Injectable Hydrogel: Amplifying the pH Sensitivity of a Triblock Copolypeptide by Conjugating the N-Termini via Dynamic Covalent Bonding.

We explore the self-assembly behavior of aqueous solutions of an amphiphilic, pH-sensitive poly(l-alanine)-b-poly(l-glutamic acid)-b-poly(l-alanine), (A5E11A5) triblock copolypeptide, end-capped by benzaldehyde through Schiff base reaction. At elevated concentrations and under physiological pH (7.4) and ionic strength (0.15M), the bare copolypeptide aqueous solutions underwent a sol-gel transition after heating and slow cooling thermal treatment, forming opaque stiff gels due to a hierarchical self-assembly that led to the formation of β-sheet-based twisted super fibers (Popescu et al. Soft Matter 2015, 11, 331-342). The conjugation of the N-termini with benzaldehyde (Bz) through a Schiff base reaction amplifies the copolypeptide pH-sensitivity within a narrow pH window relevant for in vivo applications. Specifically, the dynamic character of the imine bond allowed coupling/decoupling of the Bz upon switching pH. The presence of Bz conjugates to the N-termini of the copolypeptide resulted in enhanced packing of the elementary superfibers into thick and short piles, which inhibited the ability of the system for gelation. However, partial cleavage of Bz upon lowering pH to 6.5 prompted recovery of the hydrogel. The sol-gel transition triggered by pH was reversible, due to the coupling/decoupling of the benzoic-imine dynamic covalent bonding, endowing thus the gelling system with injectability. Undesirably, the gelation temperature window was significantly reduced, which however can be regulated at physiological temperatures by using a suitable mixture of the bare and the Bz-conjugated coplypeptide. This triblock copolypeptide gelator was investigated as a scaffold for the encapsulation of polymersome nanocarriers, loaded with a hydrophilic model drug, calcein. The polymersome/polypeptide complex system showed prolonged probe release in pH 6.5, which is relevant to extracellular tumor environment, rendering the system potentially useful for sustained delivery of anticancer drugs locally in the tumor.