Exploring the (1 -> 3)-beta-D-glucan conformational phase diagrams to optimize the linear to macrocycle conversion of the triple-helical polysaccharide scleroglucan

The immunologically important (1 --> 6) comb-like branched (1 --> 3)-beta-D -glucans scleroglucan, schizophyllan, lentinan, and others, exist mainly as linens triple-helical structures in aqueous solution. Partial interconvers ion from linear to circular topology has been reported to take place follow ing conformational transition of the triple-helical structure and subsequen t, regeneration of the tripler conformation. We here report on experimental data indicating that complete strand separation of the triple-helical stru cture is required for this interconversion. NaOH or dimethylsulfoxide was u sed to induce dissociation of rile triples at combinations of concentration s and temperatures shown by calorimetry to yield a conformational transitio n of the tripler structures. For the alkaline treatment at 55 degrees C, it is found that up, to about 30% of the material readily can be converted to the cyclic topology. This fraction increased to about 60% when the subsequ ent annealing of the scleroglucan in aqueous solution at pH 7 was carried o ut at 100 degrees C. Further increase of the annealing temperature yielded a smaller relative amount of cyclic species. The data indicate that the low er molecular weight fraction of the molecular weight distributions call be converted selectively to the I,macrocyclic topology by conditions that do n ot yield complete strand separation of the whole sample. These findings add to previous reports by providing more details about how the conditions req uired for the linear triplex to macrocycle interconversion relate to the co nformational properties of the triple-helical structure. (C) 1999 John Wile y & Sons, Inc.