Macromolecular triplex zipping observed in derivatives of fungal (1 -> 3)-beta-D-glucan by electron and atomic force microscopy

Scleroglucan, a comb-like branched (I --> 3)-beta -D-glucan, dissolves in w ater as a stiff: triple-helical structure with the single glucose branches extending from the surface. The aim of this study is to investigate structu ral changes in the triple-helical structure associated with selective chemi cal modification of the side chains. Electron and atomic force microscopy, respectively, were used to investigate the macromolecular structures of ald ehyde and carboxylated derivatives of scleroglucan-namely, scleraldehyde an d sclerox-with different degrees of substitution. Scleraldehyde was observe d to have structures resembling the tripler of the unmodified scleroglucan for all degrees of substitution up to I,a, Additionally, an increasing tend ency to aggregate for the higher degrees of substitution was observed Fully carboxylated scleroglucan, sclerox(1.0), prepared from solutions at ionic strengths below 1.0M, revealed dispersed. flexible, coil-like structures, T his indicates an electrostatic-driven strand separation of the scleroglucan triple-helical structure occurring concomitant with an increasing fraction of the side chains hearing carboxylate groups. Annealed sclerox(1.0) sampl es in aqueous 1.0 and 1.5M NaCl exhibited partly, or completely reassociate d triplex ensembles, with species ranging from apparently fully zipped line ar and circular topologies, partly zipped structures with tripler strand se paration occurring at the ends, to dispersed single-strands with random coi l-like appearance. This study shows that periodate oxidation of the sclerog lucan side chains is not a sufficient modification of the side chains to in duce dissociation of the triple-helical structure, whereas further oxidatio n of the side chains to carboxylic groups dissociates the triple-helical st ructure when the degree of substitution is above 0.6. (C) 2001 John Wiley & Sons, Inc.