Ordering in aqueous polysaccharide solutions. II. Optical rotation and heat capacity of aqueous solutions of a triple‐helical polysaccharide schizophyllan*

Abstract

Deuterium oxide solutions of schizophyllan, a triple-helical polysaccharide, undergoing an order-disorder transition centered at 17 degrees C, were studied by optical rotation (OR) and heat capacity (C(p)) to elucidate the molecular mechanism of the transition and water structure in the solution and frozen states. The ordered structure at low temperature consisted of the side chains and water in the vicinity forming an ordered hydrogen-bonded network surrounding the helix core and was disordered at higher temperature. In the solution state appeared clearly defined transition curves in both the OR and C(p) data. The results for three samples of different molecular weights were analyzed theoretically, treating this transition as a typical linear cooperative transition from the ordered to disordered states and explained quantitatively if the molecular weight polydispersity of the sample was considered. The excess heat capacity C(EX)(p) defined as the C(p) minus the contributions from schizophyllan and D(2)O was estimated. In the frozen state it increased with raising temperature above 150 K until the mixture melted. This was compared with the dielectric increment observed in this temperature range and ascribed to unfreezable water. From the heat capacity and dielectric data, unfreezable water is mobile but more ordered than free water. In the solution state, the excess heat capacity originates from the interactions of D(2)O molecules as bound water and structured water, and so forth. Thus the schizophyllan triple helix molds water into various structures of differing orders in solution and in the solid state.

Gelation behaviors of schizophyllan‐sorbitol aqueous solutions

“…helix II to ordered triple helix I. 11,19,[21][22][23][24][25] As for SPG-sorbitol aqueous solution, does the transition in DSC or ORD originate from the same molecular event as that without sorbitol? To answer this, we further decreased the sorbitol content and extrapolated it to zero.…”

“…triple helix I, [21][22][23][24][25] which has already been proven highly cooperative and almost independent of SPG concentration; 21 triple helix II to I transition causes the increase in triple helix diameter and stiffness, 19,22 and decreases the mobility of SPG chains and the surrounding water molecules, 24,25 which greatly enhances the entanglement of SPG chains; the extremely entangled SPG chains make a three-dimensional network, which confers to the SPGsorbitol system the gel-like properties. It should be emphasized that no junction zone is formed upon cooling and the gel-like properties just come from the extremely entangled network with a very long relaxation time, which will be proven by the multirun strain sweep measurements shown later.…”

“…The transition from triple helix II to I was considered as an intramolecular conformational change in which SPG side groups cooperate with water molecules to form an ordered shell-like structure surrounding the triple helix. [21][22][23][24][25] For SPG-sorbitol aqueous systems, sorbitol doesn't participate in the formation of ordered structure and just plays a role in reducing the water activity, which leads to a higher transition temperature. Thereby it is easy to understand why ⌬H p is not affected by sorbitol content.…”

“…Because the exothermic DSC peak upon cooling for SPG-sorbitol aqueous solution is considered to come from the highly cooperative transition from triple helix II to I, we can use the method described by Kitamura and Kuge, 11 Sturtevant, 52 and Yoshiba et al 25 to test its cooperativity. Figure 12 shows the linear van't Hoff plots obtained from the exothermic DSC peaks upon cooling for SPG-sorbitol aqueous solutions with sorbitol contents of 27, 42, and 51 wt%.…”

“…The triple helix below the transition temperature is stiffer, more ordered, and known as triple helix I, whereas the version above the transition temperature is relatively disordered and known as triple helix II. 11,19 Based on differential scanning calorimetry (DSC), optical rotation (ORD), and dielectric relaxation (DR) measurements as well as H-D isotope effect, Teramoto and coworkers [21][22][23][24][25] proposed a molecular model for the triple helix I, in which SPG side groups belonging to the same strand are linked together, with water molecules sandwiched between them, into a helical chain (as indicated by the dashed line in Figure 1b), and three such chains surrounding the helix core form an outer shell and give rise to the enhanced stiffness of the triple helix. It should also be pointed out that the transition from triple helix II to I depends remarkably on solvent conditions.…”