Enzyme-catalyzed regioselective modification of starch nanoparticles

The selective esterification of starch nanoparticles was performed using as catalyst Candida antartica Lipase B (CAL-B) in its immobilized (Novozym 435) and free (SP-525) forms. The starch nanoparticles were made accessible for acylation reactions by formation of Aeroaol-OT (AOT, bis(2ethylhexyl)sodium sulfosuccinate) stabilized microemulsions. Starch nanoparticles in microemulsions were reacted with vinyl stearate, ∈-caprolactone, and maleic anhydride at 40°C for 48 h to give starch esters with degrees of substitution (DS) of 0.8, 0.6, and 0.4, respectively. Substitution occurred regioselectively at the C-6 position of the glucose repeat units. Infrared microspectroscopy (IRMS) revealed that AOT-coated starch nanoparticles diffuse into the outer 50 μm shell of catalyst beads. Thus, even though CAL-B is immobilized within a macroporous resin, CAL-B is sufficiently accessible to the starch nanoparticles. When free CAL-B was incorporated along with starch within AOT-coated reversed micelles, CAL-B was also active and catalyzed the acylation with vinyl stearate (24 h, 40°C) to give DS = 0.5. After removal of surfactant from the modified starch nanoparticles, they were dispersed in DMSO or water and were shown to retain their nanodimensions.