are corrected for in rate calculations. The artificial crossing events are
generated using the closed-form solution to the most probable crossing event
described by the Onsager-Machlup action. While the best biasing methods require
the second derivative of the potential (resulting from the ``Jacobian'' term in
the action, which is discussed at length), algorithms employing solely the
first derivative do nearly as well. We discuss the importance of
one-dimensional models to larger systems, and suggest extensions to
higher-dimensional systems.Comment: version to be published in Phys. Rev.
Complications and pitfalls of lumbar interlaminar and transforaminal epidural injections
Lumbar interlaminar and transforaminal epidural injections are used in the treatment of lumbar radicular pain and other lumbar spinal pain syndromes. Complications from these procedures arise from needle placement and the administration of medication. Potential risks include infection, hematoma, intravascular injection of medication, direct nerve trauma, subdural injection of medication, air embolism, disc entry, urinary retention, radiation exposure, and hypersensitivity reactions. The objective of this article is to review the complications of lumbar interlaminar and transforaminal epidural injections and discuss the potential pitfalls related to these procedures. We performed a comprehensive literature review through a Medline search for relevant case reports, clinical trials, and review articles. Complications from lumbar epidural injections are extremely rare. Most if not all complications can be avoided by careful technique with accurate needle placement, sterile precautions, and a thorough understanding of the relevant anatomy and contrast patterns on fluoroscopic imaging
Understanding the Impacts of Molecular and Macromolecular Crowding Agents on Protein–Polymer Complex Coacervates
Complex coacervation refers to the liquid–liquid
phase separation
(LLPS) process occurring between charged macromolecules. The study
of complex coacervation is of great interest due to its implications
in the formation of membraneless organelles (MLOs) in living cells.
However, the impacts of the crowded intracellular environment on the
behavior and interactions of biomolecules involved in MLO formation
are not fully understood. To address this knowledge gap, we investigated
the effects of crowding on a model protein–polymer complex
coacervate system. Specifically, we examined the influence of sucrose
as a molecular crowder and polyethylene glycol (PEG) as a macromolecular
crowder. Our results reveal that the presence of crowders led to the
formation of larger coacervate droplets that remained stable over
a 25-day period. While sucrose had a minimal effect on the physical
properties of the coacervates, PEG led to the formation of coacervates
with distinct characteristics, including higher density, increased
protein and polymer content, and a more compact internal structure.
These differences in coacervate properties can be attributed to the
effects of crowders on individual macromolecules, such as the conformation
of model polymers, and nonspecific interactions among model protein
molecules. Moreover, our results show that sucrose and PEG have different
partition behaviors: sucrose was present in both the coacervate and
dilute phases, while PEG was observed to be excluded from the coacervate
phase. Collectively, our findings provide insights into the understanding
of crowding effects on complex coacervation, shedding light on the
formation and properties of coacervates in the context of MLOs
Systematization, description and territory of the caudal cerebral artery of the brain in broad-snouted Caiman (Caiman latirostris)
Thirty heads with the neck segment of Caiman latirostris were used. The animals were provided from a creation center called Mister Caiman, under the authorization of the Brazilian Institute of Environment and Renewable Natural Resources (Ibama). Animals were sacrificed according to the slaughtering routine of the abattoir, and the heads were sectioned at the level of the third cervical vertebra. The arterial system was washed with cold saline solution, with drainage through jugular veins. Subsequently, the system was filled with red colored latex injection. Pieces were than fixed in 20% formaldehyde, for seven days. The brains were removed, with a spinal cord segment, the duramater removed and the arteries dissected. At the level of the hypophysis, the internal carotid artery gave off a rostral branch, and a short caudal branch, continuing, naturally, as the caudal cerebral artery. This artery projected laterodorsalwards and, as it overpassed the optic tract, gave off its I (the first) central branch. Penetrated in the cerebral transverse fissure, emitting the diencephalic artery and next its II (second) central branch. Still inside the fissure, originated occipital hemispheric branches and a pineal branch. Emerged from the cerebral transverse fissure, over the occipital pole of the cerebral hemisphere. Projected rostralwards, sagital to the cerebral longitudinal fissure, as interhemispheric artery. This artery gave off medial and convex hemispheric branches to the respective surfaces of the cerebral hemispheres, anastomosed with its contralateral homologous, forming the common ethmoidal artery. This artery entered the fissure between the olfactory peduncles, emerging ventrally and dividing into ethmoidal arteries, right and left, which progressed towards the nasal cavities, vascularizing them. The territory of the caudal cerebral artery included the most caudal area of the base of the cerebral hemisphere, its convex surface, the olfactory peduncles and bulbs, the choroid plexuses and the diencephalus with its parietal organs
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