28 research outputs found
Applying a potential across a biomembrane: electrostatic contribution to the bending rigidity and membrane instability
We investigate the effect on biomembrane mechanical properties due to the
presence an external potential for a non-conductive non-compressible membrane
surrounded by different electrolytes. By solving the Debye-Huckel and Laplace
equations for the electrostatic potential and using the relevant stress-tensor
we find: in (1.) the small screening length limit, where the Debye screening
length is smaller than the distance between the electrodes, the screening
certifies that all electrostatic interactions are short-range and the major
effect of the applied potential is to decrease the membrane tension and
increase the bending rigidity; explicit expressions for electrostatic
contribution to the tension and bending rigidity are derived as a function of
the applied potential, the Debye screening lengths and the dielectric constants
of the membrane and the solvents. For sufficiently large voltages the negative
contribution to the tension is expected to cause a membrane stretching
instability. For (2.) the dielectric limit, i.e. no salt (and small wavevectors
compared to the distance between the electrodes), when the dielectric constant
on the two sides are different the applied potential induces an effective
(unscreened) membrane charge density, whose long-range interaction is expected
to lead to a membrane undulation instability.Comment: 16 pages, 3 figures, some revisio
Osmotic properties of polyethyleneglycols: quantitative features of brush and bulk scaling laws
From glycosylated cell surfaces to sterically stabilized liposomes, polymers
attached to membranes attract biological and therapeutic interest. Can the
scaling laws of polymer "brushes" describe the physical properties of these
coats? We delineate conditions where the Alexander - de Gennes theory of
polymer brushes successfully describes the intermembrane distance vs. applied
osmotic stress data of Kenworthy et al. for PEG-grafted multilamellar liposomes
[Biophys. J. (1995) 68:1921]. We establish that the polymer density and size in
the brush must be high enough that, in a bulk solution of equivalent density,
the polymer osmotic pressure is independent of polymer molecular weight (the
des Cloizeaux semi-dilute regime of bulk polymer solutions). The condition that
attached polymers behave as semi-dilute bulk solutions offers a rigorous
criterion for brush scaling-law behavior. There is a deep connection between
the behaviors of polymer solutions in bulk and polymers grafted to a surface at
a density such that neighbors pack to form a uniform brush. In this regime,
two-parameter unconstrained fits of the Alexander - de Gennes brush scaling
laws yield effective monomer lengths of 3.3 to 3.5 AA, which agree with
structural predictions. The fitted distances between grafting sites are larger
than expected from the nominal content of PEG-lipids; the chains apparently
saturate the surface. Osmotic stress measurements can be used to estimate the
actual densities of membrane-grafted polymers.Comment: 26 pages with figure
Human wealth evolution is an accelerating expansion underpinned by a decelerating optimization process
Eight centuries of yearly GDP data from three regions of Western Europe,
corresponding to present days UK, France and Sweden, gauge economic
interactions that reflect the societal structures in which they unfold. In
short, they provide an insight on human wealth evolution. Our data analysis is
carried out in two steps. First, a Monte Carlo algorithm is used to fit the GDP
time series to a piecewise continuous function comprising a sequence of
exponentials with different exponents. These arguably correspond to different
social and technological stages of societal organization.
As a function of time, human wealth evolution features an accelerating trend
and is thus an evolutionary expansion process. The intensity of human
interactions driving the evolutionary process has increased manyfold over the
centuries.
This motivates our second analysis, where `wall clock time' as independent
variable is replaced with a measure of human interactions intensity .
In terms of this interaction variable , human wealth evolution displays
two different logarithmic regimes, both decelerating, and connected by a rapid
cross-over. The latter occurs around the outbreak of World War I and coincides
with the transition in Western Europe from a mainly agricultural to a mainly
industrial and urbanized society. Finally, wealth evolution in terms of
shows the hallmarks of record dynamics optimization
Tight Coupling of Metabolic Oscillations and Intracellular Water Dynamics in <i>Saccharomyces cerevisiae</i>
We detected very strong coupling between the oscillating concentration of ATP and the dynamics of intracellular water during glycolysis in Saccharomyces cerevisiae. Our results indicate that: i) dipolar relaxation of intracellular water is heterogeneous within the cell and different from dilute conditions, ii) water dipolar relaxation oscillates with glycolysis and in phase with ATP concentration, iii) this phenomenon is scale-invariant from the subcellular to the ensemble of synchronized cells and, iv) the periodicity of both glycolytic oscillations and dipolar relaxation are equally affected by D2O in a dose-dependent manner. These results offer a new insight into the coupling of an emergent intensive physicochemical property of the cell, i.e. cell-wide water dipolar relaxation, and a central metabolite (ATP) produced by a robustly oscillating metabolic process