On the specific role of coions and counterions on kappa-carrageenan conformation

On the origin of optical rotation changes during the κ-carrageenan disorder-to-order transition

It is well established that solutions of both polymeric and oligomeric κ-carrageenan exhibit a clear change in optical rotation (OR), in concert with gel-formation for polymeric samples, as the solution is cooled in the presence of certain ions. The canonical interpretation - that this OR change reflects a 'coil-to-helix transition' in single chains - has seemed unambiguous; the solution- or 'disordered'-state structure has ubiquitously been assumed to be a 'random coil', and the helical nature of carrageenan in the solid-state was settled in the 1970s. However, recent work has found that κ-carrageenan contains substantial helical secondary structure elements in the disordered-state, raising doubts over the validity of this interpretation. To investigate the origins of the OR, density-functional theory calculations were conducted using atomic models of κ-carrageenan oligomers. Changes were found to occur in the predicted OR owing purely to dimerization of chains, and - together with the additional effects of slight changes in conformation that occur when separated helical chains form double-helices - the predicted OR changes are qualitatively consistent with experimental results. These findings contribute to a growing body of evidence that the carrageenan 'disorder-to-order' transition is a cooperative process, and have further implications for the interpretation of OR changes demonstrated by macromolecules in general.

Helicity degree of carrageenan conformation determines the polysaccharide and water interactions

The polysaccharide in solution at critical concentration, Cc (g/L), is assimilated to a nano hydrogel (nHG) made of a single polysaccharide chain. Taking as reference the characteristic temperature of 20 ± 2 °C at which kappa-carrageenan (κ-Car) nHG swelling is greater with a Cc = 0.55 ± 0.05 g/L, the temperature of the minimum deswelling in the presence of KCl was found at 30 ± 2 °C for 5 mM with a Cc = 1.15 ± 0.05 g/L but not measurable above 100 °C for 10 mM of which Cc = 1.3 ± 0.05 g/L. Lowering the temperature to 5 °C, contraction of the nHG and further coil-helix transition with self-assembly increases the sample's viscosity, which steadily evolves with time in a logarithmic scale. Accordingly, the relative increment of the viscosity per unit of concentration, R_v (L/g), should increase in agreement with increasing polysaccharide concentration. But the R_v decreases for κ-Car samples above 3.5 ± 0.5 g/L in the presence of 10 mM KCl under steady shear 15 s^-1. This reflects a decrease of κ-Car helicity degree knowing that the polysaccharide is rather hydrophilic when its helicity degree is the lowest.

Diversity of Bioinspired Hydrogels: From Structure to Applications

Hydrogels are three-dimensional networks with a variety of structures and functions that have a remarkable ability to absorb huge amounts of water or biological fluids. They can incorporate active compounds and release them in a controlled manner. Hydrogels can also be designed to be sensitive to external stimuli: temperature, pH, ionic strength, electrical or magnetic stimuli, specific molecules, etc. Alternative methods for the development of various hydrogels have been outlined in the literature over time. Some hydrogels are toxic and therefore are avoided when obtaining biomaterials, pharmaceuticals, or therapeutic products. Nature is a permanent source of inspiration for new structures and new functionalities of more and more competitive materials. Natural compounds present a series of physico-chemical and biological characteristics suitable for biomaterials, such as biocompatibility, antimicrobial properties, biodegradability, and nontoxicity. Thus, they can generate microenvironments comparable to the intracellular or extracellular matrices in the human body. This paper discusses the main advantages of the presence of biomolecules (polysaccharides, proteins, and polypeptides) in hydrogels. Structural aspects induced by natural compounds and their specific properties are emphasized. The most suitable applications will be highlighted, including drug delivery, self-healing materials for regenerative medicine, cell culture, wound dressings, 3D bioprinting, foods, etc.

Molecular dynamics simulations and X-ray scattering show the κ-carrageenan disorder-to-order transition to be the formation of double-helices

Recent molecular dynamics simulations, verified experimentally by solution-state x-ray scattering experiments, have found that κ-carrageenan chains contain helical secondary structure, akin to that found in the solid-state, even in aqueous solution. Furthermore, upon the addition of ions to single chains the simulations found no evidence that any conformational transitions take place. These findings challenge the long-held assumption that the so-called disorder-to-order transition in carrageenan systems involves a uni-molecular 'coil-to-helix transition'. Herein, the results of further molecular dynamics simulations undertaken using pairs of κ-carrageenan chains in 0.1 M NaI solutions are reported, and are validated experimentally using state-of-the-art solution-state WAXS experiments. From initially separated chains double-helices are shown to form, leading the authors to propose 'two single helices-to-stabilized double-helix' as a description of the molecular events taking place during the disorder-to-order transition.

Sulfate groups position determines the ionic selectivity and syneresis properties of carrageenan systems

The salt sensitivity and selectivity feature of α-carrageenan (α-Car) were investigated and compared with κ-carrageenan (κ-Car) and iota-carrageenan (ι-Car). These carrageenans are identified by one sulfate group on the 3,6-anhydro-D-galactose (DA) for α-Car, D-galactose (G) for κ-Car and on both carrabiose moieties (G and DA) for ι-Car. The viscosity and temperature, where order-disorder transition have been observed, were greater in presence of CaCl₂ for α-Car and ι-Car compared with KCl and NaCl. Conversely, the reactivity of κ-Car systems were greater in presence of KCl than CaCl₂. Unlike κ-Car systems, the gelation of α-Car in presence of KCl was observed without syneresis. Thus, the position of sulfate group on the carrabiose determines the importance of counterion valency too. The α-Car could be a good alternative to κ-Car to reduce the syneresis effects.

Rheological study of α- and κ-carrageenan expansion in solution as effects of the position of the sulfate group

The viscosity of carrageenan solutions in the coil state was greater for α-carrageenan (α-Car) compared with that for κ-carrageenan (κ-Car); thus, the impact of one sulfate group on 3,6-anhydro-D-galactose was compared with the impact of one sulfate group on D-galactose units of the carrabiose residues. The thermal expansion coefficient of the solutions, B₂ × T_c, characterizes the way the viscosity decreases because of extension of the physical bonds of the systems to their rupture point (T_c) under increasing temperature. The T_c and B₂ × T_c of water were equal to (100 ± 5) °C and (1.57 ± 0.05) × 10^-2/°C, respectively. The T_c of the α-Car and κ-Car systems increased after the addition of CaCl₂ and KCl, respectively, and with increasing polysaccharide concentration. However, the B₂ × T_c of the α-Car and κ-Car systems were rather sensitive to CaCl₂ and KCl, respectively. In the overall solutions examined, the expansion of α-Car systems was found to be between 1.5 × 10^-2/°C and 1.61 × 10^-2/°C, greater than the expansion of κ-Car systems, which was between 1.5 × 10^-2/°C and 1.2 × 10^-2/°C. Thus, α-Car is a good alternative to κ-Car for reducing syneresis phenomena, and its sensitivity as ι-Car to divalent cations would be due to the anhydro cycle.

Effects of gelation concentration on cyclic deformation behavior of κ-carrageenan hydrogels

The aim of this study is to elucidate the relationship between the network structure of κ-carrageenan hydrogels and their mechanical properties. First κ-carrageenan hydrogels have been prepared at different gelation concentrations, and then the mechanical behavior during four cyclic deformations has been examined at the same κ-carrageenan concentration. Young's modulus is higher for the gel prepared at 5 gL^-1 (C05) compared to that for the gel prepared at 30 gL^-1 (C30). C30 shows almost linear relation between the stress and the strain like an ideal rubber, while a residual strain appears in each cyclic deformation for C05. The extent of the residual strain depends on the maximum strain and the deformation speed, indicating that C05 deforms plastically to some extent. The residual strain for C05 decreases gradually even after a cyclic deformation and disappears in the case of a small strain as if there were a memory of the structure. The effects of the gelation concentration on the mechanical properties have been explained based on the network structure specific to κ-carrageenan hydrogels. The higher modulus for C05 has been attributed to the higher helix content and the plastic deformation of C05 to the loosely-aggregated crosslinks.

Lower critical concentration temperature as thermodynamic origin of syneresis: Case of kappa-carrageenan solution

Polysaccharide ubiquity is trimmed for applications of low syneresis impact. This syneresis may be crucial for specific applications that are very sensitive to gel dimension stability, namely, 3D scaffolds for cell culture for disease diagnosis and tissue engineering. We hypothesized that the syneresis origin results from the kappa-carrageenan (kC) polysaccharide thermodynamic instability, and we demonstrated this by measuring the critical (coil-to-coil contact) concentration as a function of temperature. The impact of 5 mM, 10 mM and 15 mM KCl salt on the critical concentration of the solution and the lower critical concentration temperature (LCCT) were particularly investigated. For the kC polysaccharide, the gelation temperature (T_g) falls at temperatures below the LCCT, which explains the shrinking or syneresis reaction of the polysaccharide gels. The gap between T_g and LCCT would be the thermomotive force of the syneresis of many colloidal gels.

Continuous flow structuring of anisotropic biopolymer particles

We review concepts and provide examples for the controlled structuring of biopolymer particles in hydrodynamic flow fields. The structuring concepts are grouped by the physical mechanisms governing drop deformation and shaping: (i) capillary structuring, (ii) shear and elongational structuring and (iii) confined flow methods. Non-spherical drops can be permanently structured if a solidification process, such as gelation or glass formation in the bulk or at the interface, is superimposed to the flow field. The physical and engineering properties of these processes critically depend on an elaborate balance between capillary phenomena, rheology, gel or glass formation kinetics, and bulk heat, mass and momentum transfer in multiphase fluids. This overview is motivated by the potential of non-spherical suspension particles, in particular those formed from 'natural' and 'sustainable' biopolymers, as rheology modifiers in food materials, consumer products, cosmetics or pharmaceuticals.

Effect of genipin, a naturally occurring crosslinker on the properties of kappa-carrageenan

Genipin-fixed kappa-carrageenan was prepared in ambient conditions in aqueous solution using genipin, a naturally occurring crosslinker with kappa-carrageenan (kappaC). The crosslinked kappaC showed the greatest swelling capacity in acidic medium having pH 1.2 when compared to those in neutral and alkaline media. Enhanced stability of the crosslinked product with respect to the non-modified kappaC was confirmed by degradation studies in Ringer's solution, rheological and thermogravimetric measurements. Genipin fixation of kappaC was confirmed by measuring the bulk density, true density, pore volume, porosity, intrinsic viscosity, UV absorbance and optical rotation of the crosslinked kappaC, and as well as by optical microscopy, SEM, and MS/MS studies. The crosslinked product may be useful as super absorbent and sustained release formulation in biomedical applications.

Non-Covalent Interactions in Polysaccharide Systems

[Chemical structure: see text] This paper describes the behavior of some polysaccharides with well-known chemical structures and in which the influence of cooperative secondary interactions play an important role. The roles played by hydrophobic and ionic interactions (including ionic selectivity) on polysaccharide conformation and gelation are discussed. Electrostatic attractions are also important in the complexes formed between surfactants and polyelectrolytes of opposite charge. Finally, van der Waals dipolar interactions and particularly hydrogen-bond formation are examined. The role of hydrogen bonds in solubility, conformation, and especially the local stiffness of polysaccharides, but also in polymer-polymer complexes frequently obtained with polysaccharides, is developed. Repeat unit for a number polysaccharides.

Sorbitan tristearate layers at the air/water interface studied by shear and dilatational interfacial rheology

The shear and dilatational rheology of condensed interfacial layers of the water-insoluble surfactant sorbitan tristearate at the air/water interface is investigated. A new interfacial shear rheometer allows measurements in both stress- and strain-controlled modes, providing comprehensive interfacial rheological information such as the interfacial dynamic shear moduli, the creep response to a stress pulse, the stress relaxation response to a strain step, or steady shear curves. Our experiments show that the interfacial films are both viscoelastic and brittle in nature and subject to fracture at small deformations, as was supported by in-situ Brewster angle microscopy performed during the rheological experiments. Although any large-deformation test is destructive to the sample, it is still possible to study the linear viscoelastic regime if the deformations involved are controlled carefully. Complementary results for the dilatational rheology in area step compression/expansion experiments are reported. The dilatational behavior is predominantly elastic throughout the frequency spectrum measured, whereas the layers exhibit generalized Maxwell behavior in shear mode within a deformation frequency regime as narrow as two decades, indicating the presence of additional relaxation mechanisms in shear as opposed to expansion/compression. If the transient rheological response from stress relaxation experiments is considered, then the data can be described well with a stretched exponential model both in the shear and dilatational deformations.

Unperturbed dimensions of Carrageenans in different salt solutions

Commercial samples of kappa-, iota-, and lambda-carrageenans were studied by means of Size Exclusion Chromatography with dual detection, i.e. employing a Refractive Index (concentration sensitive) and Multiangle Light Scattering (size sensitive) detectors. The eluent was water containing 0.1 M concentration of different ionic salts, namely LiCl, NaCl, KCl and NaI, with the exception of kappa-carrageenan that aggregates in presence of KCl. Molecular weight distributions and averages, coefficients of the scaling law of molecular dimensions and unperturbed dimensions were thus obtained from a single polydisperse sample of each polymer. Measurements were performed at 25 degrees C and all the systems were above theta conditions with values of the q exponent of the scaling law ranging from 0.51 to 0.59. Extrapolation to unperturbed conditions provides values of the characteristic ratio C(N)=56+/-1 and 40+/-5 respectively for lambda- and iota-carrageenans regardless of the ionic salt employed. However, kappa-carrageenan gives C(N)=31, 35 and 59, respectively, in presence of LiCl, NaCl and NaI, which clearly indicates that this polymer behaves on a different way in presence of NaI than with the other two salts. A tentative explanation of this difference is provided.

K+ and Na+ effects on the gelation properties of κ-Carrageenan

The effects of K(+), Na(+) ions and their mixture on the conformational transition and macroscopic gel properties of kappa-Carrageenan system have been studied using different experimental techniques. The macroscopic gelation properties of kappa-Carrageenan were found to be dependent upon cosolute type. Indeed, a more ordered and strong gel was obtained in the presence of K(+) with respect to Na(+) ions. The gel properties obtained using mixtures of two cosolutes are shown to depend on the [K(+)]/[Na(+)] ratio.

Non-equilibrium processes of interchain association induced by Cs+ ions in κ-carrageenan aqueous solutions

We present preliminary results of the investigation of interchain association processes induced by Cs+ ions in kappa-carrageenan aqueous solution. The solutions contained variable amounts of NaI and CsI, under the condition that the total concentration of 1:1 electrolyte was 0.1M. The associative processes were observed by static light scattering under isothermal conditions (at T = 25 degrees C), after cooling molecularly dispersed solutions obtained at high temperature (80 degrees C). It was found that, under all the investigated conditions of polymer concentration (from 0.2 to 2 g l(-1)) and ionic composition, the onset of time-dependent association fails to lead to an equilibrium, but proceeds up to physical gelation of the associating system. Depending on the experimental variables, however, the gelation threshold may take up to several days to be achieved.

The effect of polyelectrolyte counterion specificity, charge density, and conformation on polyelectrolyte-amphiphile interaction: the carrageenan/furcellaran-amitriptyline system

The effect of polyelectrolyte cation specificity, charge density, and conformation on the interaction between furcelleran, kappa, iota, and lambda-carrageenan, respectively, and amitriptyline, an amphiphilic cationic drug molecule, was studied by means of a dialysis equilibrium technique. The carrageenans used in this study-furcelleran, kappa, iota, and lambda-carrageenan-had a charge density corresponding to 0.69, 0.92, 1.53, and 2.07 sulfate groups per disaccharide. In general, the binding isotherms followed the order Li(+) < Na(+) < N(CH3)(4)(+) < K(+) < or = Cs(+) approximately Rb(+), i.e., the binding isotherms were shifted to higher concentrations of free amphiphile according to the sequence indicated. This affinity sequence correlates well with that found for the dextran sulfate-amitriptyline system (A. Hugerth and L.-O. Sundelöf, Langmuir 2000, 16, 313-317). The factor determining the critical aggregation concentration in the presence of Na(+) compared to K(+) was found to be as follows: the flexibility (conformation) in the case of the lower charged carrageenans, i.e., furcelleran and kappa-carrageenan, charge density for iota-carrageenan, and in the lambda-carrageenan case the difference in the ROSO(3)(-)-alkali ion specificity.

Gel--sol transition in kappa-carrageenan systems: microviscosity of hydrophobic microdomains, dynamic rheology and molecular conformation

The effect of gel-sol transition in kappa-carrageenan systems on the microviscosity of hydrophobic microdomains, as well as its relation to macroscopic rheology and molecular conformation, was studied in kappa-carrageenan systems. The microdomains were probed by 1,3-di(-1-pyrenyl)propane (P3P) for which the excimer intensity (Ie) provides relative measures of the microviscosity in the immediate probe surroundings. In particular the applicability of P3P to monitor the gel--sol transition was proved, the results showing a dramatic decrease in microviscosity in the vicinity of the transition point. The corresponding changes in rheological properties and carrageenan conformation were investigated by dynamic viscometry (DV) and optical rotation (OR), respectively. The temperature of onset of the transition as indicated by the microviscosity data (T0) was found to correlate well with the OR and DV-results. The application of microviscosity and OR-measurements allowed an estimation of the helical content at T0 to be determined. P3P-data indicate a microenvironment viscosity for the probe sites in the kappa-carrageenan system comparable to that found in SDS micelles.