Graft
copolymers consisting of two different zwitterionic blocks were synthesized
via reversible addition fragmentation chain transfer polymerization.
These polymers showed dual properties of thermo- and pH-responsiveness
in an aqueous solution. Ultraviolet–visible spectroscopy and
dynamic light scattering were employed to study the phase behavior
under varying temperatures and pH values. Unlike the phase transition
temperatures of other graft copolymers containing nonionic blocks,
the phase transition temperature of these polymers was easily tuned
by changing the polymer concentration. Owing to the biocompatible
and stimuli-responsive nature of the polymers, this system was shown
to effectively release proteins (lysozyme) while simultaneously protecting
them against denaturation. The positively charged lysozyme was shown
to bind with the negatively charged polymer at the physiological pH
(pH 7.4). However, it was subsequently released at pH 3, at which
the polymer exhibits a positive charge. Protein aggregation studies
using a residual enzymatic activity assay, circular dichroism, and
a Thioflavin T assay revealed that the secondary structure of the
lysozyme was retained even after harsh thermal treatment. The addition
of these polymers helped the lysozyme retain its enzymatic activity
and suppressed its fibrillation. Both polymers showed excellent protein
protection properties, with the negatively charged polymer exhibiting
slightly superior protein protection properties to those of the neutral
polymer. To the best of the authors’ knowledge, this is the
first study to develop a graft copolymer system consisting of two
different zwitterionic blocks that shows dual thermo- and pH-responsive
properties. The presence of the polyampholyte structure enables these
polymers to act as protein release agents, while simultaneously protecting
the proteins from severe stress.