Impact of Ethanol Precipitation Gradient on the Molecular Weight Fractionation and Rheological Behavior of Ganoderma lucidum Polysaccharides

This study investigated the impact of a sequential ethanol precipitation gradient on the molecular weight distribution, chemical composition, and macromolecular architecture of Ganoderma lucidum polysaccharides. Fractions obtained via serial precipitation (30%, 60%, and 90% ethanol: GLP30, GLP60, and GLP90, respectively) and a traditional single-step precipitation (75%: GLP75) were characterized using mainly HPSEC-MALLS-RI, GC-MS, NMR spectroscopy (HSQC-DEPT), and rheological analyses. The results demonstrated that sequential precipitation effectively fractionated the polymers based on structural profiles and molecular masses. GLP30 and GLP90 primarily comprised α- and β-D-glucans (70–74% Glc), and heteropolysaccharides with mannose and arabinose, whereas GLP60 exhibited the highest structural heterogeneity, encompassing complex heteropolysaccharides and uronic acids (5.8%). The single-step GLP75 fraction yielded a heterogeneous mixture of these distinct populations. NMR spectra showed signals that corroborated the monosaccharide compositions. High-molecular-weight fractions (GLP30 and GLP75) retained rigid triple-helical conformations, displaying non-Newtonian shear-thinning behavior, a weak-gel viscoelastic profile, and remarkably high intrinsic viscosities ([η] 39.45 and 610.73 dL/g, respectively). Conversely, the low-molecular-weight fractions (GLP60 and GLP90), although retained rigid triple-helical conformations, exhibited negligible hydrodynamic volumes ([η] < 1.6 dL/g). Overall, this research provides a highly feasible strategy to successfully separate macrofungal polysaccharides according to both their molecular weight and monosaccharide distribution, minimizing some purification steps.