A Novel Temperature-and pH-responsive Polymer-biomolecule Conjugate Composed of Casein and Poly(N-isopropylacrylamide)

A sort of novel stimuli-responsive water-soluble polymer-biomolecule conjugate, casein-g-poly(N-isopropylacrylamide) (PNIPAAm), was synthesized by free radical graft copolymerization of N-isopropylacrylamide (NIPAAm) from casein. The graft polymerization was induced by a small amount of t-butyl hydroperoxide (TBHP) in water under general condition. Free radicals on the amino groups of casein and t-butoxy radicals were generated concurrently, which initiated the graft copolymerization and homopolymerization of NIPAAm, respectively. The graft copolymers and homopolymer of NIPAAm were isolated and the chemical structure of the graft copolymer was characterized by Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The phase-transition behaviours of the graft copolymers in aqueous solutions were investigated by the turbidity measurements. Casein-g-PNIPAAm did not precipitate at around the isoelectric pH (IP) of casein at 25 C and the lowest transmittance was changed as the pH changed. These results suggest that the PNIPAAm offers a greater hindrance to the aggregation of casein at 25 C and the casein offers a hindrance to the aggregation of PNIPAAm at the temperature of lower critical solution temperature (LCST) and at the pH apart from the IP. In addition, casein almost has no effect on LCST. The graft copolymer is stimuli-sensitive with respect to both temperature and pH in aqueous solutions and the polymeric micelles with different structures can be obtained simply by changing the environmental conditions. It associates into core-shell micelles in aqueous solution with collapsed PNIPAAm as a core as well as inverse core-hair micelles with expanded casein as a core on changing temperature or pH, indicated by transmission electron microscopy (TEM). With the casein being a nature protein and the graft copolymers are metal ion-free therefore, they may have great potential for biomedical and biomaterial applications.