Observation of the Molecular Weight Change during the Helix−Coil Transition of κ-Carrageenan Measured by the SEC−LALLS Method

Preparation and characterization of curdlan with unique single-helical conformation and its assembly with Congo Red

Elucidating the structure-activity relationship of curdlan is hampered by a lack of characterization with unique specific conformations (i.e., single- or triple-helix). In this study, single-helical curdlan is generated in dilute NaOH solutions at 35-50 °C, and characterized with NMR, SAXS, and GPC. The conformational transition from coil to single-helix and the intramolecular hydrogen bond interaction are explored using NMR. It is found that the two aforementioned types of curdlan interact with Congo Red in very different ways. Single-helical curdlan can encapsulate Congo Red to form a stable, supramolecular dye assembly, which is demonstrated by the shortest distance between the H3 of curdlan and the phenyl groups of Congo Red, and also the same self-diffusion coefficients of Congo Red and curdlan. In contrast, random-coil curdlan interacts weakly with Congo Red and cannot enwrap it. This study offers insight into the specific structure-activity relationship of beta-(1,3)-glucans.

Molecular dynamics simulations of two double-helical hexamer fragments of iota-carrageenan in aqueous solution

The gelation of anionic carrageenans is known to occur through a coil-to-helix transition followed by further aggregation or association on which positive counterions play a significant role. In the present work, molecular dynamics (MD) simulations were performed on two double-helical iota-carrageenan hexamer fragments along with their sodium counterions using the Carbohydrate Solution Force Field (CSFF) in an aqueous (TIP4P) solution with the GROMACS molecular dynamics package. Results showed a counterion condensation between the two double helices and that the subsequent forces of interaction between them were predominantly attractive. By varying the distance separating the two double helices, the effect of distance on the counterion distribution and the forces of attraction was also investigated. In the presence of counterions, an increase in the forces of attraction was observed as the distance between the two double helices decreases which can be attributed to the greater counterion density between the two like-charged oligosaccharides.

Recoverable and Self-Healing Double Network Hydrogel Based on κ-Carrageenan

Combining an ionically cross-linked κ-carrageenan network with a covalently cross-linked polyacrylamide network, we have fabricated a κ-carrageenan/polyacrylamide double network (DN) hydrogel using a one-pot method. The resulting DN hydrogel exhibited a high elastic modulus of 280 kPa and a fracture energy of 6150 J/m². A model has been proposed considering unzipping of double-helical aggregates and dissociation of double helices of κ-carrageenan, which could well interpret the continuous fracture process of the first κ-carrageenan network and the toughening mechanism of the physically and chemically cross-linked κ-carrageenan/polyacrylamide DN hydrogel. Owing to the thermoreversible nature of κ-carrageenan, the DN hydrogel also exhibited the excellent recoverable property. For example, the elastic modulus and energy dissipation could be recovered to 100% and 98% after the deformed and relaxed DN samples were stored at 90 °C for 20 min. Furthermore, the temperature-dependent sol-gel transition is also related to the self-healing property of the DN hydrogel.

Supramolecular chiral self-assembly and supercoiling behavior of carrageenans at varying salt conditions

The self-assembly of anionic kappa and iota carrageenan polysaccharides in the presence of NaCl, KCl and CaCl2 is studied by high-resolution atomic force microscopy (AFM). A hierarchical supramolecular chirality amplification over various length scales is observed upon the addition of KCl, whereas in the presence of NaCl and CaCl2 the chains undergo solely a coil-helix transition with stiff kappa carrageenan and more flexible iota carrageenan helical conformations.

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.

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.

The conformational free-energy map for solvated neocarrabiose

A Ramachandran map of the conformational potential of mean force (pmf) for neocarrabiose in water was obtained using molecular dynamics (MD) simulations with umbrella sampling. The potential energy map calculated in a previous study for this molecule in vacuum exhibited a global minimum located at (phi = 81 degrees, psi = -141 degrees). However, the global minimum on the new pmf map in aqueous solution is located in an area centered around (phi = 175 degrees, psi = 180 degrees), indicating a considerable solvent shift. This new global minimum-energy solution conformation was found to correspond to the experimental value obtained from NMR-NOE measurements, and is also consistent with the experimental crystal structure for neocarrabiose and the fiber diffraction conformation for iota-carrageenan. The global minimum of the solution pmf and its local topology were found to be approximately reproduced by quick vacuum conformational energy mapping using several approximations that mimic solvation effects by de-emphasizing intramolecular hydrogen bonding.

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.

Thermoreversible gelation of kappa-carrageenan: relation between conformational transition and aggregation

We have studied, by optical rotation dispersion, light scattering and rheology, the kappa-Carrageenan system to elucidate the processes involved in gel formation (on decreasing the temperature) and gel melting (on increasing the temperature). Our results show that, on decreasing the temperature, a conformational transition from coils to double helices first occurs, followed by aggregation of the double helices into domains and gel formation at appropriate polymer concentration. Structural details of this sequence are better revealed by re-heating the system. Melting appears as a two-step process characterized by first a conformational change of helices involved in junction zones between aggregates, followed by the conformational transition of the helices inside the aggregates. These helices can regain the coil conformation only when the aggregates melt at higher temperature, in full agreement with the old 'domain' model. The full description of the sol-gel mechanism of this system can be useful in the search for new methods to control the gel texture, a relevant property for many industrial applications.

Thermodynamics of the conformational transition of biopolyelectrolytes: the case of specific affinity of counterions

A formal development of the Counterion Condensation theory (CC) of linear polyelectrolytes has been performed to include specific (chemical) affinity of condensed counterions, for polyelectrolyte charge density values larger than the critical value of condensation. It has been conventionally assumed that each condensed counterion exhibits an affinity free-energy difference for the polymer, (DeltaG(aff)). Moreover, the model assumes that the enthalpic and entropic contributions to DeltaG(aff), i.e., DeltaH(aff) and DeltaS(aff), are both independent of temperature, ionic strength and polymer concentration. Equations have been derived relative to the case of the thermally induced, ionic strength dependent, conformational transition of a biopolyelectrolyte between two conformations for which chemical affinity is supposed to take place. The experimental data of the intramolecular conformational transition of the ionic polysaccharide kappa-carrageenan in dimethylsulfoxide (DMSO) have been successfully compared with the theoretical predictions. This novel approach provides the enthalpic and entropic affinity values for both conformations, together with the corresponding thermodynamic functions of nonpolyelectrolytic origin pertaining to the biopolymer backbone change per se, i.e., DeltaH(n.pol) and DeltaS(n.pol), according to a treatment previously shown to be successful for lower values of the biopolyelectrolyte linear charge density. The ratio of DeltaH(n.pol) to DeltaS(n.pol) was found to be remarkably constant independent of the value of the dielectric constant of the solvent, from formamide to water to DMSO, pointing to the identity of the underlying conformational process.