Structure and chain conformation of five water-soluble derivatives of a beta-D-glucan isolated from Ganoderma lucidum

A linear water-insoluble (1-->3)-beta-D-glucan, coded as GL-IV-I, was isolated from the fruit body of Ganoderma lucidum by extracting with NaOH solution. Its derivatives were prepared by using sulfation, carboxymethylation, hydroxyethylation, hydroxypropylation, and methylation, respectively, and these were labeled as S-GL, CM-GL, HE-GL, HP-GL and M-GL. Five derivatives exhibited good water solubility. Their structures and chain conformations were investigated with infrared spectroscopy, elemental analysis (EA), one- and two-dimensional NMR spectroscopy, laser light scattering (LLS), and size-exclusion chromatography combined with LLS (SEC-LLS). The reactivity of the hydroxyl group of GL-IV-I was ordered as C-6>C-4>C-2 for the five derivatives. The degree of substitution (DS) of the derivatives was calculated from EA and NMR spectroscopy to be from 0.32 to 1.18. The weight-average molecular mass (M(w)) of GL-IV-I, S-GL, CM-GL, HE-GL, HP-GL, and M-GL was 13.3 x 10(4), 10.1 x 10(4), 6.3 x 10(4), 7.2 x 10(4), 5.1 x 10(4), and 14.1 x 10(4), respectively. The conformation analysis studies revealed that GL-IV-I exists as a compact coil in dimethyl sulfoxide, whereas the five derivatives are slightly expanded flexible chains in 0.9% aqueous NaCl solution.