Triple-helical Polysaccharide Schizophyllan Research Articles

Gel formation and low‐temperature intramolecular conformation transition of a triple‐helical polysaccharide lentinan in water

The gelation behavior of the triple-helical polysaccharide lentinan fractions having different molecular weights in water at 25 degrees C were studied by using a rheometer. The analysis of concentration and molecular weight dependence of shear stress and shear viscosity showed that aqueous lentinan is a typical shear-thinning fluid, possessing potential as a viscosity control agent, and that a weak gel with entangled network structure formed. The dynamic oscillatory behavior of lentinan in the temperature range of 1-15 degrees C was also investigated by rheologic method. The storage modulus G' and complex viscosity eta* increased first with decreasing temperature, and underwent a maximum centered at 7-9 degrees C, and then decreased with further decreasing temperature. This abnormal phenomenon was ascribed to formation of rigid structure in the gel state, which was confirmed by the experimental results from micro-DSC. The micro-DSC curves showed that an endothermic peak appeared at 7-9 degrees C for lentinan in water upon heating, which was attributable to the intramolecular order-disorder structure transition similar to triple-helical polysaccharide schizophyllan. Namely, at lower temperature, the side glucose residues of lentinan (triplix II) formed a well-organized triple-helical structure (triplix I) through hydrogen-bonding with the surrounding water molecules. Moreover, this conformation transition was proved to be thermally reversible.

Water Structures of Differing Order and Mobility in Aqueous Solutions of Schizophyllan, a Triple-Helical Polysaccharide as Revealed by Dielectric Dispersion Measurements

Dielectric dispersion measurements were made on aqueous solutions of a triple-helical polysaccharide schizophyllan over a wide concentration range 10-50 wt % at -45 to +30 degrees C. In the solution state, three different water structures with the different relaxation times tau were found, namely, bound water (taul), structured water (taus), and loosely structured water (tauls) in addition to free water (tauP). Structured water is less mobile and loosely structured water is nearly as mobile as free water, but bound water with taul is much less mobile, thus taul >> taus >> tauls greater, similar tauP. The order-disorder transition accompanies the conversion between structured water and loosely structured water. However, the species with taus remains even in the disordered state and constitutes part of bound water in the entire temperature range. In the frozen state, in addition to bulk water formed by partial melting, two mobile species existed, which were assigned to liquidlike bound water and found to be a continuation of bound water in the solution state. These relaxation time data are discussed in connection with the entropy levels of the four structures deduced from heat capacity data (cf. Yoshiba, K.; et al. Biomacromolecules 2003, 4, 1348-1356).

Static water structure detected by heat capacity measurements on aqueous solutions of a triple-helical polysaccharide schizophyllan.

Heat capacity measurements were made on aqueous solutions of a triple-helical polysaccharide schizophyllan by precision adiabatic calorimetry over a wide range of concentrations 30.45-90.93 wt % at temperatures between 5 and 315 K. The heat capacity curves obtained were divided into four groups depending on the weight fraction of schizophyllan w regions I-IV. In region I, triple-helices with the sheath of bound water, structured water, and loosely structured water forming layers around the helix core are embedded in free water. In region II, there is no free water, and loosely structured water decreases until it vanishes, but structured water stays constant with increasing w. In region III, bound water remains unaffected, but structured water decreases with increasing w by overlapping each other. Finally, in region IV, only schizophyllan and bound water exist, the latter decreasing upon increasing w. The maximum thickness of each layer is 0.18(3) nm for bound water, 0.13(4) nm for structured water, and 0.23(6) nm for loosely structured water, and these layers of water are at the enthalpy levels of 53%, 93.7%, and nearly 100%, respectively, between ice (0%) and free water (100%).

Chiral Interactions in Polymer Liquid Crystals Reflecting Polymer Conformations: Triple-Helical Polysaccharide Schizophyllan and Poly(γ-benzyl l-glutamate)

Cholesteric pitch P was measured on D2O solutions of a triple-helical polysaccharide schizophyllan as functions of temperature and concentration. The value of P varied with concentration and temperature and showed different concentration dependencies at lower and higher temperatures, with a sudden decrease in P in between. This is due to the order−disorder transition in schizophyllan solutions around 17 °C in D2O. These data were analyzed by a statistical theory taking into account chiral repulsive and attractive interactions proposed by Sato et al. A subtle imbalance between the attractive and repulsive interactions gave rise to a large change in P. For the schizophyllan solutions, the attractive interaction changed according to the transition, while the change of the repulsive one was less remarkable. Similar analyses were performed on poly(γ-benzyl l-glutamate) data, elucidating the roles of the two interactions in determining the cholesteric structure.

Cholesteric Structure and Order-Disorder Transition in Aqueous Solutions of Schizophyllan, a Triple-Helical Polysaccharide

Cholesteric pitch (P=2π/qc ) was measured for aqueous solutions of a triple-helical polysaccharide schizophyllan as a function of temperature (T), concentration (c) and molecular weight (M v). qc was substantially independent of M v but changed with T and c. At higher temperatures, qc changed monotonously with T but showed a sudden decrease around 7–18°C on lowering T, which was due to the order-disorder transition of aqueous schizophyllan. The concentration dependence of qc for D2O solutions in the high temperature disordered state is the same as that for H2O solutions, and interpreted theoretically using the threaded effective freely jointed chain model. On temperature jump experiments. qc did not follow immediately T, but changed with the time elapsed. The relaxation time τch was less than a few minutes. Since the conformation of the triple helix follows T much faster than τch, this relaxation comes from rotational diffusion of the triple helix.

Structured water mobile below the freezing point in aqueous solutions of a triple-helical polysaccharide schizophyllan.

Aqueous solutions of schizophyllan are known to undergo a sharp order-disorder structure transition, where the ordered structure is formed of the side chains of the polysaccharide along with nearby water molecules around its helix core. The structured water in the ordered state is positionally fixed butrotates reacting with an electric field of about 100MHz as detected by time-domain reflectometry. The same method was extended to a wide temperature range between -20 and 30°C encompassing the freezing point of about -6°C. A 17.8wt% D2O solution showed both free and structured water dispersions at 0°C, but only the latter one below the freezing point. Thus we concluded that the structured water around the helix core remains unfrozen and mobile even when the system is frozen and no free water exists.

Transient electric birefringence study of rod-like triple-helical polysaccharide schizophyllan

The rotational dynamics of the uncharged, triple-helical polysaccharide schizophyllan was determined by transient electric birefingence. The field-free decay of a sample with weight average molecular weight of 437× 10 3gmol −1 deviated from single-exponential behaviour. The relaxation spectra obtained by analysis using the CONTIN program, were bimodal, whereas analysis by the stretched exponentials also accounted well for the field-free decay of the birefringence of the polydisperse sample. The mean rotational relaxation time extrapolated to infinite pulse duration of the orienting pulse for dilute aqueous solution of the 437×10 3gmol −1 sample was determined equal to 61 μs. Analysis of the influence of chain rigidity on the rotational dynamics of the polydisperse sample indicated that the persistence length, L pe , was 70nm.

Solvent effects on the cooperative order–disorder transition of aqueous solutions of schizophyllan, a triple‐helical polysaccharide

A triple helical polysaccharide schizophyllan in aqueous solution exhibited a highly cooperative transition between ordered and disordered states associated with the conformation of its side chains and nearby water molecules. The transition was followed by optical rotation and calorimetry using water containing additives such as NaOH and DMSO as solvents. The ordered state was stabilized or destabilized depending on the kind and amount of the additive employed; in particular, the addition of DMSO had a remarkable stabilizing effect. This effect was analyzed by means of a statistical mechanical theory of linear cooperative transitions, where DMSO was assumed to interact favorably with the ordered side chains. A small amount of NaOH in a solvent mixture stabilized the ordered state and made the transition curve very gradual. No molecular mechanism was elucidated to account for the role of NaOH.