Single molecular mechanics of a cholesterol-bearing pullulan nanogel at the hydrophobic interfaces

The study of nanogel (hydrogel nanoparticle) has intensified in the last decade due to the enormous potential applications in biomimetics, biosensors, artificial muscles and drug delivery (or release) systems. Cholesterol-bearing pullulan (CHP) is composed of hydrophilic pullulan backbone and partly substituted hydrophobic cholesterol, and is capable of forming a stable hydrogel nanoparticle in aqueous solution due to the self-assembly of hydrophobic cholesterol moieties. The conformation of CHP changes dramatically at the hydrophobic interfaces. In order to understand the interfacial responses of CHP, the interaction forces of CHP nanogels to the hydrophobic HOPG (highly orientated pyrolytic graphite) or carbon-coated surfaces were measured using atomic force microscope. The freely jointed china model for CHP molecular elasticity was applied to the force—extension curves and debonding force—pull-off distance in order to estimate the contour lengths and the segment lengths of the CHP molecules. The segment length of CHP chains in aqueous solution was estimated 0.32±0.19 nm showing a very flexible chain. From our analysis of the dynamic force measurements, the debonding forces were shown to depend on the applied loading forces. The zero kinetic off-rate Koff(0) and the transition state xb were estimated to be 1.1×10−3 s−1 and 2.9 Å, respectively.