713 research outputs found
Probing complex RNA structures by mechanical force
RNA secondary structures of increasing complexity are probed combining single
molecule stretching experiments and stochastic unfolding/refolding simulations.
We find that force-induced unfolding pathways cannot usually be interpretated
by solely invoking successive openings of native helices. Indeed, typical
force-extension responses of complex RNA molecules are largely shaped by
stretching-induced, long-lived intermediates including non-native helices. This
is first shown for a set of generic structural motifs found in larger RNA
structures, and then for Escherichia coli's 1540-base long 16S ribosomal RNA,
which exhibits a surprisingly well-structured and reproducible unfolding
pathway under mechanical stretching. Using out-of-equilibrium stochastic
simulations, we demonstrate that these experimental results reflect the slow
relaxation of RNA structural rearrangements. Hence, micromanipulations of
single RNA molecules probe both their native structures and long-lived
intermediates, so-called "kinetic traps", thereby capturing -at the single
molecular level- the hallmark of RNA folding/unfolding dynamics.Comment: 9 pages, 9 figure
Calcium-free Solid Solutions in the System Ba7F12Cl2−xBrx (x<1.5), a Single-component White Phosphor Host
We have recently prepared solid solutions of Ba∼6.3Ca∼0.7F12Cl2−xBrx with x ranging from 0 to 2. In this work, the synthesis and single crystal X-ray structure of calcium-free crystals of Ba∼6.9Na∼0.2F12Br0.6Cl1.4 (space group P63/m, a=10.6024(10), c=4.2034(4)Å), Ba∼6.9Na∼0.2F12Br1.4 Cl0.6 (space group P63/m, a=10.6155(9), c=4.2355(4)Å) and Ba∼6.9Na∼0.2Br1.32Cl0.68F12 (space group P63/m, a=10.6218(9), c=4.2284(4)Å) are reported. These crystals systematically present additional electron density at the 0 0 0.25 position which is associated with the presence of small, but significant amounts of Na+ ions in the crysta
A Clinical Trial
Purpose The aim of this study was the systematic image quality evaluation of
coronary CT angiography (CTA), reconstructed with the 3 different levels of
adaptive iterative dose reduction (AIDR 3D) and compared to filtered back
projection (FBP) with quantum denoising software (QDS). Methods Standard-dose
CTA raw data of 30 patients with mean radiation dose of 3.2 ± 2.6 mSv were
reconstructed using AIDR 3D mild, standard, strong and compared to FBP/QDS.
Objective image quality comparison (signal, noise, signal-to-noise ratio
(SNR), contrast-to-noise ratio (CNR), contour sharpness) was performed using
21 measurement points per patient, including measurements in each coronary
artery from proximal to distal. Results Objective image quality parameters
improved with increasing levels of AIDR 3D. Noise was lowest in AIDR 3D strong
(p≤0.001 at 20/21 measurement points; compared with FBP/QDS). Signal and
contour sharpness analysis showed no significant difference between the
reconstruction algorithms for most measurement points. Best coronary SNR and
CNR were achieved with AIDR 3D strong. No loss of SNR or CNR in distal
segments was seen with AIDR 3D as compared to FBP. Conclusions On standard-
dose coronary CTA images, AIDR 3D strong showed higher objective image quality
than FBP/QDS without reducing contour sharpness
Thermal Fluctuations of Elastic Filaments with Spontaneous Curvature and Torsion
We study the effects of thermal flucutations on thin elastic filaments with
spontaneous curvature and torsion. We derive analytical expressions for the
orientational correlation functions and for the persistence length of helices,
and find that this length varies non-monotonically with the strength of thermal
fluctuations. In the weak fluctuation regime, the persistence length of a
spontaneously twisted helix has three resonance peaks as a function of the
twist rate. In the limit of strong fluctuations, all memory of the helical
shape is lost.Comment: 1 figur
Bending and Base-Stacking Interactions in Double-Stranded Semiflexible Polymer
Simple expressions for the bending and the base-stacking energy of
double-stranded semiflexible biopolymers (such as DNA and actin) are derived.
The distribution of the folding angle between the two strands is obtained by
solving a Schr\"{o}dinger equation variationally. Theoretical results based on
this model on the extension versus force and extension versus degree of
supercoiling relations of DNA chain are in good agreement with the experimental
observations of Cluzel {\it et al.} [Science {\bf 271}, 792 (1996)], Smith {\it
et al.} [{\it ibid.} {\bf 271}, 795 (1996)], and Strick {\it et al.} [{\it
ibid.} {\bf 271}, 1835 (1996)].Comment: 8 pages in Revtex format, with 4 EPS figure
Pain response of resistance training of the paravertebral musculature under radiotherapy in patients with spinal bone metastases – a randomized trial
BACKGROUND: To compare pain response outcomes for patients with spinal bone metastases treated with resistance training of the spinal musculature versus passive physical therapy during radiotherapy (RT). METHODS: In this randomized trial, 60 consecutive patients were treated from September 2011 until March 2013 within one of the two groups: resistance training (Arm A) or passive physical therapy (Arm B) with thirty patients in each group during RT. The course of pain according to visual analog scale (VAS), concurrent medication, and oral morphine equivalent dose (OMED) were assessed at baseline, three months, and six months after RT. Pain response was determined using International Bone Consensus response definitions. RESULTS: The course of VAS in the intervention group (Arm A) was significantly lower both during and after RT (AUC, p < .001). The use of analgetic medication showed the same result, with significantly fewer analgetics being necessary both during and after RT in arm A (p < .001). In the course of time, the OMED decreased in arm A, but increased in arm B. After 6 month, 72.2% of patients in arm A, and 22.2% in arm B were responders (p = .014). CONCLUSION: Our trial demonstrated that guided isometric resistance training of the paravertebral muscles can improve pain relief over a 6-months period in patients with stable spinal metastases. Importantly, the intervention was able to reduce OMED as well as concomitant pain medication. The trial is registered in Clinical trial identifier NCT 01409720 (http://www.clinicaltrials.gov/) since 2(nd) of August 2011
Theory of High-Force DNA Stretching and Overstretching
Single molecule experiments on single- and double stranded DNA have sparked a
renewed interest in the force-extension of polymers. The extensible Freely
Jointed Chain (FJC) model is frequently invoked to explain the observed
behavior of single-stranded DNA. We demonstrate that this model does not
satisfactorily describe recent high-force stretching data. We instead propose a
model (the Discrete Persistent Chain, or ``DPC'') that borrows features from
both the FJC and the Wormlike Chain, and show that it resembles the data more
closely. We find that most of the high-force behavior previously attributed to
stretch elasticity is really a feature of the corrected entropic elasticity;
the true stretch compliance of single-stranded DNA is several times smaller
than that found by previous authors. Next we elaborate our model to allow
coexistence of two conformational states of DNA, each with its own stretch and
bend elastic constants. Our model is computationally simple, and gives an
excellent fit through the entire overstretching transition of nicked,
double-stranded DNA. The fit gives the first values for the elastic constants
of the stretched state. In particular we find the effective bend stiffness for
DNA in this state to be about 10 nm*kbt, a value quite different from either
B-form or single-stranded DNAComment: 33 pages, 11 figures. High-quality figures available upon reques
Force and kinetic barriers in unzipping of DNA
A theory of the unzipping of double-stranded (ds) DNA is presented, and is
compared to recent micromanipulation experiments. It is shown that the
interactions which stabilize the double helix and the elastic rigidity of
single strands (ss) simply determine the sequence dependent =12 pN force
threshold for DNA strand separation. Using a semi-microscopic model of the
binding between nucleotide strands, we show that the greater rigidity of the
strands when formed into dsDNA, relative to that of isolated strands, gives
rise to a potential barrier to unzipping. The effects of this barrier are
derived analytically. The force to keep the extremities of the molecule at a
fixed distance, the kinetic rates for strand unpairing at fixed applied force,
and the rupture force as a function of loading rate are calculated. The
dependence of the kinetics and of the rupture force on molecule length is also
analyzed.Comment: Revtex file + 6 eps Figures; published in Proc. Natl. Acad. Sci. USA
98, 8608 (2001
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