Cholesterol mediates spontaneous insertion of Lycium barbarum polysaccharides in biomembrane model

In this work, the adsorptive behavior of Lycium barabrum polysaccharides (LBP) on 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine(POPC)/1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and POPC/DPPC/Cholesterol (Chol) monolayer films were investigated using the Langmuir films technique and atomic force microscopy (AFM). For the POPC/DPPC system, the pi-A isotherms shifted to larger areas and the surface pressure variations (Delta pi) in the adsorption curves (10 mN/m and 35 mN/m) increased with the increase in the LBP concentration. For the POPC/DPPC/Chol system, pi-A isotherms moved to larger areas with the addition of LBP, and the Delta pi values were also lower than those of the POPC/DPPC system. These were ascribed to hydrogen bond network formation between LBP and the lipids. The excess per molecular area (Delta A(exc)) and the excess Gibbs free energy of mixing (Delta GmixExc) revealed that the repulsion force was the interaction between POPC and DPPC molecules. Meanwhile, the repulsion force increased with the addition of LBP. The addition of cholesterol decreased the force. The interactions among molecules were also evaluated by the values of K (the speed of adsorption of LBP), A(p) (the number of each lipid molecule combining with the polysaccharide molecules in the mixed monolayer), Gamma (the surface excess concentration) and A (the area per molecule). As shown in AFM images, the morphology of the monolayer films also indicated that LBP was easily adsorbed onto the POPC/DPPC monolayer film via hydrogen bond and the weak Van der Waals interaction. In summary, cholesterol could affect spontaneous insertion of LBP into the POPC/DPPC monolayer films from phosphate buffer solution.