Competitive ion complexation to polyelectrolytes: determination of the stepwise stability constants. The Ca2+/H+/polyacrylate system

Unusual Aspects of Charge Regulation in Flexible Weak Polyelectrolytes

This article reviews the state of the art of the studies on charge regulation (CR) effects in flexible weak polyelectrolytes (FWPE). The characteristic of FWPE is the strong coupling of ionization and conformational degrees of freedom. After introducing the necessary fundamental concepts, some unconventional aspects of the the physical chemistry of FWPE are discussed. These aspects are: (i) the extension of statistical mechanics techniques to include ionization equilibria and, in particular, the use of the recently proposed Site Binding-Rotational Isomeric State (SBRIS) model, which allows the calculation of ionization and conformational properties on the same foot; (ii) the recent progresses in the inclusion of proton equilibria in computer simulations; (iii) the possibility of mechanically induced CR in the stretching of FWPE; (iv) the non-trivial adsorption of FWPE on ionized surfaces with the same charge sign as the PE (the so-called "wrong side" of the isoelectric point); (v) the influence of macromolecular crowding on CR.

Mineralized polyplexes for gene delivery: Improvement of transfection efficiency as a consequence of calcium incubation and not mineralization

Gene therapy is an emerging field in which nucleic acids are used to control protein expression. The necessity of delivering nucleic acids to specific cell types and intracellular sites demands the use of highly specialized gene carriers. As a carrier modification technique, mineralization has been successfully used to modify viral and non-viral carriers, providing new properties that ultimately aim to increase the transfection efficiency. However, for the specific case of polyplexes used in gene therapy, recent literature shows that interaction with calcium, a fundamental step of mineralization, might be effective to increase transfection efficiency, leaving an ambiguity about of the role of mineralization for this type of gene carriers. To answer this question and to reveal the properties responsible for increasing transfection efficiency, we mineralized poly(aspartic acid) coated polyplexes at various CaCl₂ and Na₃PO₄ concentrations, and evaluated the resultant carriers for physicochemical and morphological characteristics, as well as transfection and delivery efficiency with MC3T3-E1 mouse osteoblastic cells. We found that both mineralization and calcium incubation positively affected the transfection efficiency and uptake of polyplexes in MC3T3-E1 cells. However, this effect originated from the properties achieved by polyplexes after the calcium incubation step that are maintained after mineralization, including particle size increase, improved pDNA binding, and adjustment of zeta potential. Considering that mineralization can be a longer process than calcium incubation, we find that calcium incubation might be sufficient and preferred if improved transfection efficiency in vitro is the only effect desired.

Coupling of Charge Regulation and Conformational Equilibria in Linear Weak Polyelectrolytes: Treatment of Long-Range Interactions via Effective Short-Ranged and pH-Dependent Interaction Parameters

The classical Rotational Isomeric State (RIS) model, originally proposed by Flory, has been used to rationalize a wide range of physicochemical properties of neutral polymers. However, many weak polyelectrolytes of interest are able to regulate their charge depending on the conformational state of the bonds. Recently, it has been shown that the RIS model can be coupled with the Site Binding (SB) model, for which the ionizable sites can adopt two states: protonated or deprotonated. The resulting combined scheme, the SBRIS model, allows for analyzing ionization and conformational equilibria on the same foot. In the present work, this approach is extended to include pH-dependent electrostatic Long-Range (LR) interactions, ubiquitous in weak polyelectrolytes at moderate and low ionic strengths. With this aim, the original LR interactions are taken into account by defining effective Short-Range (SR) and pH-dependent parameters, such as effective microscopic protonation constants and rotational bond energies. The new parameters are systematically calculated using variational methods. The machinery of statistical mechanics for SR interactions, including the powerful and fast transfer matrix methods, can then be applied. The resulting technique, which we will refer to as the Local Effective Interaction Parameters (LEIP) method, is illustrated with a minimal model of a flexible linear polyelectrolyte containing only one type of rotating bond. LEIP reproduces very well the pH dependence of the degree of protonation and bond probabilities obtained by semi-grand canonical Monte Carlo simulations, where LR interactions are explicitly taken into account. The reduction in the computational time in several orders of magnitude suggests that the LEIP technique could be useful in a range of areas involving linear weak polyelectrolytes, allowing direct fitting of the relevant physical parameters to the experimental quantities.

Stabilization of iron oxide nanoparticles in high sodium and calcium brine at high temperatures with adsorbed sulfonated copolymers

A series of sulfonated random and block copolymers were adsorbed on the surface of ~100 nm iron oxide (IO) nanoparticles (NPs) to provide colloidal stability in extremely concentrated brine composed of 8% wt NaCl + 2% wt CaCl2 (API brine; 1.4 M NaCl + 0.2 M CaCl2) at 90 °C. A combinatorial materials chemistry approach, which employed Ca(2+)-mediated adsorption of anionic acrylic acid-containing sulfonated polymers to preformed citrate-stabilized IO nanoclusters, enabled the investigation of a large number of polymer coatings. Initially a series of poly(2-methyl-2-acrylamidopropanesulfonate-co-acrylic acid) (poly(AMPS-co-AA)) (1:8 to 1:1 mol:mol), poly(styrenesulfonate-block-acrylic acid) (2.4:1 mol:mol), and poly(styrenesulfonate-alt-maleic acid) (3:1 mol:mol) copolymers were screened for solubility in API brine at 90 °C. The ratio of AMPS to AA groups was varied to balance the requirement of colloid dispersibility at high salinity (provided by AMPS) against the need for anchoring of the polymers to the iron oxide surface (via the AA). Steric stabilization of IO NPs coated with poly(AMPS-co-AA) (1:1 mol:mol) provided colloidal stability in API brine at room temperature and 90 °C for up to 1 month. The particles were characterized before and after coating at ambient and elevated temperatures by a variety of techniques including colloidal stability experiments, dynamic light scattering, zeta potential, and thermogravimetric analysis.

A novel type of allosteric regulation: functional cooperativity in monomeric proteins

Cooperative functional properties and allosteric regulation in cytochromes P450 play an important role in xenobiotic metabolism and define one of the main mechanisms of drug-drug interactions. Recent experimental results suggest that ability to bind simultaneously two or more small organic molecules can be the essential feature of cytochrome P450 fold, and often results in rich and complex pattern of allosteric behavior. Manifestations of non-Michaelis kinetics include homotropic and heterotropic activation and inhibition effects depending on the stoichiometric ratios of substrate and effector, changes in the regio- and stereospecificity of catalytic transformations, and often give rise to the clinically important drug-drug interactions. In addition, functional response of P450 systems is modulated by the presence of specific and non-specific effector molecules, metal ions, membrane incorporation, formation of homo- and hetero-oligomers, and interactions with the protein redox partners. In this article we briefly overview the main factors contributing to the allosteric effects in cytochromes P450 with the main focus on the sources of cooperative behavior in xenobiotic metabolizing monomeric heme enzymes with their conformational flexibility and extremely broad substrate specificity. The novel mechanism of functional cooperativity in P450 enzymes does not require substantial binding cooperativity, rather it implies the presence of one or more binding sites with higher affinity than the single catalytically active site in the vicinity of the heme iron.

Calcium mediated polyelectrolyte adsorption on like-charged surfaces

Monte Carlo simulations within the primitive model of calcium-mediated adsorption of linear and comb polyelectrolytes onto like-charged surfaces are described, focusing on the effect of calcium and polyion concentrations as well as on the ion pairing between polymers and calcium ions. We use a combination of Monte Carlo simulations and experimental data from titration and calcium binding to quantify the ion pairing. The polymer adsorption is shown to occur as a result of surface overcharging by Ca(2+) and ion pairing between charged monomers and Ca(2+). In agreement with experimental observations, the simulations predict that the polymer adsorption isotherm goes through a maximum as the calcium or the polymer concentration is increased. The non-Langmuir isotherms are rationalized in terms of charge-charge correlations.