1,941 research outputs found
Molecular dynamics simulation study of the high frequency sound waves in the fragile glass former ortho-terphenyl
Using a realistic flexible molecule model of the fragile glass former
orthoterphenyl, we calculate via molecular dynamics simulation the collective
dynamic structure factor, recently measured in this system by Inelastic X-ray
Scattering. The comparison of the simulated and measured dynamic structure
factor, and the study of its properties in an extended momentum, frequency and
temperature range allows: i) to conclude that the utilized molecular model
gives rise to a dynamic structure factor in agreement with the experimental
data, for those thermodynamic states and momentum values where the latter are
available; ii) to confirm the existence of a slope discontinuity on the
T-dependence of the sound velocity that, at finite Q, takes place at a
temperature T_x higher than the calorimetric glass transition temperature T_g;
iii) to find that the values of T_x is Q-dependent and that its vanishing Q
limit is consistent with T_g. The latter finding is interpreted within the
framework of the current description of the dynamics of supercooled liquids in
terms of exploration of the potential energy landscape.Comment: RevTex, 9 pages, 10 eps figure
Phase diagram of a solution undergoing inverse melting
The phase diagram of -cyclodextrin/water/4-methylpyridine solutions,
a system undergoing inverse melting, has been studied by differential scanning
calorimetry, rheological methods, and X-rays diffraction. Two different fluid
phases separated by a solid region have been observed in the high
-cyclodextrin concentration range (150 mg/ml). Decreasing ,
the temperature interval where the solid phase exists decreases and eventually
disappears, and a first order phase transition is observed between the two
different fluid phases.Comment: 4 pages, 5 figures, accepted on Physical Review E (R
High frequency acoustic modes in liquid gallium at the melting point
The microscopic dynamics in liquid gallium (l-Ga) at melting (T=315 K) has
been studied by inelastic x-ray scattering. We demonstrate the existence of
collective acoustic-like modes up to wave-vectors above one half of the first
maximum of the static structure factor, at variance with earlier results from
inelastic neutron scattering data [F.J. Bermejo et al. Phys. Rev. E 49, 3133
(1994)]. Despite the structural (an extremely rich polymorphism and rather
complex phase diagram) and electronic (mixed valence) peculiarity of l-Ga, its
collective dynamics is strikingly similar to the one of Van der Walls and
alkali metals liquids. This result speaks in favor of the universality of the
short time dynamics in monatomic liquids rather than of system-specific
dynamics.Comment: LaTex format, 11 pages, 4 EncapsulatedPostScript figure
Relaxation processes in harmonic glasses?
A relaxation process, with the associated phenomenology of sound attenuation
and sound velocity dispersion, is found in a simulated harmonic Lennard-Jones
glass. We propose to identify this process with the so called microscopic (or
instantaneous) relaxation process observed in real glasses and supercooled
liquids. A model based on the memory function approach accounts for the
observation, and allows to relate to each others: 1) the characteristic time
and strength of this process, 2) the low frequency limit of the dynamic
structure factor of the glass, and 3) the high frequency sound attenuation
coefficient, with its observed quadratic dependence on the momentum transfer.Comment: 11 pages, 3 figure
Evidence of anomalous dispersion of the generalized sound velocity in glasses
The dynamic structure factor, S(Q,w), of vitreous silica, has been measured
by inelastic X-ray scattering in the exchanged wavevector (Q) region Q=4-16.5
nm-1 and up to energies hw=115 meV in the Stokes side. The unprecedented
statistical accuracy in such an extended energy range allows to accurately
determine the longitudinal current spectra, and the energies of the vibrational
excitations. The simultaneous observation of two excitations in the acoustic
region, and the persistence of propagating sound waves up to Q values
comparable with the (pseudo-)Brillouin zone edge, allow to observe a positive
dispersion in the generalized sound velocity that, around Q=5 nm-1, varies from
6500 to 9000 m/s: this phenomenon was never experimentally observed in a glass.Comment: 5 pages, 3 figures. To appear in Phys. Rev.
Modulation of PKM alternative splicing by PTBP1 promotes gemcitabine resistance in pancreatic cancer cells
Pancreatic ductal adenocarcinoma (PDAC) is an aggressive and incurable disease. Poor prognosis is due to multiple reasons, including acquisition of resistance to gemcitabine, the first-line chemotherapeutic approach. Thus, there is a strong need for novel therapies, targeting more directly the molecular aberrations of this disease. We found that chronic exposure of PDAC cells to gemcitabine selected a subpopulation of cells that are drug-resistant (DR-PDAC cells). Importantly, alternative splicing (AS) of the pyruvate kinase gene (PKM) was differentially modulated in DR-PDAC cells, resulting in promotion of the cancer-related PKM2 isoform, whose high expression also correlated with shorter recurrence-free survival in PDAC patients. Switching PKM splicing by antisense oligonucleotides to favor the alternative PKM1 variant rescued sensitivity of DR-PDAC cells to gemcitabine and cisplatin, suggesting that PKM2 expression is required to withstand drug-induced genotoxic stress. Mechanistically, upregulation of the polypyrimidine-tract binding protein (PTBP1), a key modulator of PKM splicing, correlated with PKM2 expression in DR-PDAC cell lines. PTBP1 was recruited more efficiently to PKM pre-mRNA in DR- than in parental PDAC cells. Accordingly, knockdown of PTBP1 in DR-PDAC cells reduced its recruitment to the PKM pre-mRNA, promoted splicing of the PKM1 variant and abolished drug resistance. Thus, chronic exposure to gemcitabine leads to upregulation of PTBP1 and modulation of PKM AS in PDAC cells, conferring resistance to the drug. These findings point to PKM2 and PTBP1 as new potential therapeutic targets to improve response of PDAC to chemotherapy.Oncogene advance online publication, 3 August 2015; doi:10.1038/onc.2015.270
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