215 research outputs found
Density Scaling and Dynamic Correlations in Viscous Liquids
We use a recently proposed method [Berthier L.; Biroli G.; Bouchaud J.P.;
Cipelletti L.; El Masri D.; L'Hote D.; Ladieu F.; Pierno M. Science 2005, 310,
1797.] to obtain an approximation to the 4-point dynamic correlation function
from derivatives of the linear dielectric response function. For four liquids
over a range of pressures, we find that the number of dynamically correlated
molecules, Nc, depends only on the magnitude of the relaxation time,
independently of temperature and pressure. This result is consistent with the
invariance of the shape of the relaxation dispersion at constant relaxation
time and the density scaling property of the relaxation times, and implies that
Nc also conforms to the same scaling behavior. For propylene carbonate and
salol Nc becomes constant with approach to the Arrhenius regime, consistent
with the value of unity expected for intermolecularly non-cooperative
relaxation.Comment: revisio
Molecular dynamics simulations of the Johari-Goldstein relaxation in a molecular liquid
Molecular dynamics simulations (mds) were carried out to investigate the
reorientational motion of a rigid (fixed bond length), asymmetric diatomic
molecule in the liquid and glassy states. In the latter the molecule reorients
via large-angle jumps, which we identify with the Johari-Goldstein (JG)
dynamics. This relaxation process has a broad distribution of relaxation times,
and at least deeply in the glass state, the mobility of a given molecule
remains fixed over time; that is, there is no dynamic exchange among molecules.
Interestingly, the JG relaxation time for a molecule does not depend on the
local density, although the non-ergodicity factor is weakly correlated with the
packing efficiency of neighboring molecules. In the liquid state the frequency
of the JG process increases significantly, eventually subsuming the slower
alpha-relaxation. This evolution of the JG-motion into structural relaxation
underlies the correlation of many properties of the JG- and alpha-dynamics.Comment: 12 pages, 6 figure
Are polar liquids less simple?
Strong correlation between equilibrium fluctuations of the potential energy,
U, and the virial, W, is a characteristic of a liquid that implies the presence
of certain dynamic properties, such as density scaling of the relaxation times
and isochronal superpositioning of the relaxation function. In this work we
employ molecular dynamics simulations (mds) on methanol and two variations,
lacking hydrogen bonds and a dipole moment, to assess the connection between
the correlation of U and W and these dynamic properties. We show, in accord
with prior results of others [T.S. Ingebrigtsen, T.B. Schroder, J.C. Dyre,
Phys. Rev. X 2, 011011 (2012).], that simple van der Waals liquids exhibit both
strong correlations and the expected dynamic behavior. However, for polar
liquids this correspondence breaks down - weaker correlation between U and W is
not associated with worse conformance to density scaling or isochronal
superpositioning. The reason for this is that strong correlation between U and
W only requires their proportionality, whereas the expected dynamic behavior
depends primarily on constancy of the proportionality constant for all state
points. For hydrogen-bonded liquids, neither strong correlation nor adherence
to the dynamic properties is observed; however, this nonconformance is not
directly related to the concentration of hydrogen bonds, but rather to the
greater deviation of the intermolecular potential from an inverse power law
(IPL). Only (hypothetical) liquids having interactions governed strictly by an
IPL are perfectly correlating and exhibit the consequent dynamic properties
over all thermodynamic conditions.Comment: 14 pages, 8 figure
Comparing dynamic correlation lengths from an approximation to the four-point dynamic susceptibility and from the picosecond vibrational dynamics
Recently a new approach to the determination of dynamic correlation lengths,
{\xi}, for supercooled liquids, based on the properties of the slow
(picosecond) vibrational dynamics, was carried out [L. Hong, V.N. Novikov, and
A.P. Sokolov, Phys. Rev. E 83, 061508 (2011)]. Although these vibrational
measurements are typically conducted well below the glass transition
temperature, Tg, the assumption is that the structure of the liquid is frozen
at Tg, so that the {\xi} characterize dynamic heterogeneity in the supercooled
liquid state. We compare {\xi} from this method to values calculated using an
approximation to the four-point dynamic susceptibility. For 26 different
materials we find good correlation between the two measures; moreover, the
pressure dependences are consistent within the large experimental error.
However, {\xi} from Boson peak measurements above Tg have a different, and
unrealistic, temperature dependence.Comment: 10 pages, 3 figure
The pleiotropic effects of the glutamate dehydrogenase (GDH) pathway in Saccharomyces cerevisiae
Ammonium assimilation is linked to fundamental cellular processes that include the synthesis of non-essential amino acids like glutamate and glutamine. In Saccharomyces cerevisiae glutamate can be synthesized from alpha-ketoglutarate and ammonium through the action of NADP-dependent glutamate dehydrogenases Gdh1 and Gdh3. Gdh1 and Gdh3 are evolutionarily adapted isoforms and cover the anabolic role of the GDH-pathway. Here, we review the role and function of the GDH pathway in glutamate metabolism and we discuss the additional contributions of the pathway in chromatin regulation, nitrogen catabolite repression, ROS-mediated apoptosis, iron deficiency and sphingolipid-dependent actin cytoskeleton modulation in S. cerevisiae. The pleiotropic effects of GDH pathway in yeast biology highlight the importance of glutamate homeostasis in vital cellular processes and reveal new features for conserved enzymes that were primarily characterized for their metabolic capacity. These newly described features constitute insights that can be utilized for challenges regarding genetic engineering of glutamate homeostasis and maintenance of redox balances, biosynthesis of important metabolites and production of organic substrates. We also conclude that the discussed pleiotropic features intersect with basic metabolism and set a new background for further glutamate-dependent applied research of biotechnological interest
Connection between dynamics and thermodynamics of liquids on the melting line
The dynamics of a large number of liquids and polymers exhibit scaling
properties characteristic of a simple repulsive inverse power law (IPL)
potential, most notably the superpositioning of relaxation data as a function
of the variable TV{\gamma}, where T is temperature, V the specific volume, and
{\gamma} a material constant. A related scaling law, TmVm{\Gamma}, with the
same exponent {\Gamma}={\gamma}, links the melting temperature Tm and volume Vm
of the model IPL liquid; liquid dynamics is then invariant at the melting
point. Motivated by a similar invariance of dynamics experimentally observed at
transitions of liquid crystals, we determine dynamic and melting point scaling
exponents {\gamma} and {\Gamma} for a large number of non-associating liquids.
Rigid, spherical molecules containing no polar bonds have {\Gamma}={\gamma};
consequently, the reduced relaxation time, viscosity and diffusion coefficient
are each constant along the melting line. For other liquids {\gamma}>{\Gamma}
always; i.e., the dynamics is more sensitive to volume than is the melting
point, and for these liquids the dynamics at the melting point slows down with
increasing Tm (that is, increasing pressure).Comment: 20 pages, 8 figures, 1 tabl
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