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

β-D-glucan (GLP20) isolated from Ganoderma lucidum fruiting bodies was successfully fractionated into five fractions with different weight-average molecular weights (M_w) through ultrasonic irradiation. The M_w, radius of gyration (R_g), hydrodynamic radius (R_h) 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 R_g = kM_w^ν and [η] = kM_w^α were calculated as 0.72 and 1.05, respectively, which confirmed the rigid chain conformation of glucans with M_w 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 M_L, 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 fractions with M_w higher than 1.82 × 10⁶ g mol⁻¹ exhibited better immunological activity. These fractions stimulated lymphocyte proliferation, promoted macrophages to form pseudopodia, and enhanced the levels of inflammatory cytokines IL-6 and TNF-α.