The dependence of the conformation of a (1→3)-β-D-glucan on chain-length in alkaline solution

(1→3)-β-D-Glucans of various degrees of polymerization were prepared by degradation of a gel-forming D-glucan with formic acid. The degraded D-glucans were separated into a water-soluble fraction (soluble D-glucan) and an insoluble fraction (insoluble D-glucan). Both D-glucans were further fractionated. The optical rotation including determination of the o.r.d. curves of the fractions and of the original gel-forming D-glucans was measured at various sodium hydroxide concentrations (0–5M). The results indicate that (1→3)-β-D-glucans of $\text{DP}$n below ca. 25 (the soluble D-glucan) took a disordered form in both neutral and alkaline solutions, whereas the D-glucans of higher $\text{DP}$n (the insoluble and the original D-glucans) took an ordered structure in dilute alkaline solution (0.1M). The proportion of ordered structure in the insoluble D-glucan increases with $\text{DP}$n to attain a maximum value at a $\text{DP}$n of around 200; this may be the lower limit of $\text{DP}$n to permit gel formation in neutral media. The formation of complexes with Congo Red in alkaline solutions by the soluble and the insoluble D-glucans supports the same conclusions.