Triple helix conformation of β-d-glucan from Ganoderma lucidum and effect of molecular weight on its immunostimulatory activity

β-d-glucan (GLP20) isolated from Ganoderma lucidum fruiting bodies was successfully fractionated into five fractions with different weight-average molecular weights (Mw) through ultrasonic irradiation. The Mw, radius of gyration (Rg), hydrodynamic radius (Rh) and intrinsic viscosity ([η]) of these fractions in phosphate buffered saline (PBS) aqueous solution were determined using HPSEC-MALLS-RI-VS system. The results indicated that β-d-glucan displayed rigid chain conformations with ρ values lager than 2.06 for the five fractions. By applying the polymer solution theory, the exponent (ν and α) values of Rg = kMwν and [η] = kMwα were calculated as 0.72 and 1.05, respectively, which confirmed the rigid chain conformation of glucans with Mw from 2.9 × 105 to 2.42 × 106 g·mol−1. According to the known theory for chains, the molar mass per unit contour length ML, persistence length q and contour length h per main-chain glucose residue were estimated to be 2150 nm−1, 128 nm and 0.30 nm, respectively, which indicated that the glucan existed as triple-helical chains in PBS aqueous solution. Bioactivity study of glucan fractions on lymphocytes and THP-1 macrophages demonstrated that all these fractions could stimulate lymphocyte proliferation, promote macrophages to form pseudopodia, and enhance the levels of inflammatory cytokines IL-6 and TNF-α. Meanwhile, the glucan fractions with Mw higher than 1.82 × 106 g·mol−1 exhibited better activity on enhancing the release of inflammatory cytokines.