Transition from Triple Helix to Coil of Lentinan in Solution Measured by SEC, Viscometry, and 13C NMR

Abstract

Lentinan, β-(1 → 3)-d-glucan with (1 → 6) branching, was isolated from Lentinus edodes. Weight-average molecular weight Mw, radius of gyration <s2>z1/2 and intrinsic viscosity [η] of Lentinan in 0.2 M NaCl aqueous solution, dimethylsulfoxide (DMSO) and water/DMSO mixtures were measured by light scattering (LS), size exclusion chromatography (SEC) combined with LS, and viscometry. The results indicated that the glucan exists mainly as triple-helical chains in 0.2 M NaCl aqueous solution and water/DMSO mixtures with over 20 wt% water content, and as single-flexible chain in DMSO. The data from SEC-LS, viscosity and 13C NMR measurements proved strongly that the helix-coil conformation transition occurred in a narrow range from 80 to 85 wt% DMSO aqueous solution, accompanying with obvious changes of Mw, <s2>z1/2, [η] as well as signals of C6 and C6s. The transition of Lentinan in water/DMSO mixture was irreversible. The difference in 13C NMR spectra for the triple-helical and coil conformations was the disappearance of the signals of C3 in β-(1 → 3)- linked backbone and the enhancement in relative intensities of glucose substituted C6s in the helix state, as well as the appearance of an asymmetric and broad peak of C6 in the intermediate of the conformation change. This suggests that the immobilization of the backbone by binding with intra- and intermolecular hydrogen bonds resulted in the loss of the signals of its carbon atoms in the triple helix state. An overcoating cylinder model composed of the β-(1 → 3)- linked backbone as helix core and the side chains as rotatable overcoat was proposed to illustrate the triple-helical conformation and its transition in the solution.