Elasticity near the vulcanization transition

Signatures of the vulcanization transition--amorphous solidification induced by the random crosslinking of macromolecules--include the random localization of a fraction of the particles and the emergence of a nonzero static shear modulus. A semi-microscopic statistical-mechanical theory is presented of the latter signature that accounts for both thermal fluctuations and quenched disorder. It is found (i) that the shear modulus grows continuously from zero at the transition, and does so with the classical exponent, i.e., with the third power of the excess cross-link density and, quite surprisingly, (ii) that near the transition the external stresses do not spoil the spherical symmetry of the localization clouds of the particles.Comment: REVTEX, 5 pages. Minor change

Polyelectrolytes in the presence of multivalent ions: gelation versus segregation

We analyze solutions of strongly charged chains bridged by linkers such as multivalent ions. The gelation induced by the strong short range electrostatic attractions is dramatically suppressed by the long range electrostatic correlations due to the charge along the uncrosslinked monomers and ions. A modified Debye-Huckel approach of crosslinked clusters of charged chains is used to determined the mean field gelation transition self-consistently. Highly dilute polyelectrolyte solutions tend to segregate macroscopically. Semidilute solutions can form gels if the Bjerrum length $l_B$ and the distance between neighboring charged monomers along the chain $b$ are both greater than the ion size $a$

Synchronization of cytoplasmic and transferred mitochondrial ribosomal protein gene expression in land plants is linked to Telo-box motif enrichment

<p>Abstract</p> <p>Background</p> <p>Chloroplasts and mitochondria evolved from the endosymbionts of once free-living eubacteria, and they transferred most of their genes to the host nuclear genome during evolution. The mechanisms used by plants to coordinate the expression of such transferred genes, as well as other genes in the host nuclear genome, are still poorly understood.</p> <p>Results</p> <p>In this paper, we use nuclear-encoded chloroplast (cpRPGs), as well as mitochondrial (mtRPGs) and cytoplasmic (euRPGs) ribosomal protein genes to study the coordination of gene expression between organelles and the host. Results show that the mtRPGs, but not the cpRPGs, exhibit strongly synchronized expression with euRPGs in all investigated land plants and that this phenomenon is linked to the presence of a <it>telo</it>-box DNA motif in the promoter regions of mtRPGs and euRPGs. This motif is also enriched in the promoter regions of genes involved in DNA replication. Sequence analysis further indicates that mtRPGs, in contrast to cpRPGs, acquired <it>telo</it>-box from the host nuclear genome.</p> <p>Conclusions</p> <p>Based on our results, we propose a model of plant nuclear genome evolution where coordination of activities in mitochondria and chloroplast and other cellular functions, including cell cycle, might have served as a strong selection pressure for the differential acquisition of <it>telo</it>-box between mtRPGs and cpRPGs. This research also highlights the significance of physiological needs in shaping transcriptional regulatory evolution.</p

Increased Anion Channel Activity Is an Unavoidable Event in Ozone-Induced Programmed Cell Death

Ozone is a major secondary air pollutant often reaching high concentrations in urban areas under strong daylight, high temperature and stagnant high-pressure systems. Ozone in the troposphere is a pollutant that is harmful to the plant. generation by salicylic and abscisic acids. Anion channel activation was also shown to promote the accumulation of transcripts encoding vacuolar processing enzymes, a family of proteases previously reported to contribute to the disruption of vacuole integrity observed during programmed cell death.-induced programmed cell death. Because ion channels and more specifically anion channels assume a crucial position in cells, an understanding about the underlying role(s) for ion channels in the signalling pathway leading to programmed cell death is a subject that warrants future investigation

Physico-chemical properties of carrageenan gels in presence of various cations.

Phase separation and gelation induced by addition of monovalent and divalent cations in iota and kappa carrageenan solutions were investigated as a function of the polymer and cation concentrations. Rheological measurements have also been carried out at a given polymer concentration. The storage modulus (G') determined at a cation/polymer ratio was always higher for kappa- than for iota-carrageenan. For iota carrageenan, G' increased slowly with the monovalent salt concentration and more quickly with the divalent salt concentration. At the opposite, for kappa carrageenan, G' increased more rapidly in the presence of KCl than with calcium or copper. Nevertheless for large salt concentrations, G' became independent of the type and concentration of cations in the kappa carrageenan solution

Effect of galactomannans on the viscoelastic behaviour of pectin/calcium networks

An investigation was carried out on the effect of the addition of galactomannans to pectin/calcium networks with different structural and rheological characteristics. For those pectin/calcium gels characterized by an elastic equilibrium modulus, the addition of the galactomannan increased both the storage and loss mod&i, especially at short time scales. This increase was greater than that which could be expected by simple additivity of the viscoelastic properties of each isolated system. The pectin/calcium network remained the continuous gel matrix controlling the viscoelastic behaviour of these systems at low frequency. For pectin/calcium systems close to the sol-gel transition or at low pH, the mixed systems evolved towards the behaviour of Viscoelastic liquids in the presence of increasing concentration of the non-gelling polymer. These overall results suggest that there is no specific interaction and that the changes in the rheological properties of the pectin gels are due to galactomannan microphase separation limited by the entrapment of these macromolecules in the pectin-calcium network