Enzyme-Catalyzed Regioselective Modification of Starch Nanoparticles

Abstract

This chapter describes the selective esterification of starch nanoparticles catalyzed by Candida antartica Lipase B (CALB) in its immobilized (Novozyme 435) and free (SP-525) forms. The starch nanoparticles were made accessible for acylation reactions by formation of Aerosol-OT (AOT, bis[2-ethylhexyl]sodium sulfosuccinate) stabilized microemulsions. Analysis of these particles by light scattering showed at the high and low ends of the distribution aggregates with many 40-nm nanoparticles and AOT-stabilized micelles with one or two 40 nm starch nanoparticles, respectively. Acylation of the starch nanoparticles in microemulsions at 40°C for 48 h with vinyl stearate, E-caprolactone and maleic anhydride gave products with degrees of substitution (D.S.) of 0.8, 0.6 and 0.4, respectively. In all cases, substitution occurred regioselectively at the C-6 position of the glucose repeat units. The acylated particles after surfactant removal retained nanodimensions and could be redispersed in DMSO or water. Infrared microspectroscopy (IRMS) showed that AOT coated starch nanoparticles can diffuse into the outer 50 μm shell of catalyst beads. The close proximity of the lipase and substrates promotes the modification reactions.