Light scattering studies of the dilute solution behaviour of κ-, ι- and λ-carrageenan

Dilute polyelectrolyte solutions: recent progress and open questions

Polyelectrolytes are a class of polymers possessing ionic groups on their repeating units. Since counterions can dissociate from the polymer backbone, polyelectrolyte chains are strongly influenced by electrostatic interactions. As a result, the physical properties of polyelectrolyte solutions are significantly different from those of electrically neutral polymers. The aim of this article is to highlight key results and some outstanding questions in the polyelectrolyte research from recent literature. We focus on the influence of electrostatics on conformational and hydrodynamic properties of polyelectrolyte chains. A compilation of experimental results from the literature reveals significant disparities with theoretical predictions. We also discuss a new class of polyelectrolytes called poly(ionic liquid)s that exhibit unique physical properties in comparison to ordinary polyelectrolytes. We conclude this review by listing some key research challenges in order to fully understand the conformation and dynamics of polyelectrolytes in solutions.

Comparative Analysis of the Functional Properties of Films Based on Carrageenans, Chitosan, and Their Polyelectrolyte Complexes

The influence of the structural features of carrageenan on the functional properties of the films was studied. The carrageenans and chitosan films, as well as three-layer films containing a polyelectrolyte complex (PEC) of the two, were prepared. The X-ray diffractograms of carrageenan films reflected its amorphous structure, whereas chitosan and three-layer films were characterized by strong reflection in the regions of 20° and 15° angles, respectively. The SEM of the cross-sectional morphology showed dense packing of the chitosan film, as well as the layer-by-layer structure of different densities for the PEC. Among the tested samples, κ/β-carrageenan and chitosan films showed the highest tensile strength and maximum elongation. Films containing the drug substance echinochrome were obtained. Mucoadhesive properties were assessed as the ability of the films to swell on the mucous tissue and their erosion after contact with the mucosa. All studied films exhibited mucoadhesive properties. All studied films exhibited mucoadhesive properties which depended on the carrageenans structure. Multilayer films are stronger than single-layer carrageenan films due to PEC formation. The resulting puncture strength of the obtained films was comparable to that of commercial samples described in the literature.

Polydispersity and negative charge are key modulators of extracellular matrix deposition under macromolecular crowding conditions

Macromolecular crowding is a biophysical phenomenon that stems from the volume excluded by macromolecules, as they undergo steric repulsion and electrostatic interactions. The excluded volume depends on the shape, size, charge and polydispersity of the molecules. Although theoretical/computational models have been used to assess the influence of macromolecular crowding in biological media, real-time experiments are scarce. Herein, we evaluated the influence of hydrodynamic radius, charge and polydispersity of (a) various concentrations of different crowders (carrageenan, Ficoll™ and dextran sulphate); (b) various molecular weights of different crowders (70, 400 and 100 kDa of Ficoll™ and 10, 100 and 500 kDa of dextran sulphate) and (c) various cocktails of the same crowders (cocktails of various concentrations of different molecular weights Ficoll™ and dextran sulphate) on extracellular matrix deposition in human dermal fibroblast culture. The use of crowding cocktails with different molecular weight/concentrations of Ficoll™ or dextran sulphate molecules led to increased polydispersity and enhanced collagen type I deposition in comparison to their mono-domain counterparts. Carrageenan, however, induced the highest deposition of collagen type I due to its negative charge and inherent polydispersity. Our data contribute to a better understanding of the influence of the biophysical properties of the crowders on extracellular matrix deposition in vitro. STATEMENT OF SIGNIFICANCE: Macromolecular crowding is a biophysical phenomenon that accelerates and enhances extracellular matrix deposition in cell culture systems. Herein, we demonstrate that negatively charged and polydispersed macromolecules or cocktails of macromolecules, as opposed to neutral and monodomain macromolecules, induce highest extracellular matrix deposition in human dermal fibroblast cultures.

Unravelling secondary structure changes on individual anionic polysaccharide chains by atomic force microscopy

The structural conformations of the anionic carrageenan polysaccharides in the presence of monovalent salt close to physiological conditions are studied by atomic force microscopy. Iota-carrageenan undergoes a coil-helix transition at high ionic strength, whereas lambda-carrageenan remains in the coiled state. Polymer statistical analysis reveals an increase in persistence length from 22.6±0.2 nm in the random coil, to 26.4±0.2 nm in the ordered helical conformation, indicating an increased rigidity of the helical iota-carrageenan chains. The many decades-long debated issue on whether the ordered state can exist as single or double helix, is conclusively resolved by demonstrating the existence of a unimeric helix formed intramolecularly by a single polymer chain.

Measuring the size of polymers with negative radii using MALS/QELS: an exploration of the thermodynamic radius

The concept of 'size' in polymer science can have several interpretations, including definitions that rely on either statistical or equivalent-hard-sphere measures of the spatial extent of macromolecules in solution. A definition such as that of the equivalent thermodynamic radius (R(T)), which relies on the second virial coefficient of the polymer solution, offers the possibility of a zero or even a negative size parameter for macromolecules, depending on whether the polymer solution is in a theta or poor thermodynamic state, respectively. Here, we present the results of multi-angle light scattering measurements of R(T) for polystyrene and poly(methyl methacrylate), showing positive, negative, and zero values for this radius, depending on dilute solution thermodynamics. These results are augmented with measurements of the hydrodynamic radius, using quasi-elastic light scattering, and with random-walk-based calculations of the root-mean-square and viscometric radii. Re-examination of the literature provides additional examples of negative radii of polymers and oligomers.

Conformational Changes in ι- and κ-Carrageenans Induced by Complex Formation with Bovine β-Casein

The formation of electrostatic complexes between beta-casein and iota- and kappa-carrageenans is well-known. However, the molecular mechanism of the complexation has yet to be determined, particularly with respect to the conformational changes of the interacting macromolecules. High-sensitivity differential scanning calorimetry was used to study beta-casein/carrageenan mixtures at different pH values (3.0 to 7.5), ionic strengths (0.03 and 0.15 M), and various molar protein/polysaccharide ratios (3-400). The effects of these variables on the temperature, enthalpy, and width of the helix-coil transition of iota- and kappa-carrageenans were investigated. Neither pH nor the protein/polysaccharide ratio influenced the transition temperature of either carrageenan in the complexes. However, the transition enthalpy of both carrageenans in complexes with beta-casein decreased to zero with both decreasing pH and increasing protein/polysaccharide ratio. This may reflect an unwinding of the polysaccharide double helix induced by beta-casein, a conformational change which is fully reversible in conditions of sufficiently high ionic strength. The interaction of beta-casein with iota- and kappa-carrageenans was approximated in terms of the model of binding of large ligands to macromolecules, that provides the binding constants for these biopolymers.

Polyelectrolyte Films Based on Polysaccharides of Different Conformations: Effects on Multilayer Structure and Mechanical Properties

Ultrathin films were prepared with cationic poly(allylamine hydrochloride) (PAH) and two anionic polysaccharides, iota- and lambda-carrageenan, of similar chemical composition but different conformations using the layer-by-layer (LbL) technique. The study of aqueous solutions of carrageenans confirms that iota-carrageenan is at room temperature in helical conformation while lambda-carrageenan is in random coil conformation. Characterization of the multilayers by ellipsometry, circular dichroism, and AFM revealed that iota-carrageenan keeps its helical conformation within the films while lambda-carrageenan chains are in random coil conformation. Investigation of the mechanical properties of the films by performing nanoindentation experiments using force spectroscopy showed clear differences between the two films based on carrageenans of different conformations.

Thermodynamics of conformational ordering of iota-carrageenan in KCl solutions using high-sensitivity differential scanning calorimetry

Thermodynamic properties of aqueous solutions of iota-carrageenan as affected by KCl (0.15-1.2 M) or iota-carrageenan (0.5-6 mg/mL) content were studied by high-sensitivity differential scanning calorimetry. The polysaccharide was found to undergo two consecutive cooperative conformational transitions, which can be represented by the scheme: [H2]2<-->2H2<-->4C where C is the random coil, H2 is the double helix, and [H2]2 is the double helix dimer. The first transition follows by the "all or none" mechanism. The profile of the second transition resembles that of a second-order phase transition. The parameter sigma (of order 1), estimated for this latter transition, suggests that the stacking effect in helices of iota-carrageenan is rather small. The cooperativity of the transition is mainly defined by the loop factor. Free energies of both transitions at 273 K were calculated as a function of salt concentration. These experimental data were found to agree with Manning's theory.

On-line direct determination of the second virial coefficient of a natural polysaccharide using size-exclusion chromatography and multi-angle laser light scattering

By combining a size-exclusion chromatographic (SEC) separation and an on-line multi-angle light scattering (MALLS) analysis, we have elaborated an original methodology permitting on-line direct determination of the second virial coefficient of molar mass fractions of polydisperse polysaccharides. By assimilating the SEC-MALLS data to a batch mode acquisition, we have obtained on-line the complete Zimm plot of the eluted fractions, leading to knowledge of their weight-average molar mass Mw, radius of gyration r(g) and second virial coefficient A2. Our methodology was successfully applied to a iota carrageenan sample in LiCl 100 mM, EDTA 1 g/l.

Relationship between conformation of polysaccharides -in the dilute regime and their interaction with a phospholipid bilayer

Interactions between polysaccharides and phospholipid bilayers have already been demonstrated in the literature but little is known about the influence of macromolecule conformations related to the solvent characteristics (pH, ions, ionic strength). In this study we have investigated the conformation of iono- and thermo-sensitive polysaccharides, iota- and kappa-carrageenans, and their interaction with a dimyristoylphosphatidylcholine (DMPC) model bilayer. The study was performed in two different media (NaCl 150 mmol/L, pH 6.5, and NaCl 300 mmol/L, pH 6.5). In the first part, the iota- and kappa-carrageenan samples have been characterized by size exclusion chromatography (SEC) coupled with a multi-angle laser light-scattering detector (MALLS). The SEC-MALLS results clearly show polysaccharide chain association at high ionic strength. In the second part, the polysaccharide-membrane interaction has been studied, using fluorescent probes embedded in the membrane. The thermotropic properties of the membrane were investigated by fluorescence depolarization of 1-(4-trimethylammonium-phenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH). The membrane surface accessibility was evaluated by fluorescence quenching of 2-(9-anthroyloxy) stearic acid (2-AS). Whatever the ionic strength tested, the polysaccharide presence notably enhances the membrane fluidity below the T(m). This sign of an interaction in the polar level of the membrane is more marked at low NaCl concentration. In contrast, the liposomes bilayer accessibility is drastically lowered when increasing the ionic strength. This is induced by macromolecular chain adsorption on the liposome surface, enhanced by the polysaccharide chain association. An ionic strength enhancement induces a conformational modification of the polysaccharide chains which modifies their ability to interact with the bilayer.

Static and dynamic light scattering of biological macromolecules: what can we learn?

Laser light scattering comes in two major 'flavors': dynamic and static. This noninvasive technique provides a means for investigating key size and shape properties of macromolecules in solution. Light scattering has long been an indispensable tool to the polymer physical chemist, and is seeing increased use in exploring properties of biological macromolecules, alone and in association. As examples, recent investigations using light scattering have clearly demonstrated the relationship between the self-association and activity of important regulatory enzymes, and examined conformational properties of DNA and polysaccharides.