Phase diagram of aggregation of oppositely charged colloids in salty water

Aggregation of two oppositely charged colloids in salty water is studied. We focus on the role of Coulomb interaction in strongly asymmetric systems in which the charge and size of one colloid is much larger than the other one. In the solution, each large colloid (macroion) attracts certain number of oppositely charged small colloids ($Z$-ion) to form a complex. If the concentration ratio of the two colloids is such that complexes are not strongly charged, they condense in a macroscopic aggregate. As a result, the phase diagram in a plane of concentrations of two colloids consists of an aggregation domain sandwiched between two domains of stable solutions of complexes. The aggregation domain has a central part of total aggregation and two wings corresponding to partial aggregation. A quantitative theory of the phase diagram in the presence of monovalent salt is developed. It is shown that as the Debye-H\"{u}ckel screening radius $r_s$ decreases, the aggregation domain grows, but the relative size of the partial aggregation domains becomes much smaller. As an important application of the theory, we consider solutions of long double-helix DNA with strongly charged positive spheres (artificial chromatin). We also consider implications of our theory for in vitro experiments with the natural chromatin. Finally, the effect of different shapes of macroions on the phase diagram is discussed.Comment: 10 pages, 9 figures. The text is rewritten, but results are not change

The Persistence Length of a Strongly Charged, Rod-like, Polyelectrolyte in the Presence of Salt

The persistence length of a single, intrinsically rigid polyelectrolyte chain, above the Manning condensation threshold is investigated theoretically in presence of added salt. Using a loop expansion method, the partition function is consistently calculated, taking into account corrections to mean-field theory. Within a mean-field approximation, the well-known results of Odijk, Skolnick and Fixman are reproduced. Beyond mean-field, it is found that density correlations between counterions and thermal fluctuations reduce the stiffness of the chain, indicating an effective attraction between monomers for highly charged chains and multivalent counterions. This attraction results in a possible mechanical instability (collapse), alluding to the phenomenon of DNA condensation. In addition, we find that more counterions condense on slightly bent conformations of the chain than predicted by the Manning model for the case of an infinite cylinder. Finally, our results are compared with previous models and experiments.Comment: 13 pages, 2 ps figure

Conformational Instability of Rodlike Polyelectrolytes due to Counterion Fluctuations

The effective elasticity of highly charged stiff polyelectrolytes is studied in the presence of counterions, with and without added salt. The rigid polymer conformations may become unstable due to an effective attraction induced by counterion density fluctuations. Instabilities at the longest, or intermediate length scales may signal collapse to globule, or necklace states, respectively. In the presence of added-salt, a generalized electrostatic persistence length is obtained, which has a nontrivial dependence on the Debye screening length. It is also found that the onset of conformational instability is a re-entrant phenomenon as a function of polyelectrolyte length for the unscreened case, and the Debye length or salt concentration for the screened case. This may be relevant in understanding the experimentally observed re-entrant condensation of DNA.Comment: 8 pages, 4 figure

In vitro rumen fermentation of diets with different types of condensed tannins derived from sainfoin (Onobrychis viciifolia Scop.) pellets and hazelnut (Corylus avellana L.) pericarps

The aim of this study was to evaluate the in vitro rumen fermentation parameters of diets including pellets of sainfoin pellets and/or hazelnut pericarps, which are two plant resources that containing different types of condensed tannins (CT) with contrasted structures, using a batch culture system during for 24 h. The treatments were a basal diet (control), the basal diet + pellets of dehydrated sainfoin (PS), the basal diet + freeze-dried hazelnut pericarps (HP), and the basal diet + PS + HP. The diets were adjusted to be isotannic (20 g/kg dry matter (DM), except for the control) and isoproteic (132 g/kg DM). Total gas and methane (CH4) productions were measured after 3.5 h and 24 h of incubation. At the end of incubation, pH, in vitro DM degradability (IVDMD) and the concentration of fermentation end-products in the medium were also measured. The CT structures of CT infrom PS and HP were very different: as PS showed a PD-dominant profilehad mostly prodelphinidins and HP showed a PC-dominant profile mostly procyanidins. After 24 h of incubation, the total gas and methane productions and IVDMD were greater for the basal diet than for the diet + HP and the diet + PS (P<0.05). The CH4 production increased significantly with the diet + HP in the presence of PEG, a compound CT-inactivating CTcompound (P<0.001), and tended to increase for the diet + PS (P<0.1). The volatile fatty acids (VFA) net productions were globally similar among treatments, while the NH3 concentration was lower for the diet + PS (with a significant PEG effect) than for the diets including HP, and was the highest for the basal diet. It was concluded that the inclusion of PS and HP in a basal diet results in lower rumen fermentability and that but their CT decreased CH4 production and protein degradability;, PS being were more efficient effective than HP for the latterreducing protein degradability

Pin1 and neurodegeneration: a new player for prion disorders?

Pin1 is a peptidyl-prolyl isomerase that catalyzes the cis/trans conversion of phosphorylated proteins at serine or threonine residues which precede a proline. The peptidyl-prolyl isomerization induces a conformational change of the proteins involved in cell signaling process. Pin1 dysregulation has been associated with some neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease and Huntington's disease. Proline-directed phosphorylation is a common regulator of these pathologies and a recent work showed that it is also involved in prion disorders. In fact, prion protein phosphorylation at the Ser-43-Pro motif induces prion protein conversion into a disease-associated form. Furthermore, phosphorylation at Ser-43-Pro has been observed to increase in the cerebral spinal fluid of sporadic Creutzfeldt-Jakob Disease patients. These findings provide new insights into the pathogenesis of prion disorders, suggesting Pin1 as a potential new player in the disease. In this paper, we review the mechanisms underlying Pin1 involvement in the aforementioned neurodegenerative pathologies focusing on the potential role of Pin1 in prion disorders