Self-assembled nanogels of cholesteryl-modified polysaccharides: Effect of the polysaccharide structure on their association characteristics in the dilute and semidilute regimes

The assembly of cholesteryl derivatives of the highly branched polysaccharide mannan Mw = (5.2 × 10⁴ g/mol) in dilute aqueous solution was investigated by ¹H nuclear magnetic resonance (NMR) spectroscopy, size-exclusion chromatography coupled with multiangle laser scattering (SEC-MALLS), dynamic light scattering (DLS), atomic force microscopy (AFM), fluorescence quenching, and fluorescence depolarization measurements. In the dilute regime, cholesteryl-bearing mannans (CHM) containing ∼1 cholesteryl group per 100 mannopyranose units formed nanogels with a hydrodynamic radius (R_H) of ∼20 run containing ∼8 macromolecules held together via hydrophobic nanodomains consisting of ∼9 cholesteryl groups. Their density (Φ_h (∼0.02) was significantly lower than the density (∼0.16) of nanogels formed by a cholesteryl derivative of the linear polysaccharide pullulan (CHP) of identical molar mass and level of cholesteryl substitution. In the semidilute regime, CHM nanogels formed a macrogel network for concentrations higher than 12.5% w/w, whereas CHP nanogels underwent macrogelation only above a threshold concentration of 8.0% w/w, as revealed by oscillatory and steady-shear viscosity measurements. The differences in the solution properties of CHM and CHP reflect differences in their assembly on the molecular level, in particular, the size and number of hydrophobic nanodomains and the hydration level. They are attributed to differences in the mobility of the cholesteryl groups which, itself, can be traced to the fact that in CHM the cholesteryl groups are predominantly linked to short oligomannopyranose branches, whereas in CHP they are linked to the polymer main chain. Our study provides a novel means to nanoengineer polysaccharide nanogels which may find unique biotechnological applications.