34 research outputs found
Probing Polyelectrolyte Elasticity Using Radial Distribution Function
We study the effect of electrostatic interactions on the distribution
function of the end-to-end distance of a single polyelectrolyte chain in the
rodlike limit. The extent to which the radial distribution function of a
polyelectrolyte is reproduced by that of a wormlike chain with an adjusted
persistence length is investigated. Strong evidence is found for a universal
scaling formula connecting the effective persistence length of a
polyelectrolyte with its linear charge density and the Debye screening of its
self-interaction. An alternative definition of the electrostatic persistence
length is proposed based on matching of the maximum of the distribution with
that of an effective wormlike chain, as opposed to the traditional matching of
the first or the second moments of the distributions. It is shown that this
definition provides a more accurate probe of the affinity of the distribution
to that of the wormlike chains, as compared to the traditional definition. It
is also found that the length of a polyelectrolyte segment can act as a crucial
parameter in determining its elastic properties.Comment: 15 pages, 19 figure
Effective interaction between helical bio-molecules
The effective interaction between two parallel strands of helical
bio-molecules, such as deoxyribose nucleic acids (DNA), is calculated using
computer simulations of the "primitive" model of electrolytes. In particular we
study a simple model for B-DNA incorporating explicitly its charge pattern as a
double-helix structure. The effective force and the effective torque exerted
onto the molecules depend on the central distance and on the relative
orientation. The contributions of nonlinear screening by monovalent counterions
to these forces and torques are analyzed and calculated for different salt
concentrations. As a result, we find that the sign of the force depends
sensitively on the relative orientation. For intermolecular distances smaller
than it can be both attractive and repulsive. Furthermore we report a
nonmonotonic behaviour of the effective force for increasing salt
concentration. Both features cannot be described within linear screening
theories. For large distances, on the other hand, the results agree with linear
screening theories provided the charge of the bio-molecules is suitably
renormalized.Comment: 18 pages, 18 figures included in text, 100 bibliog
Cellular and molecular mechanisms involved in the neurotoxicity of opioid and psychostimulant drugs
Substance abuse and addiction are the most costly of all the neuropsychiatric disorders. In the last decades, much progress has been achieved in understanding the effects of the drugs of abuse in the brain. However, efficient treatments that prevent relapse have not been developed. Drug addiction is now considered a brain disease, because the abuse of drugs affects several brain functions. Neurological impairments observed in drug addicts may reflect drug-induced neuronal dysfunction and neurotoxicity. The drugs of abuse directly or indirectly affect neurotransmitter systems, particularly dopaminergic and glutamatergic neurons. This review explores the literature reporting cellular and molecular alterations reflecting the cytotoxicity induced by amphetamines, cocaine and opiates in neuronal systems. The neurotoxic effects of drugs of abuse are often associated with oxidative stress, mitochondrial dysfunction, apoptosis and inhibition of neurogenesis, among other mechanisms. Understanding the mechanisms that underlie brain dysfunction observed in drug-addicted individuals may contribute to improve the treatment of drug addiction, which may have social and economic consequences.http://www.sciencedirect.com/science/article/B6SYS-4S50K2J-1/1/7d11c902193bfa3f1f57030572f7034