Kanji Kajiwara

Structural characteristics of carrageenan gels: temperature and concentration dependence

Abstract κ- and ι-carrageenan in aqueous solution undergoes reversible sol/gel transition by cooling or increasing concentration. The small-angle X-ray scattering profiles from carrageenans were observed at different temperatures and concentrations. The results were analyzed by means of molecular models of double helix or associated double helices. It was found that κ-carrageenan formed two or three associated double helices by gelation, although gelation took place in ι-carrageenan mostly by the transition from single chain to double helix without association. λ-carrageenan exhibited the normal scattering behavior of polyelectrolyte in solution.

Gel point in physical gels: rheology and light scattering from thermoreversibly gelling schizophyllan

Abstract The β-1.3/1.6 glucan schizophyllan is a fungal polysaccharide that is known to suppress tumor growth. Thermoreversible, optically transparent gels of schizophyllan were obtained on addition of sorbitol to aqueous solutions of schizophyllan. Gel temperatures were independent of the history of sample preparation. The gelation process was followed by low-amplitude oscillatory shear and by static and dynamic light scattering. Power law behavior of storage modulus G′(ω) and loss modulus G″(ω) was found at the gel temperature with the exponent n=0.5. The static LS measurements showed a strong irregularity at the gel temperature. The intensity time correlation functions (TCFs) as measured by dynamic LS during the gelation process were transformed into the electric field TCFs, taking into account the heterodyne contributions. At the gel temperature which was estimated by rheology, DSC and static LS, no power law was found for the TCF. The angular dependence of the intensity TCF was measured and converted to the field TCF by use of a non-ergodicity treatment and by the heterodyne approximation, respectively. Both methods result in mean relaxation times that agree quite well. Two relaxation modes were found in the whole temperature range (above and below the gel temperature) indicating the existence of inhomogeneities in the gel.

Structural characteristics of carrageenan gels: various types of counter ions

The gelation mechanism of K- and L-carrageenan with various types of monovalent counter ions are considered by means of small-angle X-ray scattering. The extent of chain aggregation was discussed with the cross-sectional radius of gyration estimated from cross-sectional Gunier plots. The gelation of K-carrageenan found to involve the multiple aggregation of double helices except for Li + and Na + -type, while τ-carrageenan gel was consisted of just double helices.

Gelation of tamarind seed polysaccharide xyloglucan in the presence of ethanol

Abstract Tamarind seed polysaccharide xyloglucan undergoes thermoreversible gelation in the presence of ethanol. Its gel–sol transition process was observed by time-resolved small-angle X-ray scattering, and the results were analyzed by assuming two phases composed of random aggregates and of single chains. The analysis has revealed that the complete dissolution of aggregates corresponds to the gel–sol transition. Here the cross-linking domain seems to be composed of random aggregates and has no ordered structure.

Inhomogeneous structure of polyurethane networks based on poly(butadiene)diol: 1. The effect of the poly(butadiene)diol content

Abstract The inhomogeneous structure of crosslinked polyurethanes based on poly(butadiene)diol (PBD), 4,4′-diphenylmethane diisocyanate (MDI) and poly(oxypropylene) triol (POPT) prepared with various PBD contents were studied by dynamic mechanical analysis (DMA) and synchrotron small-angle X-ray scattering (SAXS). An inhomogeneous two-phase structure of networks containing PBD was revealed by SAXS. One phase consists of PBD chains while the other of densely crosslinked POPT/MDI network with much higher glass transition temperature. The range of inhomogeneity depends on PBD content, and is estimated as about 70–120 A from the Bragg spacing. The scattering data are fitted using the Percus-Yevick hard sphere model.

CONFORMATION OF CYCLOMALTOOLIGOSACCHARIDE ( CYCLOAMYLOSE ) OF DP21 IN AQUEOUS SOLUTION

Abstract The conformation of a cyclomaltooligosaccharide [“cycloamylose”, cyclic (1 → 4)-α- d -glucan ] having degree of polymerization 21 was characterized by means of small-angle X-ray scattering (SAXS) with the aid of simulations of scattering profile based on molecular models. The results indicate that the cycloamylose chains adopt a circularized three-turn single helical structure with a radius of gyration of 11.5 A. The scattering profiles calculated for double helical structures with foldbacks at each end are not in good agreement with experimental profiles. Molecular dynamics simulations in water show that overall the conformation of the three-turn single-helical form is maintained at 300 K, although replacements of glucose units are observed. In one model, ninety-six water molecules are interacting with a cycloamylose molecule judging from hydrogen-bonding distances and angles. The conformation of cyclic (1 → 4)-α- d -glucan (cycloamylose) was determined via small-angle X-ray scattering (SAXS).

Micellar structure of β-casein observed by small-angle X-ray scattering

Abstract The small-angle X-ray scattering was observed from β-casein micelles in 0.2 M phosphate buffer (pH 6.7) with varying temperatures. An oblate ellipsoid of a rigid core with a thin soft layer was proposed as a probable model of the β-casein micellar structure, according to the results of the model optimization with simple triaxial bodies. Here the axial ratio was found to decrease and the micelle to become spherical when the polymerization proceeds with temperature. The consistency of the present model was examined with the results of hydrodynamic measurements published previously.

The sol-gel transition of gellan gum aqueous solutions in the presence of various metal salts

Gellan gum undergoes gelation by forming domains composed of associated double helices. Gellan gum aqueous solutions were prepared with or without adding various metal salts, and their gel-sol transitions were observed by small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC). The SAXS results are analyzed in terms of the molecular models of associated double helices, and the DSC endo-thermic peaks are attributed to the possible dissociation processes of the domains formed upon gelation. The mechanism of the gel-sol transition is discussed from the view point of the role of metal cations.

Molecular characteristics and gelling properties of the carrageenan family, 1: Preparation of novel carrageenans and their dilute solution properties

The carrageenans are a family of sulfated polysaccharides extracted from red seaweed, and are conventionally classified into three major groups as κ-, ι- and λ-carrageenan according to the maximum number (1, 2 and 3, respectively) of sulfate groups per disaccharide repeating unit in corresponding ideal structures. β- and θ- carrageenan (not available in nature) possessing none and two sulfate groups per repeating unit, respectively, were prepared from κ- and λ-carrageenan by desulfonation. Here the aqueous solutions of κ-, ι-, β- and θ-carrageenans undergo the thermoreversible gelation by lowering temperature, whereas the aqueous solution of λ-carrageenan forms no gel.Light scattering and small-angle X-ray scattering were observed from the aqueous solution of κ-, ι-, λ-, β- and θ-carrageenans, and the results were analyzed conventionally and by adapting the molecular model in order to elucidate the effect of sulfate groups and anhydrous residue on the conformational characteristics of carrageenans.

Conformation of cyclic and linear (1 → 2)-β-d-glucans in aqueous solution

Abstract The conformations of cyclic (1 → 2)-β- d -glucan chains having degree of polymerization (dp) 17 to 24 were characterized by means of small-angle X-ray scattering and Monte Carlo simulation. The results indicate that cyclic (1 → 2)-β- d -glucan chains adopt the shape of a doughnut-like ring with a thickness of about 10 A for all the samples. The diameter of the annulus for the cyclic glucan having dp 21 is estimated to be only about 4–5 A. Two linear (1 → 2)-β- d -glucans possessing dp 19 and 21 prepared by acid hydrolysis of a cyclic glucan and subsequent fractionation showed different scattering profiles from those obtained for cyclic glucans having the corresponding dp. Although the Monte Carlo simulation does not completely reproduce the scattering profiles observed by small-angle X-ray scattering, linear (1 → 2)-β- d -glucans seem to possess a characteristic cylindrical shape with cross-sectional diameters of 11.8 and 13.2 A for linear glucans of dp 19 and 21, respectively.