Ion and Temperature Sensitive Polypeptide Block Copolymer

A poly(ethylene glycol)/poly(l-alanine) multiblock copolymer incorporating ethylene diamine tetraacetic acid ([PA-PEG-PA-EDTAm) was synthesized as an ion/temperature dual stimuli-sensitive polymer, where the effect of different metal ions (Cu2+, Zn2+, and Ca2+) on the thermogelation of the polymer aqueous solution was investigated. The dissociation constants between the metal ions and the multiblock copolymer were calculated to be 1.2 x 10(-7), 6.6 x 10(-6), and 1.2 X 10(-4) M for Cu2+, Zn2+, and Ca2+, respectively, implying that the binding affinity of the multiblock copolymer for Cu2+ is much greater than that for Zn2+ or Ca2+. Atomic force microscopy and dynamic light scattering of the multiblock copolymer containing metal ions suggested micelle formation at low temperature, which aggregated as the temperature increased. Circular dichroism spectra suggested that changes in the a-helical secondary structure of the multiblock copolymer were more pronounced by adding Cu2+ than other metal ions. The thermogelation of the multiblock copolymer aqueous solution containing Cu2+ was observed at a lower temperature, and the modulus of the gel was significantly higher than that of the system containing Ca2+ or Zn2+, in spite of the same concentration of the metal ions and their same ionic valence of +2. The above results suggested that strong ionic complexes between Cu2+ and the multiblock copolymer not only affected the secondary structure of the polymer but also facilitated the thermogelation of the polymer aqueous solution through effective salt-bridge formation even in a millimolar range of the metal ion concentration. Therefore, binding affinity of metal ions for polymers should be considered first in designing an effective ion/temperature dual stimuli-sensitive polymer.