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ABSTRACT:Although schizophyllan undergoes no gelation by itself in its aqueous solution, the addition of sorbitol results in the formation of transparent gel at lower temperatures. The mechanism of the gel formation was discussed from the structural analysis of schizophyllan in the aqueous solution with and without sorbitol, based on the results of small-angle X-ray scattering. Here sorbitol was found to disentangle a part of triple-stranded helices and bridge the disentangled parts which serve as a crosslinking domain.KEY WORDS SchizophyllaniSorbitol I Small-Angle X-Ray Scattering I Gel I Viscoelasticity I Broken Rod Model I Molecular Model I Schizophyllan is an extracellular polysaccharide produced by fungus Schizophyllum commune, and is known to suppress tumor growth. It consists of (I-+ 3)-D-/3-glucan with a (1-+6)-D-{3-glucan branch per every 3 units as shown in Figure I. Schizophyllan is non-ionic and water-soluble. It assumes a triple-stranded helical conformation in aqueous solution as confirmed from the measurements of the molecular weight and viscosity 1 and by calorimetric measurements, 2 whereas in dimethyl sulfoxide (DMSO) it has a single coiled conformation. 3 Thus in water schizophyllan is rod-like and does not form gel by itself.Addition of D-sorbitol induces the thermoreversible gelation of schizophyllan aqueous solution, 4 · 5 where the gel temperature depends only on the sorbitol concentration. The present work aims to answer the question how schizophyllan forms gel in sorbitol aqueous solution. In an earlier work, 5 sorbitol was considered to reduce a water activity and thus schizophyllan triple-stranded helices aggregate to form junction zones. We examine the structural change of schizophyllan caused by solgel transition in sorbitol aqueous solution by means of small-angle X-ray scattering (SAXS). If the above conjecture on the junction zone is correct, we should observe the thicker cylindrical objects corresponding to the aggregated triple-stranded helices in gel.The work complements the rheological observation on gelation in the same system. 5 Since SAXS contains the structural information in the order of a few A to several hundreds A, the structural change of schizophyllan induced by gelation is traced in principle on a molecular level. Here the strategy is to fit the observed SAXS profile with an appropriate molecular model, which can be constructed from the crystallographic data and other results. By doing so, we may hope to visualize the gelation process of schizophyllan in sorbitol aqueous solution. ' To whom all correspondence should be addressed. 530EXPERIMENTAL D-sorbitol was dissolved in distilled and deionized water by heating the solutions to 90°C, and the solutions of four different sorbitol concentrations (0, 1. 5, 2. 7, and 4.0 moll-1 ) were prepared. After cooling the sorbitol solutions to 60°C, NaN3 (5 X 10-3 moll-1 ) was added in order to suppress bacteria growth. Schizophyllan (M w = 45 x I 0 4 ) was dissolved in those solutions to the total concentration of 16.3 mg ml-1...

The β‐1.3/1.6 glucan schizophyllan is a fungal polysaccharid that is known to suppress tumor growth. Addition of sorbitol to aqueous solutions of schizophyllan led to a thermoreversible formation of optically transparent gels. The gelation process was followed by low amplitude oscillatory shear as well as 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 an exponent n = 0.5. However, no power law was found for the time correlation functions as measured by dynamic light scattering. Two separated relaxation modes were found above and below the gel transition. The slow mode diverged below the gel temperature indicating a second critical point related to heterogeneities in the gel.

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