Interaction of αS1-Casein with Polyethylenimine

Abstract

αS1-Casein reacted with polyethylenimine, PEI 1000, flocculating at pH 9.5 but dissolved completely in 4M urea. Polyethylenimine, containing 16% nitrogen, of which 20% is primary amine, migrated to the cathode by polyacrylamide gel electrophoresis at pH 7.0 and retarded the migration of αS1-casein when run as a mixture. Polyethylenimine revealed a single broad peak at 3.1ppm in the high resolution nuclear magnetic resonance spectrometer which was almost pH insensitive. However, in the presence of αS1-casein, the PEI peak shifted to 3.3ppm at pH 9.7 and approached 3.1ppm as the pH was increased to 11.0. At pH 9.7 the resonance signals for the PEI-αS1-casein mixture were broader than those for each component indicating the formation of PEI-protein complex. A circular dichroic spectrum of αS1-casein was not affected by combining PEI, indicating no conformational change. Ultracentrifugation of the mixture of PEI 1000 and αS1-casein showed an interaction peak sedimenting slightly faster than αS1-casein.The interaction of PEI 1000 with αS1-casein was pH and concentration dependent and dissociated in urea above 4M and by increased pH, suggesting that the polymer associated at an electronegative region of the casein.