Novel polyion complex micelles entrapping enzyme molecules in the core. 2. Characterization of the micelles prepared at nonstoichiometric mixing ratios

Physicochemical properties of polyion complex (PIC) micelles formed from chicken egg white lysozyme and poly(ethylene glycol)-poly(aspartic acid) block copolymer at nonstoichiometric mixing ratios were investigated using dynamic and static light scattering as well as laser-Doppler electrophoresis measurements. The formation of spherical PIC micelles with core-shell architecture was suggested by dynamic light scattering and laser-Doppler electrophoresis measurements despite a change in mixing ratio. Estimation of the critical association concentration (cac) using Debye plots of static light scattering (SLS) confirmed that the cac values converted to the concentration of the charged segments were independent of the mixing ratio. Further, the association number of lysozyme and PEG- P(Asp), the core size, and the corona thickness were calculated by using the apparent weight average molecular weight determined from Zimm plots of SLS and the density of lysozyme and P(Asp). It was indicated that the association number of PEG-P(Asp) and the corona thickness increased with an increase in the mixing ratio. However, the association number of lysozyme and the core size kept constant and were determined to be ca. 50 molecules and ca. 7 nm, respectively. That is, the density of lysozyme in the core was also independent of the mixing ratio. Such PIC micelles entrapping enzymes in the core are expected to be available as functional materials including a carrier system in drug delivery and a nanoreactor for enzymes.