[Separation and characterization of Gastrodia elata polysaccharides based on asymmetrical flow field-flow fractionation: steric transition phenomenon].

Asymmetrical flow field-flow fractionation (AF4), a gentle tool for the separation and characterization of particles and macromolecules, has attracted increased interest in recent years owing to its broad dynamic size range and utilization of "open channel" voids in the packing or stationary phase. A steric transition phenomenon in which the sample elution mode change from the normal mode to the steric/hyperlayer mode occurs. Accurate characterization by AF4 requires the absence of steric transition, particularly when the sample has a broad size distribution, because the effect of the combination of different modes is difficult to interpret. In this study, the relative molecular mass (M), radius of gyration (R_g), and conformation of Gastrodia elata polysaccharides (GEPs) were characterized using AF4 coupled with online multi-angle light scattering (MALS) and differential refractive index (dRI) detection (AF4-MALS-dRI). Steric transition was observed during GEP separation by AF4 owing to the broad size distribution of the molecules. This phenomenon would result in the inaccurate characterization of the GEPs in terms of M and R_g because two GEP groups of different sizes may elute together. In this study, the effects of constant and exponentially decaying cross-flow rates, sample mass concentration, and spacer thickness on steric transition were systematically investigated. The results indicated that a high GEP mass concentration (i. e., 0.75 mg/mL) can lead to steric transition. The spacer thickness affected the resolution and retention time of the GEPs and changed the steric transition point (d_i). An exponentially decaying cross-flow rate not only adjusted the d_i of the polydisperse GEP samples but also improved the GEP resolution and shortened the analysis time. The influence of steric transition was solved under the following operating conditions: injected GEP mass concentration=0.5 mg/mL; injection volume=50 μL; spacer thickness=350 μm; detector flow rate=1.0 mL/min; and cross-flow rate exponentially decayed from 0.2 to 0.05 mL/min with a half-life of 2 min. Moreover, the influence of GEP origins and ultrasound treatment time on the M and R_g distributions and conformation of GEPs were investigated under the optimized operating conditions. The results showed that the M and R_g distributions of Yunnan and Sichuan GEPs decreased with increasing ultrasound time. When the ultrasound treatment time was 15 min, the Yunnan GEPs had a loosely hyperbranched chain conformation, whereas the Sichuan GEPs had a spherical conformation. When the ultrasound treatment time was increased to 30 or 60 min, the GEPs from both Yunnan and Sichuan had a hyperbranched chain conformation, indicating that ultrasound treatment resulted in GEP degradation. Under the same extraction conditions, GEPs from Yunnan had larger M and R_g values than those from Sichuan. AF4-MALS-dRI showed good repeatability for the characterization of GEPs under the optimized operating conditions. The relative standard deviations of R_g and M were 0.5% and 1.7%, respectively. The data presented in this study can be used as a starting point for in-depth studies on the structural bioactivity of GEPs.